Description Title of Invention: INFORMATION PROCESSING APPARATUS, INFORMATION PROCESSING METHOD, INFORMATION PROCESSING SYSTEM, AND COMPUTER PROGRAM PRODUCT Technical Field [0001] The present disclosure relates to an information processing apparatus, an information processing method, an information processing system, and a computer program product. Background Art [0002] Recently, receiver apparatuses capable of receiving wireless signals transmitted from satellites have been installed in moving bodies such as vehicles, mobile telephones, and the like. By using GPS (Global Positioning System) positioning, it is possible to estimate the position of the moving body in which the receiver apparatus is installed. Such position estimating technology that uses a receiver apparatus is an important base technology in a wide range of fields such as navigation, security and entertainment. However, position estimating technology based on GPS positioning requires a long time at startup for supplementary synchronization and is difficult to use inside buildings and underground that are out of range for the wireless signals from satellites. [0003] Also, as disclosed in Japanese Laid-Open Patent Publication No. 2008-104029 for example, a method of estimating the position of a wireless terminal based on the signal strength at the wireless terminal of wireless signals transmitted from base stations on a wireless LAN (Local Area Network) has been proposed. More specifically, it is possible to estimate the position of the wireless terminal based on position information of respective base stations registered in advance and the distances between the base stations and the wireless terminal calculated from the respective signal strengths of the wireless signals. Since base stations on a wireless LAN are also set up inside buildings and underground, by using this method of estimating, it is possible to carry out position estimation inside buildings and underground which has been difficult for position e s timating technology based on GPS positioning. Citation List Patent Literature [0004] PTL 1: JP 2008- 104029A Summary Technical Problem [0005] However, a case can be imagined where, due to movement of the base station or the like, the registered base station position information is erroneous. Accordingly, there is concern that the precision of position estimation carried out based on such erroneous base station position information will fall. Such concern has significantly increased in recent years due to the spread of mobile base stations that are carried by the user. [0006] The present disclosure aims to provide a novel and improved information processing apparatus, information processing method, information processing system, and program that are capable of progressively updating base station position information to values with higher reliability. [0007] According to the present disclosure, an information processing apparatus includes a base station position estimating unit that estimates position information of a base station based on signal strength measurement information collected from wireless signals transmitted from a plurality of base stations; an estimation result evaluating unit that determines a determined index corre sponding to reliability of an estimated position information for the base station; and an information management unit configured to update a stored base station position information using the estimated position information when the determined index indicates a higher reliability than a stored index for the base station. [0008] According to one aspect, the apparatus also includes a reception unit that receives from a wireless terminal signal strength measurement information; and a base station information storage unit that stores for the base station the stored base station position information, and the stored index. [0009] A feature of the apparatus is that the information management unit is configured to lower the stored index lower to indicate lower reliability when a difference between the estimated position information and stored base station position information exceeds a predetermined value. [0010] According to another aspect the apparatus includes a measurement information storage unit that stores the signal strength measurement information received by the reception unit. [00 11] Another feature is that the base station position estimating unit refers to the measurement information storage unit and estimates the estimated position information of the base station based on magnitudes of signals strengths of wireless signals received by the reception unit. [00 12] Another feature is that the estimation result evaluating unit calculates an average of the signal strengths of the wireless signals. [0013] The apparatus may also include a history information storage unit that stores history information for base stations having changes to the stored index to reflect lowered reliability; a mobile base station determining unit that determines whether the base station is a mobile base station based on the history information; and a mobile base station information storage unit that stores information regarding base stations that have been determined by the mobile base station determining unit to be mobile base stations. [0014] According to another aspect the mobile base station determining unit determines that the base station is a mobile base station where the reliability of the stored index has been lowered with a specified frequency or made higher. [0015] Optionally the apparatus includes a terminal position estimating unit that estimates a position of a wireless terminal based on measurement information received from the wireless terminal and the stored base station position information, wherein the terminal position estimating unit estimates the position of the wireless terminal using measurement results of base stations where the reliability exceeds a prede termined threshold. [0016] According to one feature the terminal position estimating unit estimates the position of the wireless terminal using measurement results of base stations whose reliability exceeds the threshold and are not stored as mobile base stations in the mobile base station information storage unit. [0017] According to another feature the information management unit is configured to lower the stored index to the pre determined threshold or below when a difference between an estimated position of the base station and a position shown by the base station position information stored in the base station information storage unit exceeds a set value. [0018] A method embodiment includes 12 estimating position information of a base station based on signal strength mea surement information collected from wireless signals transmitted from a plurality of base stations; determining with a processor a determined index corresponding to reliability of an estimated position information for the base station; and updating in a computer readable storage device a stored base station position in formation using the estimated position information when the determined index indicates a higher reliability than a stored index for the base station. [0019] The method may also include receiving from a wireless terminal signal strength measurement information; and storing in a base station information storage unit the stored base station position information, and the stored index. [0020] According to an aspect of the method, the method involves changing the stored index lower to indicate lower reliability when a difference between the estimated position information and stored base station position in formation exceeds a predetermined value. [0021] According to an aspect of the method, the method involves the estimating includes estimating the estimated position information of the base station based on magnitudes of signals strengths described by the signal strength mea surement information. [0022] The method also optionally includes storing history information for base stations having changes to the stored index to reflect lowered reliability; determining whether the base station is a mobile base station based on the history in formation; and storing information regarding base stations that have been determined to be mobile base stations. [0023] According to a computer readable storage device embodiment having instructions that when executed by a processor perform a method, the method including estimating position information of a base station based on wireless terminal signal strength measurement information of wireless signals transmitted from a plurality of base stations; determining with a processor a determined index corresponding to an estimated position information for the base station; and updating in a computer readable storage device a stored base station position in formation using the estimated position information when the determined index indicates a higher reliability than a stored index for the base station. [0024] The method may also include receiving from a wireless terminal signal strength measurement information; and storing in a base station information storage unit the stored base station position information, and the stored index. [0025] The method may also include changing the stored index lower to indicate lower reliability when a difference between the estimated position information and stored base station position information exceeds a predetermined value. [0026] One aspect is that according to the method the estimating includes estimating the estimated position information of the base station based on magnitudes of signals strengths described by the signal strength mea surement information. Advantageous Effects of Invention [0027] According to embodiments of the present disclosure described above, it is possible to progressively update base station position information to highly reliable values. Brief Description of Drawings [0028] [fig. l]Fig. 1 is a diagram useful in explaining the configuration of an information processing system according to embodiments of the present disclosure. [0029] [fig.2]Fig. 2 is a diagram useful in explaining specific examples of measurement in formation. [0030] [fig.3]Fig. 3 is a functional block diagram showing the configuration of a position e s timating apparatus according to a first embodiment of the present disclosure. [0031] [fig.4]Fig. 4 is a diagram useful in explaining specific examples of base station in formation stored by a base station information storage unit. [0032] [fig.5]Fig. 5 is a diagram useful in showing specific examples of measurement in formation stored by a measurement information storage unit. [0033] [fig.6]Fig. 6 is a diagram useful in explaining a specific example of estimating a terminal position. [0034] [fig.7]Fig. 7 is a diagram useful in explaining the concept of an estimation error. [0035] [fig.8]Fig. 8 is a diagram useful in explaining the concept of an estimation error. [0036] [fig.9]Fig. 9 is a diagram useful in explaining one example of measurement in formation used when calculating the position information and the estimation error of a base station. [0037] [fig.l0]Fig. 10 is a diagram useful in explaining one example of measurement in formation used when calculating the position information and estimation error of a base station. [0038] [fig. 1l]Fig. 11 is a diagram useful in explaining an example of updating the base station information stored by the base station information storage unit. [0039] [fig. 12] Fig. 12 is a flowchart showing the operation of the position estimating apparatus according to the first embodiment. [0040] [fig.l3]Fig. 13 is a diagram useful in showing changes in the measurement information when a base station has moved. [0041] [fig. 14] Fig. 14 is a functional block diagram showing the configuration of a position estimating apparatus according to a second embodiment. [0042] [fig. 15]Fig. 15 is a diagram useful in explaining an example of updating the base station information stored by the base station information storage unit. [0043] [fig. 16]Fig. 16 is a flowchart showing a first example operation of the position e s timating apparatus according to the second embodiment. [0044] [fig.l7]Fig. 17 is a flowchart showing a second example operation of the position e s timating apparatus according to the second embodiment. [0045] [fig.l8]Fig. 18 is a flowchart showing a third example operation of the position e s timating apparatus according to the second embodiment. [0046] [fig. 19] Fig. 19 is a functional block diagram showing the configuration of a position estimating apparatus according to a third embodiment. [0047] [fig.20]Fig. 20 is a sequence chart showing an example operation of an information processing system according to the third embodiment. [0048] [fig.21]Fig. 2 1 is a functional block diagram showing the configuration of a position estimating apparatus according to a fourth embodiment. [0049] [fig.22]Fig. 22 is a diagram useful in showing specific examples of low reliability base station information stored in a low reliability base station information storage unit. [0050] [fig.23]Fig. 23 is a diagram useful in showing a specific example of mobile base station information stored in a mobile base station information storage unit. [0051] [fig.24]Fig. 24 is a flowchart showing an example operation of a position estimating apparatus according to the fourth embodiment. [0052] [fig.25]Fig. 25 is a functional block diagram showing the configuration of a wireless terminal according to a fifth embodiment. Description of Embodiments [0053] Hereinafter, preferred embodiments of the present disclosure will be described in detail with reference to the appended drawings. Note that, in this specification and the appended drawings, structural elements that have substantially the same function and structure are denoted with the same reference numerals, and repeated explanation of these structural elements is omitted. [0054] Also, in the present specification and drawings, in some cases a plurality of structural elements that have effectively the same functional configuration are distinguished from one another by appending different letters to the same reference numeral. For example, a plurality of structural elements with effectively the same functional configuration are distinguished as necessary as the "base stations 30A, 30B, and 30C". However, when it is not especially necessary to distinguish between the plurality of structural elements with effectively the same functional configuration, the same reference numeral is used. For example, the base stations 30A, 30B, and 30C are referred to simply as the "base station 30" when it is not especially necessary to distinguish between them. [0055] Embodiments of the present disclosure will now be described in the order indicated below. 1. Fundamental Configuration of Information Processing System 2. First Embodiment: Updating Base Station Position Information in Accordance With Comparison Result for New and Old Estimation Errors 2-1. Configuration of Position Estimating Apparatus According to First Embodiment 2-2. Operation of Position Estimating Apparatus According to First Embodiment 3. Second Embodiment: Amendment of Estimation Error 3-1. Configuration of Position Estimating Apparatus According to Second Em bodiment 3-2. Operation of Position Estimating Apparatus According to Second Embodiment First Example Operation Second Example Operation Third Example Operation 4. Third Embodiment: Position Estimation of Wireless Terminal 4-1. Configuration of Position Estimating Apparatus According to Third Em bodiment 4-2. Operation of Third Embodiment 5. Fourth Embodiment: Determining Mobile Base Stations 5-1. Configuration of Position Estimating Apparatus According to Fourth Em bodiment 5-2. Operation of Position Estimating Apparatus According to Fourth Embodiment 6. Fifth Embodiment: Implementing Functions in Mobile Terminal 7. Conclusion [0056] 1. Fundamental Configuration of Information Processing System As described in detail below in the "2. First Embodiment" to "6. Fifth Embodiment" sections for example, the present disclosure has a variety of possible implementations. The "position estimating apparatus 20" or "wireless terminal 40" according to such em bodiments includes: [0057] (1) a base station information storage unit (216, 416) that stores, for each base station, base station position information and an index (estimation error) showing the reliability of the base station position information; [0058] (2) a base station position estimating unit (224, 424) that estimates position in formation of a base station based on measurement information for the signal strength of a wireless signal transmitted from the base station; [0059] (3) an estimation result evaluating unit (estimation error calculating unit 228, 428) that calculates an index showing the reliability of the position information of a base station estimated by the base station position estimating unit; and [0060] (4) an information management unit 232 (232, 234, 432) that updates the base station position information stored in the base station information storage unit using the position information estimated by the base station position estimating unit when the index calculated by the estimation result evaluating unit indicates higher reliability than the index stored in the base station information storage unit. [0061] First, the fundamental configuration that is common to the respective embodiments will be described with reference to Figs. 1 and 2. Configuration of Information Processing System [0062] Fig. 1 is a diagram useful in explaining the configuration of an information processing system 1 according to the embodiments of the present disclosure. As shown in Fig. 1, the information processing system 1 according to the embodiments of the present disclosure includes a position estimating apparatus 20 (information processing apparatus), a plurality of base stations 30, and a wireless terminal 40. [0063] Each base station 30 controls communication between communication apparatuses that are spatially distributed. As examples, the base stations 30 are capable of con trolling wireless communication between the wireless terminal 40 and another wireless terminal (not shown) that are both located in the respective signal ranges of the base stations 30 and/or controlling communication between the wireless terminal 40 and a communication apparatus that is connected by wires to a base station 30. More specifically, the base stations 30 may be base stations on a wireless LAN (Local Area Network) based on WiFi (Wireless Fidelity) Standard, LTE (Long Term Evolution) base stations, GSM (Global System for Mobile Communications) base stations, or Bluetooth (registered trademark) base stations. [0064] In addition, the base stations 30 form a wireless network by regularly transmitting a beacon signal, for example. Here, the expression "beacon signal" includes a beacon signal including a base station ID identifying each base station 30. This means that it is possible for the wireless terminal 40 to specify the base station 30 that transmitted a beacon signal that has been received from the base station ID included in the beacon signal. [0065] The wireless terminal 40 is capable of wirelessly transmitting and receiving various data in accordance with control by the base stations 30. For example, the wireless terminal 40 is capable of receiving content data from a content distribution server (not shown) and/or transmitting and receiving electronic mail to or from another wireless terminal via the base stations 30. Note that various data, for example, audio data (such as music, a performance, or a radio program), image data (such as a movie, a television program, a video program, photographs, artwork, or drawings), games, and software, can be given as examples of the content data. [0066] As examples, the wireless terminal 40 may be an information processing apparatus such as a PC (personal computer), a home video processing apparatus (a DVD recorder, video deck, or the like), a mobile telephone, a PHS (Personal Handyphone System), a mobile music player, a mobile video processing apparatus, a PDA (Personal Digital Assistant), a home game console, a mobile game console, or a home appliance. [0067] When a wireless signal (for example, a beacon signal) transmitted from a base station 30 is received, the wireless terminal 40 is also capable of measuring the signal strength of such wireless signal. The wireless terminal 40 transmits measurement information including information on the measured signal strength, the base station ID of the base station 30, and measurement position information showing the measurement position to the position estimating apparatus 20. A specific example of the measurement in formation will now be described with reference to Fig. 2. [0068] Fig. 2 is a diagram useful in explaining a specific example of the measurement in formation. As shown in Fig. 2, the measurement information includes measurement position information, a measurement time, and signal strength information for each base station. Note that in the present specification, for ease of explanation, it is assumed that the code assigned to each base station and the base station ID are the same. [0069] As mentioned earlier, the measurement position information is information showing a measurement position for wireless signals. When the wireless terminal 40 includes a position estimating function such as GPS, the measurement position information may be estimated using such position estimating function. Another conceivable imple mentation would be for the user of the wireless terminal 40 to input the measurement position information. As other examples, the measurement position information may be obtained by a method that uses base station information for mobile telephones or a method that uses various sensors. [0070] Although the measurement position information is shown in simplified form in Fig. 2, the measurement position information may be expressed by a format using latitude and longitude, a format using x and y coordinates, a format using absolute coordinates, or a format using vectors. [0071] Also, although an example where the signal strength information is expressed in "dBm" units is shown in Fig. 2, the embodiments are not limited to the illustrated example. For example, the signal strength information may be expressed by a value, such as "40%" or "80%", that is the measured value expressed as a proportion of a set value (for example, a saturation value for the signal strength). [0072] The signal strength of the wireless signal transmitted by a base station 30 falls in ac cordance with a specified rule as the distance from the base station 30 increases. That is, the signal strength of the wireless signal at the wireless terminal 40 can be converted into the distance between the base station 30 that transmitted such wireless signal and the wireless terminal 40. In this way, since the signal strength corresponds to the distance between the base station 30 and the wireless terminal 40 estimated from the signal strength, the wireless terminal 40 may treat the distance between the base station 30 and the wireless terminal 40 estimated from the signal strength as signal strength information. [0073] Returning to the description of the information processing system 1 with reference to Fig. 1, the position estimating apparatus 20 (information processing apparatus) estimates position information of the respective base stations 30 based on the mea surement information received from the wireless terminal 40. The position estimating apparatus 20 stores position information for the respective base stations 30 and updates the position information for the respective base stations 30 in accordance with e s timation results based on the measurement information. [0074] Also, a position estimating apparatus 20-3 according to a third embodiment and a position estimating apparatus 20-4 according to a fourth embodiment are capable, on receiving signal strength information for each base station 30 as a position estimation request from the wireless terminal 40, to estimate position information for the wireless terminal 40 based on the signal strength information and position information of the re spective base stations 30 that have been stored in advance. [0075] In addition, when estimating the position of the wireless terminal 40, the position e s timating apparatus 20-3 according to the third embodiment and the position estimating apparatus 20-4 according to the fourth embodiment are also capable of selectively using position information of highly reliable base stations 30 to improve the precision of the position estimation. Such embodiments are described in detail later in this speci fication. 2. First Embodiment: Updating Base Station Position Information in Accordance With Comparison Result for New and Old Estimation Errors 2-1. Configuration of Position Estimating Apparatus According to First Embodiment [0076] Fig. 3 is a functional block diagram showing the configuration of a position e s timating apparatus 20- 1 according to a first embodiment of the present disclosure. As shown in Fig. 3, the position estimating apparatus 20-1 according to the first em bodiment includes a communication unit 212, a base station information storage unit 216, a measurement information storage unit 220, a base station position estimating unit 224, an estimation error calculating unit 228, and an information management unit 232. [0077] The communication unit 212 is an interface for transmitting and receiving in formation to and from a wireless terminal 40. As one example, the communication unit 212 includes a function as a reception unit that receives measurement information from the wireless terminal 40. Note that the communication unit 212 may communicate with the wireless terminal 40 via a communication network that includes wired or wireless transfer paths. More specifically, the communication network may include a public network such as the Internet, a telephone network, or a satellite communication network, various types of LAN (Local Area Network) including Ethernet (registered trademark), a WAN (Wide Area Network), and the like. The communication network may also include a dedicated network such as an IP-VPN (Internet Protocol- Virtual Private Network). [0078] The base station information storage unit 216 stores base station information, which is made up of position information and an estimation error, for each base station 30. Here, the estimation error is an index showing the reliability of the position in formation of the base station 30 and is set so that the higher the estimation error, the lower the reliability of the position information and the lower the estimation error, the higher the reliability of the position information. Specific examples of the base station information stored in the base station information storage unit 216 will now be described with reference to Fig. 4. [0079] Fig. 4 is a diagram useful in explaining specific examples of the base station in formation stored by the base station information storage unit 216. As shown in Fig. 4, the base station information of a base station includes a base station ID that identifies such base station, position information, and an estimation error. As one example, the base station information of the base station 30A includes the base station ID "30A", the "position information A", and the estimation error "45m". Note that the concept and calculation method of the estimation error included in the base station information will be described later with reference to Figs. 6 to 8. [0080] The measurement information storage unit 220 stores measurement information received by the communication unit 212 from the wireless terminal 40. As described above with reference to Fig. 2, each piece of measurement information includes mea surement position information, a measurement time, a base station ID, signal strength information, and the like. As shown in Fig. 5, such measurement information is ac cumulated in the measurement information storage unit 220. Note that the transmission source (i.e., provider) of the measurement information is not particularly limited to the wireless terminal 40. For example, the terminal that transmitted the measurement in formation including "measurement position information 1" and the terminal that transmitted the measurement information including "measurement position information 2" shown in Fig. 5 may be the same terminal or may be different terminals. The mea surement information may also include a measurer attribute indicating whether the user of the wireless terminal 40 is walking, riding in a vehicle, or the like. Position Estimation for Base Station [0081] The base station position estimating unit 224 estimates the position information of a given base station 30 based on a plurality of pieces of measurement information relating to such base station 30. For example, the base station position estimating unit 224 may apply a weighting in accordance with the magnitude of the signal strength of the respective measurement information to the respective pieces of measurement position information included in measurement information relating to such base station 30 and use a center of gravity of the weighted measurement position information as an estimate of the position information of the base station 30. The method of estimating the position information of a base station 30 will now be described in more detail with reference to Fig. 6. [0082] Fig. 6 is a diagram useful in explaining a method of estimating the position in formation of a base station 30. Here, a case is considered where the position of the base station 30A is estimated based on the two pieces of measurement information shown in Fig. 5. In this case, the base station position estimating unit 224 applies weightings in accordance with the magnitudes of the signal strengths to the measurement position PI and the measurement position P2 when finding the center of gravity of the mea surement position PI shown by the measurement position information 1 and the mea surement position P2 shown by the measurement position information 2. [0083] Note that it is possible to convert the respective signal strengths to distances. For this reason, in Fig. 6, an example is shown where the signal strength "-90dBm" of the base station 30A at the measurement position PI has been converted to the distance "80m" and the signal strength "-70dBm" of the base station 30A at the measurement position P2 has been converted to the distance "60m". In this case, the base station position e s timating unit 224 may apply weightings in accordance with the converted distance values "80m" and "60m" to the measurement position PI and the measurement position P2. [0084] As a result, the base station position estimating unit 224 can estimate a position EP1 found by dividing the gap between the measurement position PI and the measurement position P2 in accordance with 4:3 that is the ratio of "80m" to "60m" as the position of the base station 30A. In addition, it is possible for the base station position estimating unit 224 to estimate the position of the base station 30 based on a plurality of pieces of measurement information in accordance with the generalized mathematical formulas shown below. [0085] [Math.l] i (Mathematical Formula 1) [0086] [Math.2] n = 1 distS(EP,Pi) (Mathematical Formula 2) [0087] [Math.3] V = Vi i (Mathematical Formula 3) [0088] Note that "Pi" in Formula 1 shows the 1th measurement position information. Ac cordingly, when the measurement position information is expressed by longitude and latitude, the base station position estimating unit 224 applies Formula 1 to both the longitude and the latitude. Also, as shown in Formula 2, Vi is a weighting coefficient obtained based on distS (EP,Pi) showing the distance between the wireless terminal 40 and the base station 30 estimated from the signal strength. As shown in Formula 3, V is the sum of the weighting coefficients Vi. Calculation of Estimation Error [0089] As described earlier, the estimated position of a base station 30 is found by the base station position estimating unit 224 estimating the position based on the measurement information. However, in reality a base station 30 will not always be located at the estimated position, and the estimated position is assumed to include some degree of error. Such estimation error will now be described with reference to Figs. 7 and 8. [0090] Figs. 7 and 8 are diagrams showing the concept of the estimation error. As shown in Fig. 7, even if the position EP1 of the base station 30 has been estimated by the base station position estimating unit 224, such position EP1 is thought to include an error within the range shown by the broken line, for example. [0091] In the same way, as shown in Fig. 8, in another conceivable case, the position EP2 of a base station 30 is found as an estimate based on the signal strength at the mea surement position Px (the converted distance value "40m") and the signal strength at the measurement position Py (the converted distance value "30m"). In such case also, in the same way as the example shown in Fig. 7, the position EP2 is thought to include an error within the range shown by the broken line in Fig. 8, for example. [0092] However, as shown in Fig. 8, it is thought that the error included in the position EP2 is smaller than the error included in the position EP1 shown in Fig. 7. This is because the signal strengths at the measurement position Px and the measurement position Py shown in Fig. 8 are greater than the signal strengths at the measurement position PI and the measurement position P2 shown in Fig. 7, and therefore conceivable to narrow down the position of the base station 30 further. [0093] In this way, the reliability of the position information of the base station 30 estimated by the base station position estimating unit 224 is higher the greater the signal strengths of the signals used for estimation. That is, the estimation error of the position information of the base station 30 estimated by the base station position estimating unit 224 is smaller the greater the signal strengths of the signals used for estimation. [0094] For this reason, the estimation error calculating unit 228 calculates the estimation error of the position information produced by the base station position estimating unit 224 based on the signal strengths of the signals used in estimation or converted distance values calculated from the respective signal strengths. As examples, the e s timation error calculating unit 228 may calculate an average of the converted distance values as the estimation error, calculate a median value of the converted distance values as the estimation error, or may treat a largest value out of the converted distance values as the estimation error. [0095] When the average of the converted distance values is calculated as the estimation error, in the example shown in Fig. 7, "70m" that is the average of the converted distance values "80m" and "60m" is calculated as the estimation error. Meanwhile, in the example shown in Fig. 8, "35m" that is the average of the converted distance values "40m" and "30m" is calculated as the estimation error. [0096] Note that although an example where the base station position estimating unit 224 and the estimation error calculating unit 228 calculate the position information and the estimation error of the base station 30 in question based on two pieces of measurement information has been described above, the present embodiment is not limited to such example. For example, as shown in Fig. 9, the base station position estimating unit 224 and the estimation error calculating unit 228 may calculate the position information and the estimation error based on the most recent five pieces of measurement in formation relating to the base station 30 in question. In addition, the measurement in formation used by the base station position estimating unit 224 and the estimation error calculating unit 228 may be limited to measurement information from a specified recent period (for example, within the last three months). Updating the Base Station Information [0097] The information management unit 232 updates the base station information (i.e., the position information and estimation error) stored in the base station information storage unit 216 in accordance with the estimation result produced by the base station position estimating unit 224 and the calculation result produced by the estimation error calculating unit 228. [0098] More specifically, the information management unit 232 compares the reliability of the position information relating to the base station 30 stored in the base station in formation storage unit 216 and the position information estimated by the base station position estimating unit 224. That is, the information management unit 232 compares the estimation error relating to the base station 30 in question stored in the base station information storage unit 216 and the estimation error calculated by the estimation error calculating unit 228. [0099] If it has been determined that the new estimation error calculated by the estimation error calculating unit 228 is the smaller, that is, if the reliability of the new position in formation estimated by the base station position estimating unit 224 is the higher, the information management unit 232 updates the position information and the estimation error stored in the base station information storage unit 216 for the base station 30 in question using the new position information and estimation error. Here, the in formation management unit 232 may replace the position information and the e s timation error relating to the base station 30 in question that are stored in the base station information storage unit 216 with the new position information and estimation error or may update the stored information to intermediate values or the like for both the new and old values. The updating process carried out by the information management unit 232 will now be described in detail with reference to Fig. 4 and Figs. 9 to 11. Specific Example of Updating Process [0100] Next, consider a case where the "position information A" and the estimation error "45m" shown in Fig. 4 are stored in the base station information storage unit 216 relating to the base station 30A and the base station position estimating unit 224 and the estimation error calculating unit 228 calculate the position information and the e s timation error based on measurement information relating to the five base stations 30A shown in Fig. 9. [0101] In this case, the base station position estimating unit 224 estimates the position EP1 of the base station 30A in accordance with Formula 1 given above, for example. The estimation error calculating unit 228 calculates the average value "50m" of the re spective converted distance values as the estimation error. [0102] In addition, the information management unit 232 compares the new estimation error "50m" calculated by the estimation error calculating unit 228 and the estimation error "45m" stored in the base station information storage unit 216. In this case, it is de termined that the estimation error "45m" stored in the base station information storage unit 216 is the smaller, that is, the "position information A" stored in the base station information storage unit 216 has higher reliability. For this reason, the information management unit 232 does not update the "position information A" and the estimation error "45m" relating to the base station 30A. [0103] After this, assume that the base station position estimating unit 224 and the e s timation error calculating unit 228 calculate the position information and the e s timation error based on the five pieces of measurement information at the measurement positions P2 to P6 shown in Fig. 10. [0104] In this case, the base station position estimating unit 224 estimates the position EP2 of the base station 30A in accordance with Formula 1 given above, for example. The estimation error calculating unit 228 also calculates the average value "40m" of the re spective converted distance values as the estimation error. [0105] In addition, the information management unit 232 compares the new estimation error "40m" calculated by the estimation error calculating unit 228 and the estimation error "45m" stored in the base station information storage unit 216. In such case, it is de termined that the new estimation error "40m" calculated by the estimation error cal culating unit 228 is the smaller, that is, the new estimated position EP2 estimated by the base station position estimating unit 224 has higher reliability. For this reason, the information management unit 232 updates the "position information A" and the e s timation error "45m" relating to the base station 30A to the "estimated position in formation 2" showing the new estimated position EP2 and the new estimation error "40m" as shown in Fig. 11. [0106] With this configuration, the information management unit 232 is capable of pro gressively updating the position information of each base station 30 stored in the base station information storage unit 216 to values with higher reliability. As a result, when position estimation of the wireless terminal 40 is carried out based on position in formation of the respective base stations 30 stored in the base station information storage unit 216, it is possible to improve the precision of the position estimation of the wireless terminal 40. 2-2. Operation of Position Estimating Apparatus According to First Embodiment [0107] The configuration of the position estimating apparatus 20-1 according to the first em bodiment of the present disclosure has been described above. Next, the operation of the position estimating apparatus 20- 1 according to the first embodiment will be described with reference to Fig. 12. [0108] Fig. 12 is a flowchart showing the operation of the position estimating apparatus 20-1 according to the first embodiment. First, as shown in Fig. 12, the base station position estimating unit 224 of the position estimating apparatus 20-1 estimates the position in formation of the base station 30 in question based on the measurement information stored in the measurement information storage unit 220 for such base station 30 (S304). [0109] Next, or alternatively in parallel with the position estimation by the base station position estimating unit 224 shown in S304, the estimation error calculating unit 228 calculates an estimation error of the position information estimated by the base station position estimating unit 224 based on the measurement information used in the position estimation by the base station position estimating unit 224 (S308). [0110] In addition, the information management unit 232 searches the base station in formation storage unit 216 for the estimation error relating to the base station 30 in question (S3 12) and compares the found estimation error and the new estimation error calculated by the estimation error calculating unit 228 in S308 (S3 16). [0111] After this, if the new estimation error is the smaller, the information management unit 232 updates the position information and the estimation error stored in the base station information storage unit 216 using the new position information estimated by the base station position estimating unit 224 in S304 and the new estimation error calculated by the estimation error calculating unit 228 in S308 (S320). Note that if the new estimation error is larger, the information management unit 232 does not update the position information and the estimation error stored in the base station information storage unit 216. [0112] As described above, according to the first embodiment of the present disclosure, it is possible to progressively update the position information of a base station 30 to a more suitable value based on the reliability (i.e., estimation error) of newly estimated position information relating to such base station 30. 3. Second Embodiment: Amendment of Estimation Error [0113] Next, before describing the second embodiment of the present disclosure, the de velopment behind the second embodiment of the present disclosure will be explained. [0114] In recent years, movement of base stations has increased due to the spread of mobile base stations carried by users and the presence of base stations installed in high speed trains. Also, even a base station installed in the user's home could conceivably move, such as when the user moves house. When a base station 30 moves in this way, as shown in Fig. 13, the measurement information relating to such base station 30 also changes. [0115] Fig. 13 is a diagram useful in explaining a change in the measurement information when the base station 30 has moved. As shown in Fig. 13, when a base station 30 located in the Tokyo area has been moved to the Osaka area, the measurement in formation relating to such base station 30 also changes. For example, as shown in Fig. 13, measurement information is obtained at the measurement positions P4 to P8 before movement of the base station 30 and measurement information is obtained at the mea surement positions P9 to P13 after movement of the base station 30. [0116] In such case, since the base station 30 has actually moved to the Osaka area, it is preferable to update the position information EP3 of the base station 30 to the position information EP4 for the Osaka area based on the measurement information at the mea surement positions P9 to PI3. [01 17] Here, the estimation error calculated from the measurement information at the mea surement positions P4 to P8 before movement is "20m" and the estimation error calculated from the measurement information at the measurement positions P9 to P13 after movement is "50m". This means that with the method described above in the first embodiment, since the estimation error before movement is the smaller, the position information EP3 of the base station 30 will not be updated to the position information EP4 for the Osaka area. That is, according to the method described above in the first embodiment, once the position information of the base station 30 has been estimated with a small estimation error, it will be difficult for the position information to be updated when the base station 30 has moved. [0118] For this reason, the second embodiment of the present disclosure that focuses on this situation was conceived. According to the second embodiment of the present disclosure, it is possible to appropriately update the position information of a base station 30 in keeping with movement of the base station 30. This second embodiment of the present disclosure will now be described in detail. 3-1. Configuration of Position Estimating Apparatus According to Second Embodiment [0119] Fig. 14 is a functional block diagram showing the configuration of a position e s timating apparatus 20-2 according to the second embodiment. As shown in Fig. 14, the position estimating apparatus 20-2 according to the second embodiment includes the communication unit 212, the base station information storage unit 216, the mea surement information storage unit 220, the base station position estimating unit 224, the estimation error calculating unit 228, and an information management unit 234. [0120] Since the communication unit 212, the base station information storage unit 216, the measurement information storage unit 220, the base station position estimating unit 224, and the estimation error calculating unit 228 are the structural elements described in the first embodiment, detailed description thereof is omitted here. Meanwhile the information management unit 234 according to the second em bodiment is a structural element where additional functions have been implemented in the information management unit 232 described in the first embodiment. The in formation management unit 234 according to the second embodiment will now be described in detail. [0121] The information management unit 234 determines whether the difference between the estimated position of a base station 30 based on the measurement information received from the wireless terminal 40 and the position shown by the position in formation stored in the base station information storage unit 216 for such base station 30 exceeds a set value. Here, it is believed that the difference between both positions will exceed the set value when the position information stored in the base station in formation storage unit 216 has become erroneous due to movement or the like of the base station 30 in question. [0122] Here, if the difference between both positions exceeds the set value, the information management unit 234 lowers the reliability of the position information stored in the base station information storage unit 216 for the base station 30 in question. That is, if the difference between both positions exceeds the set value, the information management unit 234 increases the value of the estimation error stored in the base station information storage unit 216 for the base station 30 in question. [0123] As one example, since the range of WiFi signals is normally said to be around 300m, the information management unit 234 may amend the estimation error stored in the base station information storage unit 216 to a larger value than 300m, such as 600m or lKm. [0124] According to this configuration, if the base station 30 has moved after the position information of the base station 30 has been estimated with a small estimation error, the estimation error will be corrected to a larger value by the information management unit 234. As a result, since it becomes easier for a new estimation error calculated by the estimation error calculating unit 228 after movement to be the smaller of the compared values, it becomes possible for the information management unit 234 to update the position information of the base station 30 stored in the base station information storage unit 216 to new position information estimated by the base station position e s timating unit 224 after movement. Specific Example of Amendment of Estimation Error [0125] Here, a specific example of amendment of an estimation error by the information management unit 234 will be described with reference to Figs. 13 and 15. [0126] Here, consider a state where, as shown in Fig. 15, "position information 3 (Tokyo)" showing the estimated position EP3 of a base station 30 and the estimation error "20m" are stored in the base station information storage unit 216 due to the measurement in formation for the base station 30 at the measured positions P4 to P8 shown in Fig. 13. If, in this state, the base station 30 is moved from the Tokyo area to the Osaka area and measurement information relating to the base station 30 is obtained at the measurement position P9, the base station position estimating unit 224 will estimate the position of the base station 30 based on the measurement information at the measured positions P5 to P9. [0127] In this case, due to the measurement position P9 being very far from the other measured positions P5 to P8, the difference between the position of the base station 30 estimated by the base station position estimating unit 224 and the position shown by "measurement information 3 (Tokyo)" exceeds the set value. For this reason, as shown in Fig. 15, the information management unit 234 amends the estimation error of the base station 30 to "5000m", for example. Note that the information management unit 234 does not change the position information of the base station 30 at this time. [0128] After this, once the measurement information at the measurement positions P9 to P13 shown in Fig. 13 has been obtained, based on such measurement information, the base station position estimating unit 224 estimates the position EP4 of the base station 30 and the estimation error calculating unit 228 calculates the estimation error "50m". [0129] Here, the estimation error "50m" newly calculated by the estimation error calculating unit 228 is smaller than the estimation error "5000m" stored in the base station in formation storage unit 216. For this reason, the information management unit 234 updates the position information stored in the base station information storage unit 216 for the base station 30 to "position information 4 (Osaka)" showing the position EP4 and updates the estimation error to "50m". [0130] In this way, according to the second embodiment of the present disclosure, when a base station 30 has moved, it is possible to update the position information stored in the base station information storage unit 216 for the base station 30 to a suitable value based on measurement information at positions near the new position after movement. 3-2. Operation of Position Estimating Apparatus According to Second Embodiment [0131] The configuration of the position estimating apparatus 20-2 according to the second embodiment of the present disclosure has been described above. Next, example op erations of the position estimating apparatus 20-2 according to the second embodiment will be described with reference to Figs. 16 to 18. First Example Operation [0132] Fig. 16 is a flowchart showing a first example operation of the position estimating apparatus 20-2 according to the second embodiment. First, as shown in Fig. 16, the base station position estimating unit 224 of the position estimating apparatus 20-2 estimates the position information of a base station 30 based on the measurement in formation stored in the measurement information storage unit 220 for the base station 30 (S324). Next, or alternatively in parallel with the position estimation by the base station position estimating unit 224 shown in S324, the estimation error calculating unit 228 calculates an estimation error of the position information estimated by the base station position estimating unit 224 based on the measurement information used in the position estimation by the base station position estimating unit 224 (S328). [0133] In addition, the information management unit 234 searches the base station in formation storage unit 216 for the position information and the estimation error relating to the base station 30 in question (S332) and compares the found estimation error and the new estimation error calculated by the estimation error calculating unit 228 in S328 (S336). [0134] After this, if the new estimation error is the smaller, the information management unit 234 updates the position information and the estimation error stored in the base station information storage unit 216 using the new position information estimated by the base station position estimating unit 224 in S324 and the new estimation error calculated by the estimation error calculating unit 228 in S328 (S340). [0135] Meanwhile, if the new estimation error is the larger (S336), the information management unit 234 determines whether the positions shown by the position in formation estimated in S324 and the position information found in S332 are further apart than the set value (S344). If the positions shown by both pieces of position in formation are further apart than the set value, the information management unit 234 amends the estimation error relating to the base station 30 stored in the base station in formation storage unit 216 to a sufficiently large value (S348). [0136] Note that the information management unit 234 may amend the estimation error relating to the base station 30 stored in the base station information storage unit 216 to a value in keeping with the magnitude of the difference between the positions shown by both pieces of position information. Also, if the positions shown by both pieces of position information are not further apart than the set value, the content of the base station information storage unit 216 is not changed. Second Example Operation [0137] Fig. 17 is a flowchart showing a second example operation of the position estimating apparatus 20-2 according to the second embodiment. In this second example operation, S324 to S332 are the same as in the first example operation, but the order in which the determinations are made differs to the first example operation. [0138] More specifically, as shown in Fig. 17, once the processing S324 to S332 has been carried out, the information management unit 234 determines whether the positions shown by the position information estimated in S234 and the position information found in S332 are further apart than the set value (S352). If the positions shown by both pieces of position information are further apart than the set value, the information management unit 234 amends the estimation error relating to the base station 30 stored in the base station information storage unit 216 to a sufficiently large value (S356). [0139] Meanwhile, if the positions shown by both pieces of position information are not further apart than the set value, the information management unit 234 compares the e s timation error found in S332 and the new estimation error calculated by the estimation error calculating unit 228 in S328 (S360). [0140] After this, if the new estimation error is the smaller, the information management unit 234 updates the position information and the estimation error stored in the base station information storage unit 216 using the new position information estimated by the base station position estimating unit 224 in S324 and the new estimation error calculated by the estimation error calculating unit 228 in S328 (S364). [0141] In this way, even if the order of the branch points in S336 and S344 of the first example operation is interchanged, it is possible to obtain the same effect as the first example operation. Third Example Operation [0142] Fig. 18 is a flowchart showing a third example operation of the position estimating apparatus 20-2 according to the second embodiment. This third example operation greatly differs to the other example operations in that the position for which the deter mination of whether the difference with the position shown by the position information stored in the base station information storage unit 216 exceeds a set value is made is not the position estimated by the base station position estimating unit 224 based on the measurement information. [0143] More specifically, as shown in Fig. 18, the information management unit 234 searches the base station information storage unit 216 for position information and an estimation error of a base station 30 relating to new measurement information (S372). After this, the information management unit 234 determines whether the position shown by measurement position information included in such measurement in formation and the position shown by the position information for such base station 30 found in S372 are further apart than a set value (S376). [0144] If the positions shown by both pieces of position information are further apart than the set value, the information management unit 234 amends the estimation error relating to the base station 30 stored in the base station information storage unit 216 to a sufficiently large value (S380). In this way, since it is not necessary for the base station position estimating unit 224 and the estimation error calculating unit 228 to estimate the position and to calculate the estimation error if the positions shown by both pieces of position information are further apart than the set value, it is possible to reduce the load of the position estimating apparatus 20-2. [0145] Meanwhile, if the positions shown by both pieces of position information are not further apart than the set value (S376), the base station position estimating unit 224 estimates the position information of the base station 30 in question based on the mea surement information stored in the measurement information storage unit 220 relating to the base station 30 (S384). The estimation error calculating unit 228 also calculates the estimation error of the position information estimated by the base station position estimating unit 224 based on the measurement information used in position estimation by the base station position estimating unit 224 (S388). [0146] Next, the information management unit 234 compares the estimation error found in S372 and the new estimation error calculated by the estimation error calculating unit 228 in S388 (S392). After this, if the new estimation error is the smaller, the in formation management unit 234 updates the position information and the estimation error stored in the base station information storage unit 216 using the new position in formation estimated by the base station position estimating unit 224 in S384 and the new estimation error calculated by the estimation error calculating unit 228 in S388 (S396). [0147] As described above, according to the third example operation, by treating a position shown by measurement position information included in the measurement information as an estimated position of a base station 30, it is possible to achieve the same effects as the other example operations while reducing the load of the position estimating apparatus 20-2. 4. Third Embodiment: Position Estimation of Wireless Terminal [0148] The second embodiment of the present disclosure has been described above. Next, a position estimating apparatus 20-3 according to a third embodiment of the present disclosure will be described in detail with reference to Figs. 19 and 20. 4-1. Configuration of Position Estimating Apparatus According to Third Embodiment [0149] Fig. 19 is a functional block diagram showing the configuration of the position e s timating apparatus 20-3 according to the third embodiment. As shown in Fig. 19, the position estimating apparatus 20-3 according to the third embodiment includes the communication unit 212, the base station information storage unit 216, the mea surement information storage unit 220, the base station position estimating unit 224, the estimation error calculating unit 228, the information management unit 234, and a terminal position estimating unit 240. [0150] Since the communication unit 212, the base station information storage unit 216, the measurement information storage unit 220, the base station position estimating unit 224, the estimation error calculating unit 228, and the information management unit 234 are the structural elements described in the second embodiment, detailed description thereof is omitted here. [0151] The terminal position estimating unit 240 estimates position information of a wireless terminal 40 in accordance with a position estimation request transmitted from such wireless terminal 40. The communication unit 212 then transmits the position in formation of the wireless terminal 40 estimated by the terminal position estimating unit 240 to the wireless terminal 40. [0152] Here, the position estimation request transmitted from the wireless terminal 40 includes signal strength information for each base station 30 obtained by signal strength measurements made at the wireless terminal 40. For example, in the position estimation request, base station IDs and signal strength information are associated with one another. [0153] The terminal position estimating unit 240 estimates position information for the wireless terminal 40 based on such position estimation request and the position in formation of the respective base stations 30 stored in the base station information storage unit 216. As one example, the terminal position estimating unit 240 estimates the position O of the wireless terminal 40 according to the mathematical formulas shown below. [0154] [Math.4] 0 = -