Selection of information units for mobile client computers DESCRIPTION Field of the invention The invention relates to the field of communications technology. It relates to a device, a computer system and a method for the selection of information units for mobile client computers according to the precharacterising clauses of claims 1, 9 and 15. Background of the invention Such a method is known, for example, from US 5,659,732. Therein, information units of a communications network, for example HTML files, which may be read over the Internet, are indexed by a so-called search engine. On the basis of a user's search request containing one or more search terms, files found by means of an index are determined. A suitably processed list of the files found and of the addresses of the found files is presented to the user, for example in the form of URLs (uniform resource locators). Given the large number of files addressable on the Internet, the number of files found is often very big. This makes it more difficult to find the files and information relevant to the user. Selection of information units for mobile client computers DESCRIPTION Technical field The invention relates to the field of communications technology. It relates to a facility, s. computer system and a. method for selecting information units i:or mobile client computers, as detailed in the preambles to claim; 1, 9 and 15. Prior art A method of this kind is known from US 5,659,732 for example. In it, information units on a communications network, such as HTML files for example, which can be read over the internet, are indexed by a so-called search emgine. In response to a search guery containing one or more query terms from a user, files which are being looked for are identified from an index. The user is presented with a suitably prepared list of the files found and their addresses, in the form of URL's (uniform resource locators) for example. Because of the vast number of files that can be accessed on the internet, the number of files found is often very large. This makes; it more difficult for the files and information which are relevant to the user to be found. It is also a generally familiar idea Eon: motor cars or aircraft seats to be fitted with client computers provided with HTML browsers to allow information from tire internet to be displayed. In these applications too there is the problem described above of the user being swamped with information, In addition, the idea is generally known of equipping cars or aircraft seats with client computers with HTML browsers for displaying information from the Internet. These applications too are plagued by the above-described problem of user information, overload. Summary of the invention It is therefore the object of the invention to provide a device, a computer system and a method for the selection of information units for mobile client computers of the above-mentioned type, which reduces the amount of information presented to a user and increases the relevance of the selected information units for the user. This object is achieved with a device, a computer system and a method for the selection of information units having the features of claims I, 9 and 15. In the method according to the invention, the selection of information units for mobile client computers is thus effected in that geographic location information is assigned to each of the individual information units and this location information and the geographic position of a client computer are take?) into account for selection. This makes it possible to limit the selection of information units to those information units which relate to a particular geographic area, in particular information units may be selected which are located in the vicinity of the position of the client computer. for the files and information which &re relevant to the user to be found. It ia also a generally familiar idea for motor cars or aircraft .seate to be fitted with client computers provided with HTML browsers to allow information from the irte.rnet to be displayed, In these applications too there is ths problem described above of the.- user being swamped with information. in EP 0 765 535 Al is described a vehicle navigation system in which a map section stored on a CD-ROM in the client is determined from GPS, positioning, information. On -he basis of a district of a town selected by a user, the name of the district i> transmitted to the server, whereupon tlie lattiar determines from the district information unita assigned ta, for example. entertainment offerings. It is not however possible in thig way for suitable information unite to be selected automatically and without any interaction wikh a user. Description of the invention The cbject of the invention is therefore ;o provide a facility, a computer system and a method :!or selecting information units for mobile client computers, of the a.bove-mentioned kind, which reduce the amount of information which is preaanted to a user and which increase the? reluveuica to the user of the information units selected. This object is achieved by a facility, a computer systsm and a method for selecting information units which have the features detailed in claims 1, 9 and 15. Hence, in the method according to the invention th« selection of information units for mobil«« client computers is effected by assigning geographical locacional information in .the form of a geographical longitude and latit.ude to each of the individual information units and by makincr allowance for this locaticnal information and the geographical position of the cliant computer In the form of a geographical long:.tude and_latitude in making the selection. In this way it becomes possible to restrict the selection of information units to those units which raleta to a sriven geographical area, in particular, information units which are situated close to the position of the client computer can be selected. A substantial advantage of the method according to the invention is that the selection of information units is limited to a given geographic area and a large amount of irrelevant information is thereby eliminated. In this way, the computational effort involved in preparing, transmitting and presenting the information is also reduced. In a variant of the invention, the client computer is located in a means of transport, for example a train, aeroplane or car. In a further variant of the invention, the position of the client computer which is taken into account is not necessarily the current one, but may be a past or anticipated future position of the client computer or the means of transport. In a further variant of the invention, the information units are image files, which are generated by web cameras or "WebCams" and are continually updated. Further preferred embodiments are revealed by the dependent claims. Brief description of the drawings The invention is explained in more detail below with reference to a preferred exemplary embodiment, which is illustrated in the attached drawings, in which: to be it is current position but can be a past an expect eel future position of the client computer or th means of transport. In a further variant of the invention tha in format ion units are image files which are generated by so-called web cameras or "webcams" and are continually updated, Other preferred embodiments can be seen from the dependent claims. Brief description of the drawings The invention is explained in detail below by reference to a preferred embodiment which is shown, in the accompanying drawings. In the drawings: Fig.l is a schematic representation of communications links and elements which the method according to the invention uses, Fig. 2 is a representation of information units which are selected in a variant of the invention, and Fig. 3 shows a section of a Voronoi diagram. The reference numerals used in, the drawings and their meanings are listed as a group in the List of reference numerals. The same elements are always given tha same reference numerals in the drawings. Ways of implementing the invention Fig 1 is a schematic representation of dements and links which the method according to the invention preferably uses. The link, should be thought of as bi-directional communications links in Fig.l, a Plurality of sources of information, i.e. sources 2 of information units, one or more server computers, referred to for short as servers 3, and one or more client computers, rsfsrrsd to for short as clients 4, are connected Figure 1 is a schematic representation of communications connections and elements using the method according to the invention; Figure 2 is a representation of selected information units according to a variant of the invention; and Figure 3 is a portion of a Voronoi diagram. The reference numerals used in the drawings and their meaning are listed in the list of reference numerals. In principle, identical elements in the Figures are provided with identical reference numerals. Description of preferred embodiments Figure 1 is a schematic representation of elements and connections which make preferential use of the method according to the invention. The connections should be understood to be bidirectional communications connections. In Figure 1, a plurality of information sources, i.e. sources 2 of information units, one or more server computers, or servers 3 for short, and one or more client computers, clients 4 for short, are connected together via a communications network 1. A client 4 comprises an arithmetic and logic unit, a display unit and a communications unit for communicating with the communications network 1. A client 4 preferably also comprises means of determining its geographic position and means for a user to input information. One or more first clients 4 are connected to the communications network 1 via a standard interface 5 of the communications network 1. One or more second clients 4' are connected via a special together by means of a communications network 1. A client 4 a processing unit, a display unit and a communications unit communicating with the communications network 1. A client 4 preferably also has means for determining its geographleal position and means for the input of information by a user. One or more first clients 4 are connected to the cornrtiunications network 1 via a standard interface 5 belonging to the communications network 1, One or more second clients 4' are connected via a special interface 6 to a coupling un:.t 7 which is connected in turn to the communications network 1. one or more third clients 4'" do not communicate via the communications network 1 but via special interfaces 6 with the server 3 . Other, fourth clients 4' ' ' are connected to a shared proxy server 8 on a means of transport. 9, which in turn is connected to the communications network. There; are various technological variants that can be employed ' to implement the above-mentioned elements? of the invention. The communications network 1 may ::or example be the internet, which is based on the TCP/IP protocol, or the worldwide web which is based on the internet. The sources 2 may £or example be files in the HTML format or a format derived therefrom, or image files, which can be accessed by means of the communications network 1, The clients 4 are portable, being for example a PDA (personal digital assistant) or a motile phone with an integral web browser, or are fitted in a means of transport such as a motor car, train, bicycle, aircraft, airship, boat or submarine. Browser software -isod by the client 4 is fully HTML-compatible or else is tailored1, to efficiently display image or textual information, The standard interface 5 is preferably a TCP/IP (Transmission Control Protocol/Internet Protocol) link, It can for example be implemented in transparent from by more of a. telephone network or a wireless tranemiecion facility. The special interface 6 is for example a private interface 6 to a coupling unit 7, which is in turn connected with the communications network 1. One or more third clients 4" communicate with the server 3 not via the communications network 1 but via special interfaces 6. Further, fourth clients 4"' are connected with a common proxy server 8 of a means of transport 9 which is in turn connected with the communications network 1. There are various technological variants which may be used to achieve the above-mentioned elements of the invention. The communications network 1 is for example the Internet, based on the TCP/IP protocol, or the Internet-based World Wide Web. Sources 2 are for example files in HTML format or a format derived therefrom, or image files, which may be accessed by the communications network 1. Clients 4 are portable, for example a PDA (personal digital assistant) or a mobile telephone with built-in web browser or they are built into a means of transport, for example a car, train, bicycle, aeroplane, airship, boat or submarine. The browser software of the client 4 is fully HTML-capable, or is tailor-made for efficient presentation of image or text information. The standard interface 5 is preferably a TCP/IP (Transmission Control Protocol/Internet Protocol) connection. This may for example be obtained transparently over a telephone network or a wireless transmission device. The special interface 6 is for example a private communications network or a non-standardised communications protocol or a proprietary interface. Two advantageous variant embodiments of the invention are described below. Common to both variants is the fact that the server 3 collects information from a plurality of sources 2 and assigns geographic location information to communications network or a non-standardised". protocol or a proprietary interface. Two advantageous variant implementations of invention are described, below. What is common to both variants;, that server 3 collects information from a plurality of screes 2 assigns geographical locational information to each source 2 This, assignment is performed by, fcr example, extract geographical locational information which is already explicitly present in a source or implicitly present in "a text at source, by analysing an IP address associated with source, or by manual input by a user. A proposal for how information units; which are made available over the internet can be provided with explicit information on a geographical location is made in the "w3c Proposed Recommendation 03, March 1999" under the title "Resource Description Framework (PDF) Schema Specification." by Dan Brickley and R.V.Guha, which cart be found at http://www.w3.org/TR/PR-rdf-schema. In a first advantageous variant of the invention, the invention is used for informing and entertaining passengers on a means of public transport such as the railway, a bus or coach or an aircraft, in this case the passengers ars shown images by means of the display unit of the client 4, the images making reference to the route followed by the means of transport. These images are for example generated and continually updated by so-called, web cameras or "webcams" acting as sources 2. For this purpose, the client 4 makes contact periodically with the server 3 and transmits to the server 3 an enquiry arid a past, current or future position of the means of transport. The position, i.e. the geographical location, of the means o:; transport is found by means of a GPS (global positioning system} receiver for example. In the case of a railway vehicle, signalling imc. route data can also be used to determine position and in the casse of an each source 2. This assignment is effected for example by extraction of geographic location information, which is already explicitly present in a source or implicitly present in a source text, by analysis of an IP address associated with the source or by manual input by an operator. One proposal as to how information units made available over the Internet may be provided with explicit information about a geographic location is made in the "W3C Proposed Recommendation 03 March 1999" entitled "Resource Description Framework (RDF) Schema Specification" by Dan Brickley and R.V. Guha, which may be found at http://www.w3.org/TR/PR-rdf-Schema. In a first advantageous variant of the invention, the invention serves to inform and entertain passengers in a public means of transport such as a train, bus or aeroplane. The passengers are presented with images by means of the display unit of the client 4, wherein the images are related to the route of the means of transport. For example, these images are generated by sources 2 in the form of web cameras or "WebCams" and are continually updated. To this end, the client 4 periodically makes contact with the server 3 and transmits to the server 3 an enquiry and a past, current or future position of the means of transport. The position, i.e. the geographic location of the means of transport, is determined for example by means of a GPS (Global Positioning System) receiver, while signalling and route data may also be used to determine the position in the case of a rail vehicle or corresponding navigation data in the case of an aeroplane. aircraft appropriate navigational data can also be used for this purpose. A first form of the enquiry asks about a web camera which is closest to the position of the means of transport. The server 3 then either transmits to the client 4 a reference or an address, such as a URL (uniform resource locator) address, of a web camera, whereupon the client 4 loads the image data from the web camera, or the server itself loads the image data from the web cameria and transmits it to the client: 4, In both cases it is advantageous if the server checks whether t;he image data has changed since the last enquiry, thus making it unnecessary for data to be transmitted to the client 4. Periodic enquiries show either new images from the same web camera or images from a different web camera if the means of transport is approaching the latter. This first form of enquiry is advantageous if a plurality of, or all, the passengers view the images displayed by a client 4. The image data comprises not only the image information proper but also data such as parameters of the image, time of recording or comments. A second form of enquiry asks for all the web cameras which are situated in a preferably rectangular section of the area surrounding the position of the means of transport, This section is for example defined by the geographical latitude and longitude of its centre and by the width ,and height of the section, in kilometres for example. The server 3 then transmits to the client 4 as references for the sources 2 the addresses of all the web cameras which are situated in this section together with their geographical location, i.a. their geographical latitude and longitude for example, and reduced or thumbnail images 22 from the relevant web cameras. The client 4 superimposes these reduced images on a map 21 of the surrounding area and shows the resulting image on the display unit of the client 4. An example of a representation of this kind, is shown A first form of the enquiry requests a web camera closest to the position of the means of transport. The server 3 then transmits to the client 4 either a reference or an address, for example a URL (Uniform Resource Locator) address, of a web camera, whereupon the client 4 loads the image data from the web camera, or the server itself loads the image data from the web camera and transmits the image data to the client 4. In both cases, it is advantageous for the server to check whether the image data have changed since the last enquiry, such that data transmission to the client 4 may be dispensed with. Periodic enquiries thus show either new images from the same web camera, or images from another web camera, if the means of transport is approaching it. This first form of the enquiry is advantageous, when several or all the passengers view the displayed images of a client 4. In addition to the actual image information, image data also contain data such as, for example, parameters of the image, recording time or commentary text. A second form of the enquiry requests all the web cameras which are located in a preferably rectangular portion of the area surrounding the position of the means of transport. For example, this portion is described in terms of the latitude and longitude of its central point and of the breadth and length of the portion, for example in kilometres. The server 3 then transmits to the client 4 as references to sources 2 all the addresses of web cameras which are located in this portion, together with the geographic location, i.e. for example the latitude and longitude, and scaled-down or thumbnail images 22 from the corresponding web cameras. The client 4 superimposes these scaled-down images on a map 21 of the surrounding area and in Fig.2. The reduced images 22 are advantageo i.e. they have a reference assigned to them. tleference is for example the eiddress of a web camera from this representation the user selects which image he would like to see enlarged. He makes the selection by for example cking on a reduced image 22 with a graphic input unit or bouching a touch-sensitive surface of the display unit. The in selected. is loaded by the client 4 from the web camera, or server 3 loads the image data from the web camera and cransmi it to the client 4. This second form of enquiry is advantages if for example each client 4 is assigned to an individual passenger. Advantageously, the means of transport is fatted with a proxy server 8. This stores the replies to all the enquiries from, a.11 the clients 4 in a circulating memory sc thai images and addresses which are asked for by a. plurality of passengers are transmitteid to the means of transport only once . By compressing the image data and if the transmission interfaces! 5 and S have sufficient bandwidth available, it is also possible for video images to be shown rat'.ier than stills . The server 3 maintains data structures) which allow the enquiries from the clients 4 to be answered efficiently: To determine the position of the web carrera closest to the means of transport when the enquiry is of the first form, use is made of Voronoi diagrams for example. Fig. 3 shows a section of a two-dimensional Voronoi diagram. In a two-dimensional Voronoi diagram, a plane which contains a set of given points 31 is partitioned into a set of regions 32 in such a way that the distance between any point in a region and one of the given points 31 is less than the distance to any of the other given points 31. In the present invention, the positions of web cameras constitute the given points 31. It is found from the position of a client 4 in which region the client 4 is; situated and hence which is the closest web camera. displays the resultant image on the display unit of the client 4. An example of such a display is illustrated in Fig 2. The scaled-down images 22 are advantageously hyperlinks, i.e. a reference is assigned to them. The reference is for example a web camera address. Using this display, the user selects which image he/she wishes to see on an enlarged scale. This selection is made, for example, by clicking on a scaled-down image 22 with a graphic input device or by touching a touch-sensitive surface of the display unit. The selected image is loaded by the client 4 from the web camera, or the server 3 loads the image data from the web camera and transmits the image data to the client 4. This second form of enquiry is advantageous, when for example each client 4 is assigned to an individual passenger. The means of transport is advantageously provided with a proxy server 8. This stores the responses to all the enquiries from all the clients 4 in a cyclical memory, such that images and addresses requested by a plurality of passengers are only transmitted to the means of transport once. It is also possible to display video images instead of still images by compressing image data and if the bandwidth of the transmission interfaces 5 or 6 is sufficient. The server 3 supports data structures which allow efficient responses to be made to the enquiries from the clients 4: For example, to determine which web camera is closest to the position of the means of transport, according to the first form of enquiry, Voronoi diagrams are used. Figure 3 shows a portion of a two-dimensional Voronoi diagram. In a two-dimensional Voronoi diagram, a plane, which comprises a The use of a Voronoi diagram typically entai, the use a. corresponding data structure for sea:rchinj. latter is generally an acyclic directed graph with a height V the of o(log :n) where n is the size of the Voror.oi diagL i.e. number of given pointis in it, and the maximum rrumbef branc.lri.es radiating from each node in the graph :.s constant. it is tlntis possible in logarithmic time to find the associated .region in the Voronoi diagram for a point (x,y) a:nd hence closest. of the given points. The search structure is; typically also used to add new points or to remove points. Both cause changes to the Voronoi diagram and the search structure-; but the cost involved is less than would be entailed by re-determining then in tineir entirety. To determine all the points which are situated in a given rectangular section of a plane for the socond form of the enquiry, two-dimensional range trees (referred to below as RT's) for example are used. An RT for two-dimensicnal positional coordinates (x,y) has an RT for a first co-ordinate (x), with each node in the RT which is not a leaf having an RT with a second co-ordinate (y) for all the points in the subtree of tae node. A two-dimensional RT for a set of points in the Euclidean plane is a data structure which efficiently supports window eneuiries. A window enquiry means the listing of the points which are situated in a given rectangular section of the plane whose outlines sire parallel to the axes of the plar.e, Efficient means a poly logarithmic cost, i.e. a cost of the order of 0((log n)c) where n is the number of points and c is a constant. RT's are implemented as static,, semi-dynamic or dynanic structures, in the case of static structures all the points, tire known before the structure is calculated. In the case: of semi-dynamic structures points can be added efficiently and in the case of dynamic structures points can be added and removed efficiently. large number of given points 31, is subdivided into a large number of regions 32 in such a way that the distance from each point of a region to one of the given points 31 is smaller than the distance to each of the other given points 31. In the present invention, the positions of web cameras correspond to the given points 31. It is determined from the position of a client 4 in which region the client 4 is located and which is therefore the closest web camera. The use of a Voronoi diagram is typically associated with a corresponding search data structure. This is as a rule a directed acyclic graph with a height of the order of 0(log n), wherein n is the quantity, i.e. the number, of given points of the Voronoi diagram, and the maximum number of outgoing branches of each node is constant in the graph. It is thus possible to find in logarithmic time the associated cell in the Voronoi diagram for a point (x,y) and thereby the closest of the given points. The search structure is typically also used to add or remove new points. Both lead to changes in the Voronoi diagram and the search structure, but with less effort than is required for complete redetermination thereof. To determine all the points which are located in a given rectangular portion of a plane, according to the second form of enquiry, two dimensional range trees (hereinafter RT) are used, for example. An RT for two-dimensional local coordinates (x,y) comprises an RT for a first (x) coordinate, wherein each node of the RT, which is not a leaf, comprises an RT with second (y) coordinates of all the points in the sub-tree of the node. A two-dimensional RT for a point quantity in the Euclidean plane is a data structure which efficiently supports window enquiries. A to e, on The data structures/ i.e. the Vcrono:L diareu or two-dimensional RT, are continually updated by accepting -new cameras that become available and cutting out ones that cease be available, thus ensuring optimum availability of the dat offer. Since the number n of web cameras me.y be considerable, these operations need to be efficiently performed, at poly logarithmic cost for example. The process of finding web cameras from the large and constantly changing number of URL's may or example be automated by examining the environment 'of images which ,are included in web pages for keywords such as "webcam" or "camera" and by examining images for periodic changes. This process prcduces a set of web cameras cf potential interest, When web cameras ar