SURFACE VEHICLE VERTICAL TRAJECTORY PLANNING CLAIM OF PRIORITY This application claims priority under 35 USC §119(e) to U.S. Patent Application Serial No. 09/535,849, filed on March 27, 2000, now abandoned, and U.S. Patent Application Serial No. 10/368,187, filed on February 18, 2003, the entire contents of which are hereby incorporated by reference. BACKGROUND OF THE INVENTION The invention relates to active vehicle suspensions, and more particularly to active vehicle suspension systems including vertical trajectory planning systems. SUMMARY OF THE INVENTION It is an important object of the invention to provide an improved active vehicle suspension. According to one aspect of the invention, a vehicle suspension system for a surface vehicle having a payload compartment and a surface engaging device includes a controllable suspension element for applying a force between the payload compartment and the surface engaging device, and a profile storage device, for storing a plurality of profiles of paths. The profiles include vertical deflection data. The system further includes a profile retrieving microprocessor, coupled to the controllable suspension element and to the profile storage device, for retrieving from the profile storage device one of the profiles, the one profile corresponding to the path on which the vehicle is traveling. In another aspect of the invention, in a vehicle for operating on a path, the vehicle having a payload compartment and a surface engaging device, an active vehicle suspension includes a force applying element coupling the payload compartment and the surface engaging device, for applying a force between the payload compartment and the surface engaging device to vary the vertical position of the payload compartment relative to the surface engaging device, a profile storage device for storing a vertical profile of the path, and a trajectory plan development subsystem, communicatingly coupled to the force applying element and to the profile storage device, for developing a trajectory plan responsive to the stored profile and for issuing commands to the force applying element, the commands corresponding to the trajectory plan. In another aspect of the invention, a method for operating an active vehicle suspension system in a surface vehicle having a data storage device includes the steps of: determining the location of the surface vehicle; determining if there is stored in the surface vehicle a vertical trajectory plan corresponding to the location; responsive to a determination that there is stored in the vehicle suspension system the vertical trajectory plan, retrieving the plan; executing the plan. In another aspect of the invention, a method for operating an active vehicle suspension in a surface vehicle having a sensing device to sense the vertical profile of a path and a data storage device, includes the steps of sensing a vertical profile of a path; recording the profile; and comparing the recorded profile with profiles stored in a database to find if the sensed profile matches one of the stored profiles. In another aspect of the invention, an active suspension system for a surface vehicle for operating on a path, includes an active suspension; a profile sensor for sensing a profile of the path; a path profile storage device for storing a database of path profiles; and a path profile microprocessor, coupled to the storage device and to the profile sensor, for comparing the sensed profile with the database of profiles. In another aspect of the invention, an active suspension system for a surface vehicle includes an active suspension; a locator system for determining the location of the surface vehicle; a trajectory plan storage device, for storing a database of trajectory plans corresponding to locations; and a trajectory plan microprocessor for determining if the database contains a trajectory plan corresponding to the determined location, for retrieving the corresponding trajectory plan, and for transmitting to the active suspension instructions, based on the corresponding trajectory plan. In another aspect of the invention, a method for determining the location of a surface vehicle includes storing a plurality of profiles of paths, the path profiles associated with locations and containing only vertical deflections of the path, measured at increments; sensing vertical deflection of a path on which the vehicle is currently traveling; and comparing the sensed vertical deflections with the path profiles. In another aspect of the invention, a method for developing a trajectory plan for use with a vehicle that includes a vehicle suspension system that includes a trajectory planning system for developing a trajectory plan, a controllable suspension element for urging a point on the vehicle to follow the trajectory plan includes recording a profile that includes data points, the data points representing positive and negative vertical deflections of a travel path; smoothing data of the profile, the smoothing providing positive and negative values; and recording the smoothed data as the trajectory plan. In another aspect of the invention, a method for developing a trajectory plan for use with an active vehicle suspension includes operating the vehicle on a section of road; recording data points representative of a profile of the section of road; and smoothing the data to provide the trajectory plan. The smoothing preserves positive and negative values of the data points. In another aspect of the invention, a method for operating a vehicle that includes a controllable suspension element, a microprocessor, and sensors for measuring at least one of vertical deflection, force applied by the controllable suspension, vertical velocity, and vertical acceleration, includes storing a library of a plurality of profiles; driving the vehicle over a road section and recording data measured by the sensors to provide measured data; and comparing the measured data with the plurality of profiles to determine a degree of match. In another aspect of the invention, a method for developing an optimized trajectory plan for a vehicle that includes a controllable suspension element includes a first developing, by a microprocessor, using a first characteristic value, of a first trajectory plan corresponding to a profile; a first executing, of the first trajectory plan, the first executing including recording performance data corresponding to the first trajectory plan; a first modifying, of the first characteristic value to provide a second characteristic value; a second developing, using the second characteristic value, by the microprocessor, of a second trajectory plan corresponding to the profile; a second executing, of the second trajectory plan, the second executing including recording a measure of performance data corresponding to the second trajectory plan; a first comparing of the performance data corresponding to the executing of the first trajectory plan and the performance data corresponding to the executing of the second trajectory plan to determine better performance data; and a first storing, as a current characteristic value, of a one of the first characteristic value and the second characteristic value corresponding to the better performance data. In another aspect of the invention, a method for developing a trajectory plan for use by a vehicle including a payload compartment, a wheel, a plurality of sensors for measuring a corresponding plurality of states of the vehicle, and a controllable suspension element for exerting force between the wheel and the payload compartment, includes recording a profile includes data points measured by the sensors, the data points representing positive and negative vertical values; storing the profile as one of a series of commands causing the controllable suspension element to exert a force, and a series of states of the vehicle as measured by at least one of the sensors. In another aspect of the invention, an active vehicle suspension for a surface vehicle including a payload compartment and a surface engaging device, the vehicle for operating along a path, includes a controllable suspension element for modifying the displacement between the payload compartment and the surface engaging device responsive to vertical displacements in the path; and a trajectory developing subsystem for issuing commands to the controllable suspension element causing the controllable suspension element to exert a force to modify the displacement between the payload compartment and the surface engaging device prior to the surface engaging -device-encountering the verticaltlisplacenient. In another aspect of the invention, a method for operating a vehicle including a payload compartment and a front surface engaging device and a rear surface engaging device, the vehicle further including a suspension system, the suspension system including a front controllable suspension element for exerting a force between the front surface engaging device and the payload compartment to modify the distance between the front surface engaging device and the payload compartment, the front controllable suspension element having a centered position, the front controllable suspension element including a centering subsystem for urging the front controllable suspension element toward the centered position, the suspension system further including a rear controllable suspension element for exerting a force between the rear surface engaging device and the payload compartment to modify the distance between the rear surface engaging device and the payload compartment, the rear controllable suspension element having a centered position, the rear controllable suspension element including a controllable centering subsystem for urging the rear controllable suspension element toward the centered position, includes operating the vehicle on a road segment including disturbances so that the front surface engaging device encounters the disturbances before the rear surface engaging device and so that the front controllable suspension element exerts forces responsive to the disturbances; determining the amplitude of one of the road disturbances, responsive to a determining that the amplitude of the one of the disturbances is less than a first threshold amount, disabling the rear suspension element centering subsystem. In another aspect of the invention, a surface vehicle includes a payload compartment; a front surface engaging device; a rear surface engaging device; and a suspension system including a front controllable suspension element for exerting a force between the front surface engaging device and the payload compartment to modify the distance between the front surface engaging device and the payload compartment, the front controllable suspension element having a centered position, the front controllable suspension element including a centering subsystem for urging the front controllable suspension element toward the centered position, the front controllable suspension element further includes a measuring system to measure the amplitude of a road disturbance encountered by the front surface engaging device; and a rear controllable suspension element for exerting a force between the rear surface engaging device and the payload compartment to modify the distance between the rear surface engaging device and the payload compartment, the rear controllable suspension element having a centered position, the rear controllable suspension element including a controllable centering subsystem for urging the rear controllable suspension element toward the centered position; and controlling circuitry, responsive to the measuring system, for disabling the rear suspension element centering subsystem. In another aspect of the invention, a method for operating a vehicle including a payload compartment and a first surface engaging device and a second surface engaging device, the vehicle further including a suspension system, the suspension system including a first controllable suspension element for exerting a force between the first surface engaging device and the payload compartment to modify the distance between the first surface engaging device and the payload compartment, the suspension system further including a second controllable suspension element for exerting a force between the second surface engaging device and the payload compartment to modify the distance between the second surface engaging device and the payload compartment, each of the first controllable suspension element and the second suspension element including associated sensors to measure at least one of vertical acceleration, vertical velocity, vertical road deflection, suspension displacement, and force applied by the controllable suspension includes operating the vehicle on a road segment having disturbances so that the first surface engaging device encounters the disturbances before the second surface engaging device; measuring, by the sensors associated with the first controllable suspension element, the disturbances; based on the measuring, causing the second controllable suspension element to exert a force related to the disturbance before the second surface engaging device encounters the disturbance. In another aspect of the invention, a method for operating a vehicle including a payload compartment and a first surface engaging device and a second surface engaging device, the vehicle further including a suspension system, the suspension system including a first controllable suspension element for exerting a force between the first surface engaging device and the payload compartment to modify the distance between the first surface engaging device and the payload compartment, the suspension system further including a second controllable suspension element for exerting a force between the second surface engaging device and the payload compartment to modify the distance between the second surface engaging device and the payload compartment, each of the first controllable suspension element and the second •suspension* element includes associated sensors to measure at least one of vertical acceleration, vertical velocity, vertical road deflection, suspension displacement, and force applied by the controllable suspension, the method includes operating the vehicle on a road segment having disturbances so that the first surface engaging device encounters the disturbances before the second surface engaging device; measuring, by the sensors associated with the first controllable suspension element, the disturbances; and based on the measuring, causing the second controllable suspension element to exert a force related to the disturbance before the second surface engaging device encounters the disturbance. In still another aspect of the invention, a method for operating a vehicle including a payload compartment and a surface engaging device the vehicle further including a suspension system, the suspension system including a controllable suspension element for exerting a force between a surface engaging device and the payload compartment to modify the distance between the surface engaging device and the payload compartment, the surface controllable suspension element having a centered position, the controllable suspension element including a reactionary operating mode and a trajectory plan operating mode, the method includes driving the vehicle on a road segment having vertical disturbances;determining the amplitude of the disturbances; responsive to a determining that an amplitude of a one of the disturbances is less than a first threshold amount, operating the controllable suspension element in the reactionary mode; responsive to a determining that the amplitude of the one of the disturbances is greater than the first threshold amount and less than a second threshold amount, disabling the centering system; and responsive to a determining that the amplitude of the one of the disturbances is greater than the second threshold amount, causing the controllable suspension to exert a force related to the one of the disturbances before the surface engaging device encounters the disturbance. Other features, objects, and advantages will become apparent from the following detailed description, which refers to the following drawings in which: BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING FIG. 1 is a diagrammatic view of a vehicle having a controllable suspension; FIG. 2a is a partially block diagram, partially diagrammatic representation of a "controllable suspension according to the invention; FIG. 2b is a partially block diagram, partially diagrammatic representation of a controllable suspension according to the invention; FIG. 3 is a diagrammatic view of the operation of a prior art active suspension; FIGS. 4a - 4c are diagrammatic views of the operation of an active suspension according to the invention; FIG. 5 is a diagrammatic view of the operation of the operation of an active suspension according to the invention; FIGS. 6a, 6b, and 6c are flow diagrams illustrating the operation of a suspension system according to the invention; FIG7 is a diagrammatic view illustrating a method of trajectory plan development; FIG. 8 is a diagram illustrating a method of collecting data in accordance with the invention; FIG. 9 is a block diagram of a process for optimizing a trajectory plan; FIGS. lOa-lOc are diagrams of a vehicle operating on a road surface in accordance with the invention; and FIGS, lla-llc are diagrams of a vehicle operating on a road surface in accordance with the invention. DETAILED DESCRIPTION With reference now to the drawings and more particularly to FIG. 1, there is shown a diagrammatic view of a vehicle 10 according to the invention. A suspension system includes surface engaging devices, such as wheels 14 connected to payload compartment 16 (represented diagrammatically as a plane) of the vehicle by a controllable suspension element 18. In addition, the suspension system may include conventional suspension elements (not shown), such as a coil or leaf spring arrangement or damper. While one embodiment of the invention is an automobile, so that the surface engaging devices are wheels and the payload includes passengers, the invention may also be practiced in other types of vehicles, such as cargo carrying vehicles. Payload compartment 16 may be a planar structure or may be enclosed on some or all sides. The surface engaging devices may include tracks or runners. The invention may also be practiced in vehicles that engage the surface through some form of levitation, such as magnetic or pneumatic levitation, so that the surface engaging devices include devices that do not require physical contact with the surface, and so that the surface may include tracks or open terrain. For simplicity of explanation, the invention will be described as embodied in an automobile. Controllable suspension elements 18 may be one of a variety of suspension elements that receive, or are capable of being adapted to receive, control signals from a microprocessor and to respond to the signals. Controllable suspension elements 18 may be components of a conventional active suspension system, in which the controllable suspension elements can respond to the control signals by varying the vertical displacement between the passenger compartment 16 and wheel 14 by applying a force. Suitable active suspension systems are described in U.S. Pat. Nos. 4,960,290 and 4,981,309 incorporated by reference herein. The force may be transmitted through some element such as a linear or rotary actuator, ball screw, pneumatic system, or hydraulic system, and may include intervening elements between the wheel and the force producing element. The controllable active suspension may also comprise an adaptive active vehicle suspension such as described in U.S. Pat. No. 5,432,700, in which signals maybe used to modify adaptive parameters and gains. Controllable suspension elements 18 may also be components of a conventional suspension system, which apply forces between passenger compartment 16 and wheel 14 reactively, in response to vertical forces resulting from wheel 14 passing over uneven surfaces. In conventional suspension systems, the controllable suspension elements may respond to the control signals by extending or compressing a spring, by changing a damping rate, or in other ways. By way of example, the invention will be described in an embodiment in which the controllable suspension element is an active suspension element. Referring now to FIG. 2a, there is shown a block diagram of a suspension according to the invention. Controllable suspension element 18 is coupled to a microprocessor 20 which is in turn coupled to profile storage device 22 and optional locator system 24. The suspension system further .includes sensors 11, 13, and 15 associated with payload compartment 16, controllable suspension elements 18, and wheels 14, respectively. Sensors, 11, 13, and 15 are coupled to microprocessor 20. Locator system 24 may receive signals from an external source, such as a positioning satellite 23. For convenience, only one of the controllable suspension elements 18 is shown. The remaining wheels 14, controllable suspension elements 18, and the respective sensors 11,13, and 15 are coupled to microprocessor 20 substantially as shown in FIG. 2a. Microprocessor 20 may be a single microprocessor as shown. Alternatively, the functions performed by microprocessor 20 may be performed by a number of microprocessors or equivalent devices, some of which can be located remotely from vehicle 10, and may wirelessly communicate with components of the suspension system, which are located on vehicle 10. Profile storage device 22 may be any one of a number of types of writable memory storage, such as RAM, or mass storage devices such as a magnetic or writable optical disk. Profile storage device 22 may be included in the vehicle as shown, or may be at some remote location, with a broadcasting system for wirelessly communicating path profile data to the vehicle. Locator system 24 may be one of a number of systems for providing longitudinal and latitudinal position, such as the Global Positioning System (GPS) or an inertial navigation system (INS). Locator system 24 may include systems, which provide for user input to indicate location and may also include profile matching systems that compare the profile of the path being driven by the vehicle with the profiles stored in memory storage. In one embodiment, the path being driven on is a roadway. However, the invention may be used in other types of vehicles that do not operate on roadways, such as open terrain vehicles and vehicles that operate on rails. The path can be typically defined by a location and a direction. By way of example, the invention will be described as embodied in an automobile for operating on a roadway. A suspension system incorporating the invention may also include a trajectory planning subsystem, which includes (referring to FIG. 2a) microprocessor 20, profile storage device 22, and locator system 24. Locator system 24 detects the location of the vehicle, and microprocessor 20 retrieves a copy of the profile of the road, if available, from a plurality of profiles stored in profile storage "device 22.^Microprocessor 20 calculates or retrieves a trajectory plan responsive to the road profile, and issues control signals to controllable suspension element 18 to execute the trajectory plan. The profile retrieval, trajectory calculation, and suspension control may be performed by a single microprocessor as shown, or may be done by separate microprocessors if desired. The trajectory plan development process is described more fully in connection with FIGS. 6a and fib. If controllable suspension element 18 is an active suspension acting reactively to road forces, microprocessor 20 may issue an adjusted control signal to controllable suspension element 18 based in part on the road profile. In a typical form, a road profile includes a series of vertical (z-axis) deflections from a reference point. The z-axis deflection measurements are typically taken at uniform distances from the location taken in the direction of travel. A road profile can also contain additional data such as x-axis and y-axis deflection; compass heading; steering angle; or other information such as may be included in navigation systems, such as commercially available vehicle navigation products. The additional data may involve greater processing capability of microprocessor 20 and profile storage device 22, but may be advantageous in using "dead reckoning" or pattern matching techniques described below to more precisely locate the vehicle or in uniquely associating a road profile with a location. Additionally, the additional data may be advantageous in determining, for example, the degree to which traction should be considered in developing the trajectory. A trajectory plan is a pre-determined path in space of a point or set of points on the payload compartment. To control the pitch of the vehicle, the trajectory may represent at least two points, respectively forward and rearward in the payload compartment. To control the roll of the vehicle, the trajectory plan may represent at least two points, one on each side of the vehicle. In a four wheeled vehicle, it may be convenient to use for trajectory plan development four points in the payload compartment, one near each wheel. Pairs of the points could be averaged (such as averaging the two points on each side of the vehicle to consider roll in the development of the trajectory plan, or averaging the two points in the front and the rear, respectively, to consider pitch in the development of the trajectory plan). For simplicity of explanation, the invention will be described in terms of a single point. The microprocessor issues control signals to controllable suspension element 18 to cause the vehicle to follow the trajectory plan. More detail on trajectory plans and the execution of trajectory plans are set forth in the examples that follow. The trajectory plan may take a number of factors into account, for example matching the pitch or roll of the vehicle to the pitch or roll expected by the passengers; minimizing the vertical acceleration of the payload compartment; maximizing the stroke of the suspension available to absorb bumps or dips, (hereinafter "disturbances") in the road; minimizing the amplitude or occurrence of accelerations of an undesirable frequency, such as frequencies around 0.1 Hz, which tends to induce nausea; maximizing tire traction; or others. The trajectory plan may also include "anticipating" a disturbance in the road and reacting to it before it is encountered, as will be described below in the discussion of FIG. 5. Further, particularly if the suspension system includes a conventional spring to support the weight of the car and the operation of the active suspension element extends or compresses the conventional spring, the trajectory plan may take power consumption into account. Referring now to FIG. 2b, there is shown another embodiment of the invention incorporating a trajectory plan storage device 25. Elements of FIG. 2b are similar to elements of FIG 2a, except profile device 22 of FIG. 2a is replaced by a trajectory plan storage device 25. Trajectory plan storage device 25 may be any one of a number of types of writable memory storage, such as RAM, or mass storage devices such as a magnetic or writable optical disk. Trajectory plan storage device 25 may be included in the vehicle as shown, or may be at some remote location, with a broadcasting system for wirelessly communicating path profile data to the vehicle. Operation of the embodiment of FIG. 2b is similar to the operation of the embodiment of FIG. 2a, except that microprocessor 20 retrieves and calculates trajectory plans that are associated with locations rather than being associated with profiles. Another embodiment of the invention includes both the profile storage device of FIG. 2a and the trajectory plan storage device of FIG. 2b. In an embodiment including both profile storage device 22 and trajectory plan storage device 25, the storage devices may be separate devices or may be different portions of a single memory device. Operation of embodiments including trajectory plan storage device 25 are described further in the discussion of FIG. 6c. FIG. 3 shows an example of the operation of a conventional active suspension without a trajectory planning subsystem. In FIG. 3, when front wheel 14f encounters sloped section 41, controllable suspension element 18f exerts a force to shorten the distance between payload compartment 16' and front wheel 14f. When the rise r due to the slope approaches the maximum lower displacement of the suspension element, suspension element 14f is "nosed in" to slope 41, and in extreme cases may reach or approach a "bottomed out" condition, such that there is little or no suspension travel left to accommodate bumps in the rising surface. Many suspension systems have centering subsystems to retain available suspension travel and to prevent the suspension element from bottoming out or topping out. Centering subsystems urge the suspension toward a centered position if the suspension approaches a bottomed out or topped out position. Spring systems inherently have a centering system, because the force exerted by a spring is proportional to the extension or compression of the spring. A "centered" position is typically the condition of the suspension when there is no upward or downward force, other than the weight of the vehicle, on the suspension. The centered position is not necessarily a position at which there is equal suspension available for upward and downward disturbances. Referring now to FIGS. 4a - 4c, there is shown an example of the operation of an active suspension according to the invention. Microprocessor 20 of FIG. 2a furnishes a computed trajectory plan 47, which closely matches the road surface, including sloped section 41, and issues appropriate control signals to controllable suspension elements 18f and 18r to follow the trajectory plan. In this example, the trajectory plan can be followed by exerting no force to shorten or lengthen the distance between wheels 14fand 14r and payload compartment 16, or if the suspension system includes a conventional spring, the trajectory plan can be followed by exerting only enough force to counteract acceleration resulting from force exerted by the spring. In FIG. 4b, when the vehicle has reached the same position in the road as in FIG. 3, payload compartment 16 is tilted slightly. In FIG. 4c, the payload compartment is tilted at an angle