1. Field of the Invention
The present invention relates to a navigation system for detecting the position of a moving body to inform a user of the current position of the moving body, and a map display apparatus using the navigation system.
2. Description of Related Art
Various types of navigation systems have been recently known, and these navigation systems are used while mounted on moving bodies such as vehicles, ships, etc. When such a navigation system is mounted on a moving body, it performs arithmetic and logic processing on information provided from various sensors to detect the current position of the moving body and display the current position on a screen of the navigation system.
This type of navigation system includes a position detector for detecting the absolute position of the moving body, a storage device for storing map data which include two-dimensional vector data obtained by projecting roads and points on the ground, such as structures, etc., onto a mesh-segmented plane using a universal transverse Mercator projection method, and character data associated with the vector data, an input device for receiving external instructions (commands), and a display device for reading out desired vector data from the mesh-segmented map stored in the storage device in accordance with an instruction input from the input device, to perform data conversion processing on the data, and then displaying the map on the display.
The data conversion processing includes shift conversion processing for changing the display position on a map, scale conversion (enlarging/reducing) processing for displaying a map on any scale, and rotational coordinate-transformation processing for changing a display direction on the map. With these processings, a plan view map is obtained by drawing the ground through an orthogonal projection in a vertical direction.
As described above, when a map is displayed on the screen, the conventional navigation system as described above adopts a plan-map display mode in which the ground is drawn in the vertical orthogonal projection style. Therefore, in order to simultaneously display two spots which are far away from each other, a reduction-scale of the map must be necessarily increased, and thus detailed information cannot be displayed on the screen.
An object of the present invention is to provide a bird""s-eye view forming method for displaying a map of a plane (ground) on the basis of a bird""s-eye view corresponding to a projected plan of the plane (ground) which is obtained by projecting the plane (ground) on any plane between the plane (ground) and a view point when the plane (ground) is viewed from the view point located at any height from the plane (ground), a map display apparatus using the bird""s-eye view forming method, and a navigation system using the map display apparatus.
In order to attain the above object, a bird""s-eye view forming method for forming, from map data, drawing (display) data for a map which is to be represented by a bird""s-eye view, is characterized in that coordinate data contained in the map data are perspectively converted to drawing (display) data based on a bird""s-eye view on a desired projection plane with a view point being set to a desired position (i.e., the coordinate data of the map data on a vertical orthogonal projection plane are perspectively converted to drawing (display) data on a desired projection plane based on a bird""s-eye view).
The bird""s-eye view forming method preferably includes a step of receiving an input of the position of the view point, and a step of determining the projection plane so that the drawing (display) positions of two predetermined spots (for example, a current position and a destination) which are obtained through a perspectively projecting transformation on the basis of the coordinates of the two spots in the map data and the position of the view point, are set to predetermined positions. (Here, the perspectively projecting conversion is defined as such a data conversion that data on a plan view are converted to data on a bird""s-eye view through a perspective projection operation. Therefore, by using the perspectively projecting conversion, a plan-view map is converted to a bird""s-eye view map. In the following description, the perspectively projecting conversion is merely referred to as xe2x80x9cperspective projectionxe2x80x9d).
Alternatively, the bird""s-eye view forming method may include a step of receiving an input of a scale, and a step of determining the position of the view point and the projection plane so that the drawing (display) positions of the two predetermined spots (for example, a current position and a destination) which are obtained by performing the perspective projection on the basis of the coordinates of the two spots in the map data and the input scale, are set to predetermined positions, and a scale of a drawn (displayed) map is equal to the input scale.
Further, the bird""s-eye view forming method may include a step of receiving an input of a projection angle which is defined as an angle at which a plane defined for the map data and the projection plane intersect each other, and a step of determining the projection plane on the basis of the input projection angle and the set view point position.
Still further, according to the present invention, there are provided a map display apparatus for displaying a bird""s-eye view by using the bird""s-eye view forming method as described above, and a navigation system for displaying a map by using the map display apparatus.
According to the bird""s-eye view display apparatus and the navigation system, map drawing means includes coordinate transforming means, and the coordinate transforming means performs the perspective projection (conversion) on the map data to convert a plan-view map to a bird""s-eye view map, and to display the converted map on a screen. Therefore, according to the present invention, a user can obtain a bird""s-eye view display which is easy to see and in which the user can easily recognize the positional relationship of spots displayed on the map. According to the bird""s-eye view forming method of the present invention, the view point can be freely set, so that it can meet user""s need.
According to the present invention, a navigation system which is excellent in operational performance and convenient for a user can be obtained if it is designed to overcome the following first to seventh problems.
Firstly, in the case where the processing speed of scroll processing in a bird""s-eye view display mode is lower than that in a plan display mode, the operational performance may be degraded when, the scroll processing is frequently used to search for a desired spot on the bird""s-eye view display (first problem).
Therefore, it is preferable in the present invention that the plan view (plan map) display mode and the bird""s-eye view display mode are freely switchable. With this switching operation, both the plan view display and the bird""s-eye view display can be made in accordance with a user""s requirement, and thus the convenience is enhanced.
In the above case, a second problem occurs, namely, that it is difficult for the user to grasp the positional relationship between two different type of view when display is switched between the plan view display and the bird""s-eye view display. This is because the position of the same spot to be displayed on the screen varies significantly due to variation of the view point with respect to the map, when the display switching operation is performed, and further, spots which have not been displayed or have been displayed until the display switching time are respectively displayed or are not displayed at the display switching time.
Therefore, if the present invention is designed so that the display is freely switchable the plan view display and the bird""s-eye view display, it is further preferable that an intersecting angle between a plane containing the map data and a plane onto which the map is projected, is gradually increased or reduced in time series. That is, according to the present invention, it is preferable that the view point is smoothly shifted during the conversion between the plan view display and the bird""s-eye view display. With this operation, the shift between the plan view and the bird""s-eye view is smoothly performed, and thus the user can easily recognize the positional relationship between spots displayed on the plan view map and the same spots displayed on the bird""s-eye view map.
Thirdly, when the map is displayed in the bird""s-eye view display mode, if the view point serving as a parameter to the perspective projection is fixed to a position, a mark indicating the current position which is displayed on the screen is shifted on the map while the current position is shifted. In this case, if the marked current position deviates from a displayed map area, the mark of the current position is not displayed (third problem).
Therefore, according to the present invention, the view point from which to obtain the bird""s-eye view may be fixed to a specific position which is set to be away from the current position at a fixed distance and in a fixed direction at all times. Furthermore, the height of the view point may be varied in accordance with an external operation. If the view point is fixed to the specific position as described above, the bird""s-eye view is displayed such that the current position is fixed to a point on the screen at all times, so that the user can easily grasp the current position. Furthermore, if the position of the view point can be set to a user""s desired position, the convenience can be further enhanced.
Fourthly, when two spots are displayed on the same frame, it is sufficient in the conventional plan view display to renew the scale in accordance with the distance between the two points. However, in the bird""s-eye view display of the map, the map cannot be optimally displayed by merely renewing the scale (fourth problem).
Therefore, according to the present invention, an instruction regarding the positions of two spots (for example, the current position and the destination) may be accepted, and the view point and the intersection angle between the map-projected plane (bird""s-eye view) and the plane containing the map data (plan view) may be determined so that the two spots are displayed at predetermined positions on the screen, whereby the bird""s-eye view display is performed. With this operation, the positional relationship between the two spots can be easily recognized. Further, even when the positions of the two spots vary, these are displayed on the same frame at all times, so that the user can grasp the positional relationship between the two spots with no complicated operation.
According to the present invention, in the bird""s-eye view display mode, line backgrounds such as roads, railroad lines, etc., plane backgrounds such as rivers, green zones, etc., and character strings may be perspectively converted and drawn on the bird""s-eye view map. However, if the character strings are perspectively converted, the shape of a character is smaller and is more deformed as it becomes farther away from the view point. On the other hand, if it is in the vicinity of the view point, it is enlarged. Thus the character strings may be illegible in these cases (fifth problem).
Therefore, it is preferable that the coordinate transforming means of the present invention does not perform the perspective projection on a character image. In this case, characters contained in the bird""s-eye view are displayed at the same size, so that the character strings are easily legible.
Further, when a lot of character data are displayed, character strings are displayed while overlapping with one another. Particularly in the bird""s-eye view, an angle of depression with respect to the view point is smaller at a far-distance point from the view point and thus the reduction scale is large at that point, so that the data amount of character data to be displayed per unit area is increased. Accordingly, the character data (strings) are liable to be overlapped with each other at the far-distance point (sixth problem).
Therefore, it is preferable that as the position of character data contained in the map data becomes nearer to the view point, drawing (display) judgment means of the present invention displays the character data more preferentially (i.e., increases the display priority rank of the character data). With this operation, character data having a higher display priority rank may be displayed while being superposed on character data having a lower display priority rank, so that the character string can be prevented from being deficient and it can be made legible.
When the current position is located in the vicinity of the bottom side of the display frame or the view point, it is preferable that the display height or the distance from the view point is set as a criterion for determining the display priority rank. Further, when two or more character strings are displayed while overlapping with one another, a character string which is displayed at a lower display height (which is the height from the bottom side of the display frame) on the screen may be displayed more preferentially (i.e., so that the character string is not covered by the other character strings, but is superposed on the other character strings). In this case, the character data near to the current position are displayed while being prevented from being covered by the other character strings and thus missed. With this display operation, the deficiency (missing) of the character information (string) near to the current position which the user has a greater need for can be prevented, and the character string can be made legible.
As described above, when the bird""s-eye view display is performed, the data amount of line data, plane background data and character data to be drawn per unit area is increased in a region which is far away from the view point (hereinafter referred to as xe2x80x9cfar-distance regionxe2x80x9d, and this region is a region having a small angle of depression). Accordingly, when the character data of these data are drawn while superposed on the other data, the line data and the plane background data are covered by the character data in the far-distance region, so that it may be difficult to check the shapes of roads, etc. in the far-distance region (seventh problem).
Therefore, the drawing judgment means of the present invention preferably eliminates, from targets to be drawn (displayed), those character strings which are obtained through the perspective projection of the map and which are located at positions higher than a predetermined display height from the bottom side of the display frame. Or the drawing judgement means may eliminate those character strings which are defined at position where is further than a fixed distance from the view point. As described above, the character strings are not displayed (drawn) in a region which is located above the predetermined height or is more than at a predetermined distance away, that is, in a region where the data amount to be drawn per unit area is considerably increased due to the reduction in angle of depression in the bird""s-eye view display mode. Therefore, a road display, a plane background display, etc. are not covered by the character string display, so that the recognition of roads in the far-distance regions can be prevented from being impaired even in the bird""s-eye view display mode.