Navigation system using angular rate sensor

In a navigation system by which vehicle travel locations are detected by an angular rate sensor and a distance sensor and the detected vehicle travel locations are corrected so as to be displayed along a map road, when a detected vehicle travel angle exceeds a predetermined value under the condition that the vehicle is out of a route judge area set at each branch point, an abnormal travel is detected and alarmed while displaying an abnormal vehicle location. Further, when a travel route change cannot be determined after the vehicle has passed through the set route judge area, an abnormal location is also displayed and alarmed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a navigation system using an angular rate 
sensor, and more specifically to a navigation system by which automotive 
vehicle travel locations are detected by an angular rate sensor and a 
distance sensor and the detected vehicle locations are projected on a 
displayed road map so that the vehicle travel display moves along a 
predetermined route on the map. 
Since this system can be operative independently without need of other 
auxiliary means (e.g. radio signals), this system is effective in an urban 
area or other areas subjected to radio interference where radio navigation 
systems cannot operate reliably. 
2. Description of the Prior Art 
The applicant has already proposed a dead reckoning and map correlation 
system for automotive vehicle tracking, which uses an angular rate sensor 
and a distance sensor, in Japanese Unexamined Published (Kokai) Patent 
Application No. 60-48600, entitled vehicle position detecting system. In 
this system, current vehicle locations can be intermittently detected on 
the basis of vehicle travel distance data and vehicle travel angle data 
(detected by an angular rate (velocity) sensor); and the calculated 
vehicle locations are projected onto roads of a digital map (including 
intersections and inflections of roads) previously prepared by inputting 
digital map data to a CPU through a keyboard so that the vehicle travel 
motion can track a road displayed on the map. 
In the prior-art navigation system, a route R is calculated on the basis of 
map information data including intersections, inflections, and point 
numbers adjacent to succeeding points all stored in a memory unit, and the 
current vehicle travel location is displayed along the calculated route R. 
Further, as shown in FIG. 1, a route judge area JE is formed at each 
branch point P.sub.1 in order to determine a direction along which the 
vehicle travels. That is to say, a vehicle travel angle is integrated 
within this route judge area, and the integrated vehicle travel angle is 
compared with the angles .theta..sub.2 and .theta..sub.3 of the routes 
P.sub.2 and P.sub.3 with respect to a horizontal line X. 
Therefore, in the prior-art navigation system, although the vehicle travel 
locations can be displayed along the route R stored in the memory unit, 
when the vehicle turns unexpectedly into a gasoline station, for instance, 
since the vehicle location is still displayed only on the route R obtained 
by the map, there exists a problem in that the unexpected vehicle travel 
is displayed as if the vehicle travel in a straight line. 
In addition, when the vehicle travel route change is not correctly detected 
at a branch point, there exists a problem in that vehicle travel locations 
are not correctly displayed along the route R, and there exists a 
deviation from an expected route on the map. 
SUMMARY OF THE INVENTION 
With these problems in mind, therefore, it is the primary object of the 
present invention to provide a navigation system which can detect such an 
abnormal vehicle travel deviation from the road map or a travel route 
change not correctly detected at a branch point in order to produce an 
alarm signal, and display the detected abnormal condition so that the 
driver can take appropriate corrective action quickly. 
To achieve the above-mentioned object, the navigation system for displaying 
travel locations of a vehicle according to the present invention 
comprises: (a) means for detecting vehicle travel angle; (b) means for 
detecting vehicle travel distance; (c) means, coupled to said vehicle 
travel angle detecting means and said vehicle travel distrance detecting 
means, for calculating vehicle locations on the basis of detected vehicle 
travel angles and distances; (d) means for storing map information data 
including branch points; (e) means, coupled to said map information data 
storing means, for setting a route judge area at each branch point to 
determine a route along which the vehicle travels for a branch point; (f) 
means, coupled to said vehicle distance detecting means, said map storing 
means and said route judge area setting means, for determining vehicle 
passing into the set route judge area when a distance between a current 
vehicle location and a succeeding branch point becomes shorter than a 
predetermined value and vehicle passing through the set route judge area 
when a distance between a position, at which vehicle enters the set route 
judge area, and a current vehicle location becomes longer than a 
predetermined value; (g) means, coupled to said vehicle travel angle 
detecting means and said route judge area pass determining means, for 
determining travel route at each branch point by comparing a travel angle 
detected by said travel angle detecting means with map data stored in said 
map data storing means when the vehicle has passed through the set route 
judge area; (h) means, coupled to said vehicle location calculating means 
and said travel route determining means, for correcting calculated vehicle 
locations so as to be located along a road on a displayed map; (i) means, 
coupled to said vehicle location correcting means and said route judge 
area pass determining means, for detecting an abnormal travel when a 
detected vehicle travel angle exceeds a predetermined value under the 
condition that said route judge area pass determining means determines 
that the vehicle is out of the set route judge area; (j) means, coupled to 
said abnormal travel detecting means, for storing a current vehicle travel 
location and generating an alarm signal when said abnormal travel 
detecting means detects an abnormal travel; and (k) means, coupled to said 
location storing and alarm generating means and said map data storing 
means, for displaying a map stored in said map data storing means, vehicle 
locations calculated by said calculating means, the stored vehicle located 
at which an abnormal travel is detected, and abnormal vehicle locations 
deviated from a map route. 
Further, when the travel route determining means cannot detect a travel 
route change after the route judge area pass determining means detects 
that the vehicle has passed through the set route judge area, the location 
storing and alarm generating means stores the current abnormal location 
and generates an alarm signal. 
Therefore, in the navigation system of the present invention, even if the 
vehicle drops in at a gasoline station, for instance, since the turning 
location is stored and alarmed and vehicle locations to and from the 
gasoline station are displayed by distinguishable dots, the driver can 
easily restart the normal operation along the map when the vehicle has 
returned to the predetermined route again.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
With reference to the attached drawings, a basic configuration of the 
navigation system using angular rate sensor will be described hereinbelow 
in more detail. 
FIG. 3 shows a schematic block diagram showing a basic configuration of an 
embodiment of the system according to the present invention. In FIG. 3, 
the navigation system comprises travel angle detecting means A for 
detecting a travel angle (.theta.=.intg.wdt) of an automotive vehicle on 
the basis of a signal W from an angular rate sensor; travel distance 
detecting means B for detecting a travel distance of the vehicle; location 
calculating means C for calculating a current vehicle location on the 
basis of the signals from the travel angle detecting means and the travel 
distance detecting means; map memory means D for storing survey map data 
including various point coordinates of intersections, branches and 
inflections of roads; route judge area setting means E for setting a route 
judge area at each branch point to determine a route along which the 
vehicle travels at a branch point; route judge area pass determining means 
F for determining that a vehicle passes into a set route judge area when a 
distance between a current vehicle location and a succeeding branch point 
becomes shorter than a predetermined value and that a vehicle passes 
through the set route judge area when a distance between a position at 
which vehicle passing into the set route judge area is determined and a 
current vehicle location becomes longer than a predetermined value; travel 
route determining means G for determining a vehicle travel route by 
comparing a travel angle detected by the travel angle detecting means with 
map data stored in the map data storing means when the vehicle has passed 
through the set route judge area; location correcting means I for 
correcting the calculated vehicle location on the judged map route; 
abnormal travel detecting means J for detecting an abnormal travel when a 
detected vehicle travel angle exceeds a predetermined value under the 
condition that the route judge area pass determining means determines that 
the vehicle is out of the set route judge area; location storing and alarm 
generating means K for storing a current vehicle travel location and 
generating an alarm signal when the abnormal travel detecting means 
detects an abnormal travel; and displaying means H for displaying a map 
stored in said map data storing means, vehicle locations, stored vehicle 
location at which an abnormal travel is detected, and abnormal vehicle 
locations deviated from a map route. 
FIG. 4 shows a block diagram of the system of the present invention. In the 
drawing, the system comprises a travel distance sensor 11 for generating a 
signal indicative of the vehicle distance; an angular rate sensor 12 such 
as a vibrating gyrosensor for detecting vehicle angular velocity; and a 
CPU 13 for receiving signals outputted from the travel distance sensor 11 
and the angular rate sensor 12. The CPU 13 operates in accordance with 
with programs stored in a ROM 14 to execute various operations by reading 
various data from the RAM 15 and writing the processed data in the RAM 15. 
Further, the CPU 13 is connected to a keyboard 16 for entering data, a 
display unit such as CRT, and a memory 18 for storing survey map 
information data. 
FIG. 5 shows a diagram for assistance in explaining the map correction 
method of the present invention. A point P(x, y) 1 is first initialized to 
P.sub.00 (X.sub.00, Y.sub.00). An actual travel route 2 shown by dashed 
line is calculated by the distance sensor and the angular rate sensor, and 
compared with map data R.sub.0. As a result, vehicle locations are 
displayed on a map on a CRT screen using symbols. The broken line 2 is a 
route detected by the sensors and projected to the map route R for 
correction and this is not displayed on the CRT map. Data for the 
displayed map are prepared based on a survey map including intersections 
or inflections of roads (referred to as points and straight lines). The 
portion between two points is approximate by a straight line. The map is 
digitally coordinated for each point and stored in the ROM. 
In order to remove errors due to the straight line approximate of the map, 
a certain range referred to as "route judge area JE" is set up at each 
point at the rate proportional to the distance between two points P.sub.0 
and P.sub.1. In this area JE, judgement of a change in vehicle travel 
direction and correction of errors are implemented. Within the area judge 
area, the route or location calculated by the distance sensor and the 
angular rate sensor is displayed without coordinate correction to the map. 
However, when the vehicle passes out of the area judge area, the symbol 
indicating the vehicle location is displayed along the map after 
correction. 
The map data stored in the memory 18 are position data expressed by X-Y 
coordinates of branch points such as intersections, inflections of roads 
as shown in FIG. 5 and includes the number of the points and the numbers 
of other points adjacent thereto. 
In more detail, the position data of a route as shown in FIG. 5 are as 
follows: 
______________________________________ 
Point No. X Y Adjacent point Nos. 
______________________________________ 
P.sub.0 X.sub.0 
Y.sub.0 P.sub.1, P.sub.4 
P.sub.1 X.sub.1 
Y.sub.1 P.sub.0, P.sub.1, P.sub.3 
P.sub.2 X.sub.2 
Y.sub.2 P.sub.1 
P.sub.3 X.sub.3 
Y.sub.3 P.sub.1 
. . . . 
. . . . 
. . . . 
P.sub.n 
______________________________________ 
In the table, the coordinates of the point P.sub.0 are (X.sub.0, Y.sub.0) 
and the adjacent points thereof are P.sub.1 and P.sub.4. The coordinates 
of the point P.sub.1 are (X.sub.1, Y.sub.1) and the adjacent points 
thereof are P.sub.0, P.sub.1 and P.sub.3 and so on. 
The operation of the system will be explained in accordance with a 
flowchart shown in FIGS. 6(A), 6(B) and 7 by taking an example where a 
vehicle travels from a point P.sub.00 (X.sub.00, Y.sub.00) to a point 
P.sub.1 (X.sub.1, Y.sub.1) along a route R.sub.0 connecting two points 
P.sub.0 (X.sub.0, Y.sub.0) and P.sub.1 (X.sub.1, Y.sub.1) on a map shown 
in FIG. 5. Further, in FIG. 5, the dashed lines indicate calculated 
vehicle route (not displayed). 
The initial location P.sub.00 (X.sub.00, Y.sub.00) and two points P.sub.0 
(X.sub.0, Y.sub.0) and P.sub.1 (X.sub.1, Y.sub.1) along a route R.sub.0 
are entered through the keyboard 16 and stored in the RAM 15 for 
initialization (in step S.sub.1). The entered current position and map 
data including roads and place names are displayed (in step S.sub.2). In 
this step S.sub.2, the current vehicle location P(x, y) is corrected on 
the route R. 
A gradient .theta..sub.0 of the route R.sub.0 with respect to a base line 
L.sub.H (positive in the counterclockwise direction) is calculated on the 
basis of two points P.sub.0 (X.sub.0, Y.sub.0) and P.sub.1 (X.sub.1, 
Y.sub.2) as follows: 
##EQU1## 
In the same way, gradients .theta..sub.2 and .theta..sub.3 with respect to 
the base line L.sub.H are calculated on the basis of the two adjacent 
points P.sub.1, P.sub.2, and P.sub.3 (in step S.sub.3). 
In the succeeding step, a distance L.sub.n between P.sub.00 and P.sub.1 is 
calculated as follows (in step S.sub.4): 
##EQU2## 
Thereafter, an area judge value L.sub.c and an area pass judge value MM are 
determined in order to set a route judge area JE (in step S.sub.4). 
Thereafter, each data is set (in step S.sub.5) as follows: 
.delta.S.sub.n (sampling distance)=0, .delta..theta..sub.n =0, 
X.sub.n =X.sub.00, Y.sub.n =Y.sub.00, .theta..sub.n =.theta..sub.0 
Further, the calculated data .theta..sub.0, .theta..sub.2, .theta..sub.3, 
L.sub.n, .delta.S.sub.n =0, .delta..theta..sub.n =0, X.sub.n =X.sub.00, 
Y.sub.n =Y.sub.00 are all stored in the RAM 15 for sampling calculation of 
each vehicle travel distance .delta.S.sub.n and each vehicle travel angle 
.delta..theta..sub.n. (in step S.sub.5). 
The next sampling calculation (step S.sub.6) can be implemented in 
accordance with a subroutine as shown in FIG. 7. 
Each travel distance signal S.sub.se from the travel distance sensor 11 and 
each travel angle signal .theta..sub.se (.intg.wdt) from the angular rate 
sensor 12 are both inputted (in step S.sub.61). 
Further, the obtained travel distance S.sub.se and the travel angle 
.theta..sub.se are integrated to obtain .delta.S.sub.n =S.sub.se and 
.delta..theta..sub.n =.theta..sub.se (in step S.sub.62), and then CPU 
check whether a predetermined time (e.g. 1 sec) has elapsed (in step 
S.sub.63). If YES, the integrated angle value .delta..theta..sub.n is 
added to the current angle .theta..sub.n-1 stored in the RAM 15 to obtain 
an updated angle value .theta..sub.n. The obtained value .theta..sub.n is 
stored in the RAM 15. On the basis of the current angle value 
.theta..sub.n, the current corrected location P.sub.n (X.sub.n, Y.sub.n) 
can be obtained as follows (in step S.sub.64): 
EQU X.sub.n =X.sub.n-1 +.delta.S.sub.n cos .theta..sub.n 
EQU Y.sub.n =Y.sub.n-1 +.delta.S.sub.n sin .theta..sub.n 
where P.sub.n-1 (X.sub.n-1, Y.sub.n-1) are coordinates stored in the RAM 15 
as the preceding location such as P.sub.00 (X.sub.00, Y.sub.00). By the 
above correction, it is possible to correctly track the vehicle travel 
trace along the road on the map. 
In the same way, on the basis of the calculated angle value .theta..sub.1, 
the current non-corrected (sensor-detected) location P(x.sub.n, y.sub.n) 
can be obtained as follows (in step S.sub.64): 
EQU x.sub.n =x.sub.n-1 +.delta.S.sub.n cos .theta..sub.n 
EQU y.sub.n =y.sub.n-1 +S.sub.n sin .theta..sub.n 
By the above calculation, it is possible to display the current vehicle 
location as it is without correction on the map. 
In the above-mentioned tracking on the map, the tracking display moves 
.delta.S.sub.n by .delta.S.sub.n from a designated point P.sub.00 along a 
route R.sub.0 connecting betwen P.sub.00 and P.sub.1 (in step S.sub.64). 
Thereafter, the integrated travel distance .delta.S.sub.n and the 
integrated travel angle .delta..theta..sub.n are reset when the vehicle 
reaches a route judge area (in step S.sub.65). 
Control checks whether it is necessary to change the map now displayed on 
the basis of the current location coordinates P.sub.n (X.sub.n, Y.sub.n) 
and/or (x.sub.n, y.sub.n) (in step S.sub.66). If YES, the map is changed 
(in step S.sub.67). If NO, control skips steps S.sub.67 to step S.sub.68. 
Control checks whether flag. is 1 (in step S.sub.68). This flag D 
determines coordinates of the current location to be displayed. If YES 
(D=1 in step S.sub.68), the current location is displayed by the 
non-corrected coordinates (x.sub.1, y.sub.1) based upon the sensors. If NO 
(D=0 in step S.sub.68), the current location is displayed by the corrected 
coordinates (X.sub.1, Y.sub.1) based upon the correcting calculation. That 
is, the displayed coordinates are determined by this flag D. Thereafter, 
control returns to step S.sub.7 shown in FIG. 6(A). 
Returning to FIG. 6(A), CPU checks whether the number of routes selectable 
at the next point P.sub.1 (i.e. the number of branch points) is more than 
1 (in step S.sub.7). If NO, flag B is set to 1 (in step S.sub.8) and 
proceeds to step S.sub.9. With reference to FIG. 6(B), control checks 
whether the integrated travel distance .delta.S.sub.n is zero (in step 
S.sub.9). 
Control checks whether .delta.S.sub.n is zero (in step S.sub.9). If NO, 
since this indicates that the vehicle is traveling, L.sub.n-1 
-.delta..sub.n =L.sub.n is set (in step S.sub.10). This indicates to 
obtain an updated distance L.sub.n+1 between the point P.sub.n and the 
succeeding point P.sub.1 by subtracting a travel distance increment 
.delta.S.sub.n from the distance L.sub.n between P.sub.00 and P.sub.1 in 
FIG. 5. 
Control checks whether the distance L.sub.n is equal to or smaller than 
L.sub.c (e.g. 20 m) indicative of route judge area JE (in step S.sub.11). 
If NO, since this indicates that L.sub.n is longer than 20 m, control 
proceeds to the step S.sub.12. If YES, since this indicates that L.sub.n 
is shorter than 20 m and the vehicle approaches an intersection, flap E=0 
is set (in step S.sub.13). This flag E is a flag indicative of abnormal 
travel. When initialized, this flag is E=0. However, when the vehicle 
deviates abnormally from the map route due to an abnormal travel, this 
flag is set to E=1. 
Then, control sets the travel distance to L.sub.m =0 and the flag to D=1 
(in step S.sub.14). L.sub.m is a travel distance after the vehicle enters 
the route judge area and D=1 indicates that the vehicle travel distances 
are displayed by the non-corrected coordinates (x.sub.n, y.sub.n). If NO 
(in step S.sub.11), control checks whether flag E=1 (in step S.sub.12). 
Since E (abnormal travel) is initialized to 0, control proceeds to step 
S.sub.15. However, if E=1, control proceeds to step S.sub.19. 
Control checks travel angle changes .vertline..theta..sub.n .vertline. by 
sampling travel angle data obtained in step S.sub.62 of the subroutine 
shown in FIG. 7 (in step S.sub.15). 
If .vertline..theta..sub.n .vertline..gtoreq.15 degree/sec, this indicates 
that an abnormal travel occurs, because no travel angle change occurs as 
long as the vehicle travels along a straight line according to a 
prearranged plan. Control checks whether the warning flag W is 1 (in step 
S.sub.16). If W=1, since control has already detects an abnormal travel, 
the warning flag is set to W=1 (in step S.sub.23) and control produces a 
warning signal and display the current location (in step S.sub.24), 
returning to step S.sub.6 shown in FIG. 6(A). If W=0 (in step S.sub.16), 
the abnormal flag is set to E=1 and the integrated travel angle is set to 
.theta..sub.m =0 (in step S.sub.17). Further, the current corrected 
location, the current non-corrected location, and two point numbers on 
both sides of the route are stored in a non-volatile memory. Travel angle 
.theta..sub.m is integrated (in step S.sub.19) and travel angle change 
rate .vertline..theta..sub.n .vertline. is checked (in step S.sub.20). The 
vehicle turning right or left can be determined by checking the angle 
change rate. For instance, if the angle change rate is less than 2 
deg/sec. control determines that the vehicle has been turned. Control 
checks whether the vehicle travel deviates from the route on the basis of 
the integrated travel angle .theta..sub.m (in step S.sub.21). For 
instance, if .theta..sub.m exceeds 60 degrees, an occurrence of vehicle 
travel deviation is determined. If YES (in step S.sub.21), control sets 
the warning flag to W=1 (in step S.sub.23) and informs the driver of the 
abnormal travel and displays the current vehicle location stored in step 
S.sub.18 (in step S.sub.24). Therefore, as far as the driver does not 
correct the operation, the warning flag is not reset, so that warning is 
kept produced and the non-corrected vehicle locations are displayed being 
deviated from the map route. 
Further, when the vehicle enters a route judge area and this is detected 
(in step S.sub.11), the abnormal travel flag is reset (E=0) into the 
initial condition (in step S.sub.13) when no travel deviation is detected 
in step S.sub.21, the abnormal travel flag is reset to E=0 into the 
initial condition (in steps S.sub.22). 
If YES (in step S.sub.9), since this indicates that the vehicle is kept 
stopped, control checks whether the abnormal travel flag is E=1 (in step 
S.sub.25). If YES, control proceeds to step S.sub.21 ; if NO, control 
proceeds in this step S.sub.25 to step S.sub.6. Further, control proceeds 
to step S.sub.6 when control determines that travel angle change rate is 
15 deg/sec or less (in step S.sub.15) or 2 degree/sec or more (in step 
S.sub.20). 
With reference to FIG. 6(A) again, steps S.sub.26 and after will be 
described hereinbelow. 
Returning to the step S.sub.11 again, control checks whether the updated 
distance L.sub.n becomes less than an area judging valve L.sub.c (e.g. 20 
m) indicative of the range of the route judge area JE (in step S.sub.11). 
If NO, control proceeds to the step S.sub.12 as described above. If YES, 
control sets the abnormal flag to E=0 (in step S.sub.13) and proceeds to 
the succeeding step because this indicates that the vehicle approaches a 
branch point. Therefore, the L.sub.m (distance between the current point 
to the branch point P.sub.1) is set to zero and the flag D is set to 1 in 
order to display the current location on the basis of the non-corrected 
location on the basis of the non-corrected location (x.sub.n, y.sub.n) (in 
step S.sub.14). Control proceeds to the step S.sub.26 (the same as step 
S.sub.6 shown in FIG. 7). In this step S.sub.26, the sampling calculation 
starts. 
In the succeeding step, control sets L.sub.m-1 +.delta.S.sub.n =L.sub.m (in 
step S.sub.27). That is, in FIG. 5, a travel distance .delta.S.sub.n is 
integrated to obtain a distance away from a point P.sub.a where the 
vehicle enters the route judge area JE. Thereafter, control checks whether 
L.sub.m exceeds an area pass judge value MM (e.g. 30 m) (in step 
S.sub.28). If NO, control returns to the step S.sub.26 to continuously 
obtain travel distance .delta.S.sub.n. If YES, since this indicates that a 
distance L.sub.m away from a position P.sub.a where the vehicle enters 
into the route judge area JE exceeds an area pass judge valve MM (e.g. 30 
m) at the branch point P.sub.1, control determines that the vehicle has 
passed through the branch point P.sub.1. 
After control determines that the vehicle has passed through the point 
P.sub.1 as L.sub.m &gt;MM in step S.sub.28, control proceeds to step 
S.sub.29. If L.sub.m &lt;MM, control returns to step S.sub.26 to continue 
integrating calculation of distance and angle for determining the route 
area. 
Control sets flag D to 0 (in step S.sub.29) to display the current location 
on the basis of coordinates (X.sub.n, Y.sub.n) of corrected data, because 
the location passed out of the area JE. 
Control checks whether the flag B is 1 (in step S.sub.30). If NO 
(B.noteq.1), since this indicates that there exists plural branch points, 
control proceeds to the succeeding step to determine a route along which 
the vehicle turns (in step S.sub.31). For doing this, the current vehicle 
travel angle .theta..sub.n is compared with the angle .theta..sub.2 
(corresponding to the forward point P.sub.2) and the angle .theta..sub.3 
(corresponding to the forward point P.sub.3) both previously obtained in 
step S.sub.3. When the vehicle angle .theta..sub.n is close to the angle 
.theta..sub.2, for instance, it is determined that the vehicle turns along 
the road R.sub.2 (in step S.sub.31). On the other hand, if YES in step 
S.sub.39 (B=1), since this indicates that the number of branches is one 
and therefore it is unnecessary to determine the route (in step S.sub.31), 
and control directly proceeds to the succeeding step S.sub.32. 
Control reinitializes the current vehicle location and determines an 
updated initial location P.sub.01 (X.sub.01, Y.sub.01) (in step S.sub.32). 
The updated initial location P.sub.01 is displayed (in step S.sub.23). 
Thereafter, control returns to step S.sub.3. 
The distance S.sub.1 between P.sub.c (X.sub.c, Y.sub.c) and P.sub.n 
(X.sub.n, Y.sub.n) is calculated, and then the initialized vehicle 
location P.sub.01 (X.sub.01, Y.sub.01) can be calculated as 
##EQU3## 
The above initialized location P.sub.01 (X.sub.01, Y.sub.01) is determined 
as a new location P.sub.00 (X.sub.00, Y.sub.00) for the succeeding 
tracking operation. 
Thereafter, control returns from the step S.sub.33 to the step S.sub.3 to 
repeat the similar tracking operation. In this case, the point P.sub.1 is 
regarded as the point P.sub.0 ; the point P.sub.2 is regarded as the point 
P.sub.1 ; the route R.sub.0 is regarded as the route R.sub.2 in the 
succeeding tracking operation. 
In the above-mentioned system, when a vehicle traveling from a point 
P.sub.0 to a point P.sub.1 turns to the left to drop in at a gasoline 
station, for instance, as shown in FIG. 2(A), a warning is produced and 
the left-turn location is automatically displayed on the map by a black 
dot. Further, thereafter, the vehicle travel locations are displayed on 
the map as shown by white dots along the dashed line. Therefore, when the 
vehicle returns back to and starts again along the map route, the driver 
can easily determine a restart position on the map route. 
Further, when no left-turn signal is produced after the vehicle has passed 
through the route judge area JE of a point P.sub.1 in FIG. 2(B), since a 
large travel angle change is detected and a warning is produced, the drive 
can know an erroneous display at early stage, and therefore he can make an 
appropriate location correction without lasting sight of the vehicle 
travel location. 
As described above, in the navigation system of the present invention, when 
the vehicle turns unexpectedly to the left or right at positions of a 
straight road, since a warning is produced; the travel angle is monitored; 
and the turning point is stored, the driver can easily initialize the 
restart point when the vehicle returns to the map route. 
Further, when vehicle turning angle data are not read within a route judge 
area due to a difference in distance between a route judge area on the map 
and an actual travel location, route judgement error is immediately warned 
to the driver, the driver can find the route judgement error in its early 
stages, and therefore take an appropriate action.