In general , Global Positioning System (GPS) has been widely used in Vehicle Navigation Systems. It helps the users to determine the vehicle position or provides users with proper maneuver instruction .Because of some biases, such as SA effect, buildings canyon around cities , thus those affect and block GPS signals .In addition , the road network of the digital Map is not accurate enough. These cause vehicle position not to locate precisely on the road network of the digital map. Therefore a map-matching algorithm is necessary. An optimal algorithm is developed through the analysis of some driving conditions (such as turning. U-turn or drive to parking lots).The algorithm is presented.
Assuming that GPS point is G1 with coordinate (XG1, YG1), from-node of
road is S1 with coordinate (XS1, YS1), to-node of road is E1 with coordinate (EE1, YE1).
The value of K can be determined.
where θ is the angle between vector S1G1 and S1E1
1) if 0<= K <=1 then the prependicular distance between G1 and the road is:
( (XG1-XN)^2+(YG1-YN)^2)^1/2
The (Xn, Yn) are the coordinates of the projected point of the road:
XN=K*(XE1-XS1)+ X S1
YN=K*(YE1-YS1)+ YS1
2) If K <>
3) If K > 1, then the vehicle location is not on the candidate road, and the location is away from the to-note E1
in case like the following:
the candidate road of G1 is either line1 or line2. The possibility of making wrong decision is very high. To solve this problem we consider the vehicle traveling direction. In digital road map database the true azimuth (which is a mathematical concept defined as the angle)of road is recorded, and NMEA data also includes the vehicle direction. Therefore, we can compare these two any try to select the candidate road.
After selection of the candidate road, We assume current vehicle location is in G1, next vehicle location is G2, if the distance between G1, and E1, (end-node of road segment) are close to a pre-set value, the next turning candidate road will be Line1, Line2 and Line3. G1E, is the azimuth of traveling direction from G1, to E1, point, G2E, is the azimuth from E1, to G2, point. These values of azimuth will be used for judging which road is the candidate. If driver has a U-turn, the from-node and to-node of road segment must be switched. Assuming current vehicle location is at G1, point, the location of former epoch is G0, endnode of road segment is E1,
ΔHEADING = G1a - G0a
G1a: azimuth of G1E1,G0a : azimuth of G0E1
If ΔDISTANCE is negative and larger then a tolerant value and
ΔHEADING > Tolerance, we presume, we presume vehicle has a U-Turn.
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