ISPRS, Vol.34, Part 2W2, "Dynamic and Multi-Dimensional GIS”, Bangkok, May 23-25, 2001 
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2.3 THE LAA ALGORITHM 
After these preparing work, we come to the algorithm. The basic 
of LAA is to find a road nearest to the vehicle point and put the 
corrected point on the line segment that represents the road. We 
can get the position of the corrected point by calculating the 
vertical point of a point to a line. The details of the algorithm is as 
following. 
Step 1 
Draw a circle, taking the vehicle point as the central point and 
selecting a suitable value (According to precision of GPS 
receiver GN-77, 100m is OK.) as radius. The circle should not be 
displayed on the interface. Judge whether the circle intersects 
with objects in the crossroad layer, if there are more than one 
object, then reduce the radius until there is only one crossroad in 
the circle. Put the corrected point on the position of the 
crossroad and get a next point and return to step 1. If there isn’t 
any object in the circle, go to step 2. 
Step 2 
Judge the relationship between the circle (Whose radius may 
change) and the objects in the road layer. If there are more than 
one object in the road layer that intersect with the circle, then 
reduce the radius, else increase the radius, until there is only 
one object that satisfies the condition. Record the name of the 
road. 
Step 3 
If the name recorded is the same as the one of last time, then go 
to step 4; otherwise we know that the vehicle is nearer to another 
road than the former road. But we don’t know whether the 
vehicle turns (Look at Fig. 3 and Fig. 4), so we just ignore this 
point, get next point and return to step 1. 
Step 4 
Use the method in step 2, get an object in the linear layer which 
is nearest to the vehicle point. 
Step 5 
In a Maplnfo map, we are able to get the end points of a line 
segment object. Look at Fig. 5, if we know the coordinates of 
point A, B and C, it is easy to know the coordinates of point D. 
We can calculate the vertical point of the vehicle point to the line 
segment in the linear layer which is nearest to the vehicle point. 
It is the corrected point we want. 
Step 6 
Put a point on the corrected vertical point, get a next point, return 
to step 1. 
2.4 THE BLAA ALGORITHM 
In LAA Algorithm, when we determine the relationship between 
the road layer, the linear road layer, the crossroad layer and the