Map matching is a technique to improve and correct dead reckoning (see the earlier section "Augmenting GPS With Dead Reckoning"). It uses distinctive features of a mobile device's movement and a road network, for example, to find a corresponding point. This comparison takes place when conspicuous movements such as turning take place. A second method of map matching calculates the distances between the estimated position and the edges of the polygon the device is traveling on. This method is primarily used to relocate position before the estimated position moves too far from the road network.
In mobile location services, estimated position might be generated by GPS or A-GPS, and then sent from the mobile device to the processing center via SMS. This generates what is sometimes called a string of pearls, used for map matching (see Figure 5.10). Recent developments include algorithms to allow map matching for personal navigation by extending the techniques used for map matching in vehicle navigation.
Figure 5.10. Map Matching.