AVL system is a successful application of the integration of GIS and GPS. When GPS data is displayed on a GIS map, logically the vehicle point should be displayed on the roads. But because of the effect of errors, there are always warps between the true vehicle point and the displayed point on the map. It is necessary to correct these warps. An algorithm—Buffer Linear Abstraction Algorithm (BLAA) is put forward. This algorithm improves a former algorithm—Linear Abstraction Algorithm (LAA). Experimental result shows that BLAA has high precision, and at the same time it is more efficienty.