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 warps between the true vehicle point and the displayed point on the map. It is badly necessary to correct these warps. An algorithm named Linear Abstractin Algorithm ( LAA) was put forward. It was realized through Visual Basic and Maplnfo's OLE Automation technology. Experimental result shows that LAA has correspondingly perfect precision, and the shortcoming of the algorithm is also pointed out.