Abstract: At the background of digitation for map scanning,the elemental elements of map and the topologic relationship is discussed.And it also introduced the idea that the topologic relationship of subgraph is built before creating the topologic relationship of graph with the operation of union.In the mode of realization,the relationship of subgraph can be formed with seed filling,using raster data structure;in order to build graph relationship,adopting the idea of area object matching,we can sequently search and build it.At last,all have come into use in the form of automation.