Calculating River Length Based on Topographic Data

River length has been the particular interest of people. Topographic data, which are obtained from field survey or digitizing from topographies, can serve as ideal data source for river length calculating. The topographic data are vector-based data, which represent river entities as polygonal line. Depending on data scale, rivers are represented in topographic data as single-line (narrow) or double-line (wide). Unlike single-line rivers, double-line rivers can’t be used to computing length directly. So we constructed a tree-like single-line river network by replacing double-line rivers and lakes with their centerlines. The exact river length is determined by the position of the headwater and the position of the embouchure. The headwater is difficult to be located in the topographic data, for relatively large rivers usually have numerous tributaries. There are presently three principles to accord when determining the mainstream and the main headwater: a) Length; b) Orientation; c) Amount of water. We use the multiple criteria decision approach, which is a well-known general approach, to determine the best one from many choices based on the three factors and their weights.