Implementing the automatic extraction of ridge and valley axes using the PPA algorithm in Grass GIS

The profile recognition and polygon breaking (PPA) algorithm allows to extract the ridge and valley axes automatically over wide areas maps. The algorithm simulates the human thinking in the line drawing process. First a process of profile recognition takes all the points close to the possible axes and connects them as a belt of closed polygons. The polygons are then broken into a continuous line and a smoothing algorithm adjusts the line. The algorithm, originally coded in Fortran, has been adapted to be used for raster analysis in Grass. The module takes a raster map, typically a DEM, and outputs a vector file representing the ridge or the valley axes. Various testings show the features of the algorithm and the implementation into Grass GIS.