DESCRIPTION scans the named input raster map layer, extracts points, lines or area edge features from it, converts data to GRASS vector format.


The program extracts data from a GRASS raster map layer and stores output in a new GRASS vector file.

Lines assumes that the input map has been thinned using r.thin. extracts vectors (aka, "arcs") from a raster map. These arcs may represent linear features (like roads or streams), or may represent area edge features (like political boundaries, or soil mapping units).

r.thin and may create excessive nodes at every junction, and may create small spurs or "dangling lines" during the thinning and vectorization process. These excessive nodes and spurs may be removed using v.clean.

Areas first traces the perimeter of each unique area in the raster map layer and creates vector data to represent it. The cell category values for the raster map layer will be used to create attribute information for the resultant vector area edge data.

A true vector tracing of the area edges might appear blocky, since the vectors outline the edges of raster data that are stored in rectangular cells. To produce a better-looking vector map, smoothes the corners of the vector data as they are being extracted. At each change in direction (i.e., each corner), the two midpoints of the corner cell (half the cell's height and width) are taken, and the line segment connecting them is used to outline this corner in the resultant vector map. (The cell's cornermost node is ignored.) Because vectors are smoothed by this program, the resulting vector map will not be "true" to the raster map from which it was created. The user should check the resolution of the geographic region (and the original data) to estimate the possible error introduced by smoothing. extracts only area edges from the named raster input file. If the raster map contains other data (i.e., line edges, or point data) the output may be wrong.


For feature=line the input raster map MUST be thinned by r.thin; if not, may crash.


Bill Brown

Mike Baba
DBA Systems, Inc.
10560 Arrowhead Drive
Fairfax, Virginia 22030

Original version of r.poly:
Jean Ezell and Andrew Heekin,
U.S. Army Construction Engineering Research Laboratory

Modified program for smoothed lines:
David Satnik, Central Washington University
Updated 2001 by Andrea Aime, Modena, Italy

Original, r.line and r.poly merged and updated to 5.7 by Radim Blazek

Last changed: $Date$