## DESCRIPTION

r.surf.contour creates a raster elevation map from a rasterized contour map. Elevation values are determined using procedures similar to a manual methods. To determine the elevation of a point on a contour map, an individual might interpolate its value from those of the two nearest contour lines (uphill and downhill).

r.surf.contour works in a similar way. Initially, a vector map of the contour lines is made with the elevation of each line as an attribute. When the program v.to.rast is run on the vector map, continuous "lines" of rasters containing the contour line values will be the input for r.surf.contour. For each cell in the input map, either the cell is a contour line cell (which is given that value), or a flood fill is generated from that spot until the fill comes to two unique values. So the r.surf.contour algorithm linearly interpolates between contour lines. The flood fill is not allowed to cross over the rasterized contour lines, thus ensuring that an uphill and downhill contour value will be the two values chosen. r.surf.contour interpolates from the uphill and downhill values by the true distance.

### Parameters:

input=name
  Name of an existing raster map that contains a set of initial category values (i.e., some cells contain known elevation values (denoting contours) while the rest contain NULL values or zeros (0)).
output=name
  Name to be assigned to new output raster map that represents a smooth (e.g., elevation) surface generated from the known category values in the input raster map layer.

## NOTES

r.surf.contour works well under the following circumstances: 1) the contour lines extend to the the edge of the current region, 2) the program is run at the same resolution as that of the input map, 3) there are no disjointed contour lines, and 4) no spot elevation data BETWEEN contour lines exist. Spot elevations at the tops of hills and the bottoms of depressions, on the other hand, improve the output greatly. Violating these constraints will cause non-intuitive anomalies to appear in the output map. Run r.slope.aspect on r.surf.contour results to locate potential anomalies.

The running of r.surf.contour is very sensitive to the resolution of rasterized vector map. If multiple contour lines go through the same raster, slight anomalies may occur. The speed of r.surf.contour is dependent on how far "apart" the contour lines are from each other (as measured in raster cells). Since a flood fill algorithm is used, the program's running time will grow exponentially with the distance between contour lines.

## BUGS

r.surf.contour has not been fully updated for NULL support and still considers a value of "`0`" to be NULL. Thus any contour lines at 0 elevation (e.g. the coastline) will be ignored. In such cases converting any 0 values in the input map to -1 with r.mapcalc may be a suitable work-around.

Currently r.surf.contour will only produces CELL (integer) map output. If you would like a finer grade output map (i.e. floating point) it is recommended to multiply the input map by 100 (for example) using r.mapcalc, then divide the resultant r.surf.contour output map by 100.0, again with r.mapcalc.

Volunteers are sought to remedy both these issues.

## SEE ALSO

r.mapcalc, r.slope.aspect, r.surf.idw, r.surf.idw2, v.digit, v.surf.idw, v.surf.rst, v.to.rast

## AUTHOR

Chuck Ehlschlaeger, U.S. Army Construction Engineering Research Laboratory

Last changed: $Date: 2006/04/18 05:06:17 $