## 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).
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.

An existing MASK raster map is respected for both reading input and writing output.

## 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.

## EXAMPLE

Example to create contour lines from elevation model, then recreating DEM from these contour lines along with differences analysis (North Carolina sample data set):

    g.region raster=elevation -p

    # get minimum elevation value
    r.univar elevation

    # generate vector contour lines
    r.contour input=elevation output=contours_5m step=5 minlevel=50

    # rasterize contour lines
    v.info -c contours_5m
    v.to.rast input=contours_5m output=contours_5m use=attr attribute_column=level

    # generate DEM from rasterized contour lines
    r.surf.contour input=contours_5m output=elevation_from_cont5m

    # calculate difference map
    r.mapcalc "diff = elevation - elevation_from_cont5m"
    r.colors diff color=differences

    # analyze differences statistically
    r.univar diff

## SEE ALSO

r.mapcalc, r.slope.aspect, r.surf.idw, wxGUI vector digitizer, v.surf.idw, v.surf.rst, v.to.rast

Overview: Interpolation and Resampling in GRASS GIS

## AUTHOR

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