Usage: gdal_contour [-b <band>] [-a <attribute_name>] [-3d] [-inodata]
[-snodata n] [-f <formatname>] [-i <interval>]
[-off <offset>] [-fl <level> <level>...]
<src_filename> <dst_filename>
gdal_contour -a elev dem.tif contour.shp -i 10.0
1.5.1.