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