Get negative DEM data.

def get_negative_dem(raw_dem, neg_dem):
        """Get negative DEM data."""
        origin = RasterUtilClass.read_raster(raw_dem)
        max_v = numpy.max(origin.data)
        temp = origin.data < 0
        neg = numpy.where(temp, origin.noDataValue, max_v - origin.data)
        RasterUtilClass.write_gtiff_file(neg_dem, origin.nRows, origin.nCols, neg, origin.geotrans,
                                         origin.srs, origin.noDataValue, origin.dataType)