gdal_contour dtm input signed / unsigned flag

[moellney]

Sometimes we have files that produce negative dtm values at sea-level but put unsigned integer data in png / tiff file format.

When e.g. producing a contour at 100m we get a log of small circle at the sea, because gdal_contour has no chance to see, that the extremle high numbers are negative values in effect (due to the coding of negative values in binary notation).

So how to about a flag that tells gdal_contour to reinterpret the binary data singed instead of unsigned?

[moellney]

DTM Data received from SRTM NASA WMS server saved as tiff.