Replying to rouault:
The TIFF file isn't valid if it is meant at having signed values. Namely the value of the SAMPLEFORMAT tag is 1 (SAMPLEFORMAT_UINT) in your example, whereas it should be 2 (SAMPLEFORMAT_INT).
This can be corrected by example with the tiffset utility of libtiff
tiffset -s 339 1 2 Hongkong_test1.tif
Thanks for the hint.
In tiff is seems to be possible to signal signed or unsigned. Your example works and gives nice results with gdal_contour.
But do you know if png supports signed integer?
Let me cite from the capabilities file of the nasa wms:
<Layer queryable="0">
<Name>worldwind_dem</Name>
<Title>SRTM derived global elevation, 3 arc-second</Title>
<Abstract>
A global elevation model, prepared from the 3 arc-second SRTM dataset by filling some
of the problem areas. Prepared by the NASA Learning Technologies.
The default style is scaled to 8 bit, non-linear.
It is possible to request the elevation data in meters by the short_int tyle and
requesting PNG format. The resulting PNG file will be a unsigned 16 bit per pixel
image. The values are then the elevation in meters.
Values are signed 16 bit integers, but PNG will present them as unsigned, leading to a
few areas with very large values (65000+)
For elevation values in feet, request PNG format with the style feet_short_int.
</Abstract>
