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GeoTIFF GDAL_NO_DATA_VALUE anomaly

tjhb
tjhb Global Mapper UserTrusted User
edited February 2013 in Elevation Data
Hello Mike,

There's an anomaly with the interpretation and display of some source tiles of SRTM v4.1 (from CGIAR).

In general, sea is encoded as missing data. For most tiles, the GDAL_NO_DATA_VALUE value is -32768. These tiles show correctly.

But for 25 tiles, all of which show only a small, low island surrounded by sea, the GDAL_NO_DATA_VALUE is 255. (Perhaps someone was determined to use unsigned integers for these tiles somewhere in the workflow.)

Global Mapper interprets this value correctly when calculating metadata (since the maximum elevation calculated for these tiles is always less than 255).

But it seems to ignore it for the display of data (and for colouring and shading). The 255 values are all shown as valid elevations, rather than as sea.

Here is one of the 25 tiles.
srtm_20_18.zip

I had always assumed it was a data fault at the CGIAR end, but apparently it is just an unusual encoding.

Regards,
Tim Baigent

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