GeoTIFF GDAL_NO_DATA_VALUE anomaly

tjhb

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

global_mapper

Tim,

I took a look and found that the GDAL_NO_DATA_VALUE was only being obeyed when it was lower that the valid range and not above it. I've got it fixed now. I have placed a new build at http://www.bluemarblegeo.com/downloads/global-mapper/global_mapper14.zip with the latest changes for you to try. Simply download that file and extract the contents into your existing v14.xx installation folder to give it a try. If you are using the 64-bit v14 version there is a new build at http://www.bluemarblegeo.com/downloads/global-mapper/global_mapper14_64bit.zip .

Thanks,

Mike
Global Mapper Guru
gmsupport@bluemarblegeo.com
http://www.bluemarblegeo.com/

tjhb

Thanks very much Mike. Perfect now.