[Gdal-dev] negative nodata value loaded as positive in gdal 1.4.0

[Maciej Sieczka]

Hernán De Angelis wrote:

> I am having a weird problem when importing SRTM data using gdal_translate or 
> r.in.srtm in GRASS (which also uses gdal_translate)

FWIW, this is not correct. r.in.srtm is a script which uses r.in.gdal,
not gdal_translate.

However, both r.in.gdal and gdal_translate have the problem Hernán
describes, so propapably this is an issue with GDAL itself; unless an
issue with some problematic SRTM tiles, ?. I'm not up to tracing it
down exactly, but anybody who can please read below.

> nodata values are loaded 
> as 32768 (positive) even when -32768 is explicitly set in the script. After 
> import gdalinfo reports nodata as -32768, but r.info (in GRASS), cmm (in 
> OSSIM) and OpeneEV all report these cells as 32768 (positive). I am using 
> gdal 1.4.0. Is anyone else having this problem as well?

I confirm. I can reproduce this with some 3" tiles, eg:
ftp://e0srp01u.ecs.nasa.gov/srtm/version2/SRTM3/Eurasia/N49E023.hgt.zip

I checked whether this is related to the SRTM version - it's not; same
problem with the version 1 at:
ftp://e0srp01u.ecs.nasa.gov/srtm/version1/Eurasia/N49E023.hgt.zip

After importing the tile into GRASS with r.in.srtm (ie. r.in.gdal), the
max value of the output raster is 32768 - wrong. Should be circa 1200.
Min is OK - 153.

I have also checked with gdal_translate, and the same issue remains.
The value range is reported as 153-32768, see:

$ gdalinfo N49E023.bil -mm
Driver: EHdr/ESRI .hdr Labelled
Size is 1201, 1201
Coordinate System is `'
Origin = (22.999583333333334,50.000416666666666)
Pixel Size = (0.000833333333333,-0.000833333333333)
Corner Coordinates:
Upper Left  (  22.9995833,  50.0004167)
Lower Left  (  22.9995833,  48.9995833)
Upper Right (  24.0004167,  50.0004167)
Lower Right (  24.0004167,  48.9995833)
Center      (  23.5000000,  49.5000000)
Band 1 Block=1201x1 Type=UInt16, ColorInterp=Undefined
    Computed Min/Max=153.000,32768.000
  NoData Value=-32768

I have put the problematic SRTM V2 tile N49E023 (converted into
bil+hdr) for download here:

http://kufaya.googlepages.com/srtm_v2_N49E023_bil_hdr.tar.bz2

Using GDAL 1.4 CVS 2007-01-14 (today).

Maciek