[FWTools] Gdaltranslate ESRI grid to GeoTIFF: pixel values are blown up.

Frans Knibbe frans at geodan.nl
Wed Oct 4 08:53:38 EDT 2006

Hello list,

Frank has had a look at the sample data now. The conclusions are that 
gdaltranslate did not actually blow up the pixel values. It produces a 
TIFF file that is almost the same as a TIFF file produced by ArcMap, 
only it does not produce an associated *.aux file. ArcMap uses 
information from this *.aux file to get the minimum and maximum pixel 
values. If I delete the aux file, ArcMap reports the same minimum and 
maximum values for both TIFFs (ArcMap-produced and 
gdal_translate-produced) . Both those values are wrong. The nodata value 
is -3,4E38. The information that -3,4E38 is the nodata value gets lost 
somehow when translation to TIFF is done. So the TIFF has pixel values 
between the real minimum and maximum ( -6.96 and 30.5 for example), plus 
some pixels with value -3,4E38.

I was misled by ArcMap not showing the right information for TIFF files 
that doe not have an *.aux file. I will now further investigate the 
problem of drawing the TIFFs in mapserver.



Frank Warmerdam wrote:
> Frans Knibbe wrote:
>> Hello,
>> I am trying to prepare elevation grids for display and query with the 
>> UMN MapServer. The source data are ESRI floating point grids. As I 
>> understand geoTIFF to be a good format to use with MapServer I use 
>> gdaltranslate (from FWTools 1.0.5) to convert the grids to geoTIFFs. 
>> The problem is that in the resulting geoTIFF images the pixel values 
>> are somehow blown up. Gdalinfo does not report the minimum and 
>> maximum values of the GeoTIFFs, but opening a file in ArcMAP reveals 
>> the minimum value to be 1.17E-38 and the maximum value to be 3.4E38, 
>> when the original values were -6.96 and 30.5.
>> When I use ArcMap to convert the grids to TIFF, the pixel values do 
>> seem correct, but somehow those TIFFs are not drawn by MapServer. 
>> MapServer does draw the files made by gldaltranslate. Can anyone help 
>> me with getting the pixel values right?
> Frans,
> If you could create a small arcinfo grid, and convert it to geotiff both
> ways, and then send me the two outputs and the input all zipped up I can
> try to analyse the difference.
> I vaguely suspect the problem is that ArcMap isn't getting min/max 
> metadata
> value from the GDAL produced GeoTIFF and that is resulting in default
> scaling.  But it is hard to be sure.
> It would be best to submit this as a bug report against GDAL in the GDAL
> bugzilla.
> Best regards,

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