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

[Bryan Keith]

Frans,

Frank knows about a gdal bug working with floating point no data values. 
  He asked me to write up a bug in bugzilla, but I haven't got around to 
it yet (poor excuse).  It has to do with comparing (and casting?) 
floating point no data values with the raster values.  You can read more 
here:

http://lists.maptools.org/pipermail/gdal-dev/2006-July/009573.html

Bryan

Frans Knibbe wrote:
> 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.
> 
> Greetings,
> 
> Frans
> 
> 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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