[gdal-dev] gdalinfo and scale/offset

[Joaquim Luis]

Even

Thanks for pointing into this that I was not aware of as it would be the 
main point of my answer to Kyle's question.
But still, as it is referred in the ticket (sorry for lousy formatting 
but I never learn how to do it better) the main issue occurred in the 
conversion to another format (geotiff for that matter). So though an 
option exists ('unscale') to account for offset/scale the natural 
expectancy is that the conversion does not change the data values.

Redoing the tickets example we can see (not shown than because I 
thought  it of lesser interest)

C:\SVN\mironeWC\src_C\t>gdalinfo lixo.tiff -mm
Driver: GTiff/GeoTIFF
Files: lixo.tiff
Size is 21, 21
Coordinate System is `'
Origin = (-10.500000000000000,10.500000000000000)
Pixel Size = (1.000000000000000,-1.000000000000000)
Metadata:
   NC_GLOBAL#Conventions=COARDS/CF-1.0
   NC_GLOBAL#title=lixo.grd
   NC_GLOBAL#history=grdmath -R-10/10/-10/10 -I1 X Y MUL = lixo.grd
   NC_GLOBAL#GMT_version=4.5.4
   z#long_name=z
   z#_FillValue=1.#QNAN0e+000
   z#actual_range=-1, 1
   z#scale_factor=0.01
   x#long_name=x
   x#actual_range=-10, 10
   y#long_name=y
   y#actual_range=-10, 10
Image Structure Metadata:
   INTERLEAVE=BAND
Corner Coordinates:
Upper Left  ( -10.5000000,  10.5000000)
Lower Left  ( -10.5000000, -10.5000000)
Upper Right (  10.5000000,  10.5000000)
Lower Right (  10.5000000, -10.5000000)
Center      (   0.0000000,   0.0000000)
Band 1 Block=21x21 Type=Float32, ColorInterp=Gray
     Computed Min/Max=-100.000,100.000


It's true that we can still see a trace of the previous info about the scale

   z#actual_range=-1, 1
   z#scale_factor=0.01

but a user would need to be very very attentive to realize that and to 
know that the data values were off by an 0.01 factor.

I think that in these situations the scale factor would better be 
applied by default (like with the gdal_info example)

Joaquim

> Kyle,
>
> Frank added in GDAL 1.7 a '-unscale' option to gdal_translate, so people can
> always use gdal_translate -unscale to apply the offset and scale (they can even
> do that do a VRT file to save disk space), and then use gdalinfo on the result.
>
> Extract from the gdal_translate man page:
>
> "-unscale : Apply the scale/offset metadata for the bands to convert scaled
> values to unscaled values.  It is also often necessary to reset the output
> datatype with the -ot switch."
>
> Is there a need for such an option in gdalinfo ? I have no strong opinion
> about this. An issue I see is that usually -stats record the computed stats in
> a .aux.xml file for later retrieval. The interaction with a -unscale option
> would be tricky... What happens if the user computes with -unscale and then do
> gdalinfo without it... Or the other way round.
>
> Le jeudi 28 octobre 2010 19:58:55, Kyle Shannon a écrit :
>    
>> Hello,
>> I have been working  on ticket #3797.  In the example given, gdalinfo is
>> the calling the netcdf driver.  I agree with Frank's opinion on this in
>> ticket #1660:
>>
>> Note that GDALRasterBand has methods to get the offset and scale. The
>> normal
>>
>>      
>>> GDAL practice would be to return them via those methods - not to apply
>>> them on the fly. Then it is up to the caller to do so if they wish.
>>>        
>> gdal shouldn't be in charge of scaling the data to what may be a different
>> data type, or unpacking it.  But in this case, gdalinfo is the caller.
>> Should the functionality of the scaling be added to gdalinfo?  Maybe
>> optionally reporting the stats as scaled data?  Any thoughts?
>>
>> # ============================
>> Kyle Shannon
>> Physical Science Technician
>> RMRS Fire Sciences Lab
>> Fire, Fuels&  Smoke - RWU 4405
>> 5775 Highway 10 W.
>> Missoula, MT 59808
>> (406)829-6954
>> kshannon at fs.fed.us
>> # ============================
>>      
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