[gdal-dev] gdal_merge: stacking and band statistics

[Peter J Halls]

Jason,

    as a heavy ArcGIS user and teacher, as well as GDAL, I think you may have 
missed something about ArcGIS.  I have not tested the *outputs* of statistics 
calculations between GDAL and ArcGIS, however what you describe seems to be a 
display default.

    When any image is opened from disk in ArcGIS the default display is a 
black-to-white stretch: it is necessary to go to the Symbology menu and select a 
more appropriate scheme.  However, the internal Statistics tool in ArcMap sets a 
categorical contrast colour scheme by default.  I think this is the difference 
you are observing.

    Whether the two results are equivalent or different cannot really be 
determined by display in this way, rather use the ArcGIS Raster Calculator to 
subtract one image from the other and then examine the residual results: if the 
images are equivalent the output cells should have the same value - although 
this value may not be zero.

Best wishes,

Peter

Jason Roberts wrote:
> Matt,
> 
>  
> 
> Although I have not worked with multi-band HFA format, I have experimented
> fairly extensively with single-band, GDAL-produced, HFA-format files in
> ArcGIS 9.3.1 SP1. Maybe my experience will be interesting to you anyway.
> 
>  
> 
> My experience was that calculating the statistics with GDAL and with ArcGIS
> was not equivalent. For example, when I calculated the statistics with GDAL
> for integer rasters, they did not show up with a random color classification
> in ArcGIS by default. They appeared to use a black-to-white stretch instead.
> This was not desirable for categorical data (e.g. landcover values). But if
> I calculated the statistics using ArcGIS, they did show up with random
> colors.
> 
>  
> 
> Unfortunately I did not take extensive notes on this, summarizing my
> experience in my own code with this comment:
> 
>  
> 
>         # Do not use GDAL to calculate statistics.
> 
>         # Even though GDAL can do it faster than the geoprocessor, we
> 
>         # prefer to use the geoprocessor to maximize the compatibility
> 
>         # of the output raster with ArcGIS. For example, I do not know
> 
>         # how to get GDAL to build an ArcGIS-compatible histogram.
> 
>         # Without the histogram ArcGIS defaults to displaying integer
> 
>         # rasters using a black-to-white stretch rather than a unique
> 
>         # value classifier with random colors. ArcGIS users will
> 
>         # expect colors; to get the necessary histogram, we have to
> 
>         # calculate statistics with the geoprocessor.
> 
>  
> 
> I also recall there being some other differences, possibly with floating
> point files, too. I'm sorry I did not take better notes.
> 
>  
> 
> I found the whole thing a bit bizarre because I thought that ArcGIS 9.3.1
> used GDAL internally to manipulate HFA files. But maybe not in all
> circumstances. And 9.3.1 has an ESRI-customized version of GDAL 1.4, so it
> is not like they are running the latest GDAL code.
> 
>  
> 
> Anyway, hope this helps,
> 
>  
> 
> Jason
> 
>  
> 
> From: gdal-dev-bounces at lists.osgeo.org
> [mailto:gdal-dev-bounces at lists.osgeo.org] On Behalf Of Matt Hanneman
> Sent: Wednesday, October 20, 2010 4:51 PM
> To: gdal-dev at lists.osgeo.org
> Subject: [gdal-dev] gdal_merge: stacking and band statistics
> 
>  
> 
> Hi,
> 
> Can anyone help a newbie? I have been trying to calculate statistics with
> gdal_merge when merging/stacking several tiffs together into a HFA format
> (see below for syntax). However, when loaded into ArcGIS it reports that
> there are no statistics calculated for any of the bands. Will the
> 'STATISTICS' option not work when stacking multiple bands? As you can
> probably tell I am fairly new to this, so forgive my ignorance.
> 
> gdal_merge -co STATISTICS=YES -v -o "E:/Temp/A2E.img" -of HFA -separate
> "A.TIF" "B.TIF" "C.TIF" "D.TIF" "E.TIF" "F.TIF"
> 
> Thanks,
> 
> Matt 
> 
>  
> 
> 
> 
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-- 
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Peter J Halls, GIS Advisor, University of York
Telephone: 01904 323806     Fax: 01904 323740
Snail mail: IT Services, University of York, Heslington, York YO10 5DD
This message has the status of a private and personal communication
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