[Gdal-dev] Scaling of heights in gdaladdo

[Robert Osfield]

Hi All,

I have been testing out my 3D database generation codes on some sample 
datasets and have found a few issues with generation of/and using overviews.  
The problem can be seen when looking at the min max values before and after 
running gdaladdo.  For the test below the min/max values start of as 0 and 
43929.00 and end up being 0 and 18039 respectively.  I would expect a certain 
amount rounding, but not reduction by 2.4 times.

The test dataset ps_height_4k.png from the Puget Sound dataset  

	http://www.cc.gatech.edu/projects/large_models/ps.html

The tests were done with CVS version of gdal, just ater the 1.2 release.

The steps I took are first to convert ps_height_4k.png to a tif to allow 
overviews to be generated:

	> gdal_translate ps_height_4k.png ps_height_4k.tif

Then check the min max:

	> gdalinfo ps_height_4k.tif -mm
	Driver: GTiff/GeoTIFF
	Size is 4097, 4097
	Coordinate System is `'
	Corner Coordinates:
	Upper Left  (    0.0,    0.0)
	Lower Left  (    0.0, 4097.0)
	Upper Right ( 4097.0,    0.0)
	Lower Right ( 4097.0, 4097.0)
	Center      ( 2048.5, 2048.5)
	Band 1 Block=4097x1 Type=UInt16, ColorInterp=Gray
	    Computed Min/Max=0.000,43929.000

Then generate the a single overview

	> gdaladdo -r average ps_height_4k.tif 2

Then check the min max once more:

	> gdalinfo ps_height_4k.tif -mm

	Driver: GTiff/GeoTIFF
	Size is 4097, 4097
	Coordinate System is `'
	Corner Coordinates:
	Upper Left  (    0.0,    0.0)
	Lower Left  (    0.0, 4097.0)
	Upper Right ( 4097.0,    0.0)
	Lower Right ( 4097.0, 4097.0)
	Center      ( 2048.5, 2048.5)
	Band 1 Block=4097x1 Type=UInt16, ColorInterp=Gray
	    Computed Min/Max=0.000,18039.000
	  Overviews: 2049x2049

Is this a bug, feature, something I've done wrong at my end?

Thanks for any help you can provide,
Robert.