gdalinfo -stats and numeric imprecision on dataset with nodata

[maphew]

When I run gdalinfo -stats on a 32bit 1-band tiff (elevation model) it renders the image unviewable in Arcmap 9.3.1sp1. Arccatalog & Arcmap report all the usual info as for as datatype presence of pyramids and so on, it just draws as white. There is no error message.

Before calculating statistics Arcmap reports high/low values in the form of:

    High : 3.40282e+038
    Low : 1.17549e-038

and after gdalinfo -stats it becomes:

    High : 1952.5
    Low : -3.40282e+038

After playing with Arcmap symbology and using contrast stretch, std devations, etc. The Low changes to:

    Low : -340282346638529990000000000000000000000.000000

Arccatalog reports NoData: -3.4028235e+038

The raster can be made to display by instructing Arcmap to use "Stretched>Min/Max>Edit-High-Low" and change the bottom end to the real minimum value, or at least 0. This is not a real solution as it's only good for that particular Arcmap session. Deleting the "foobar.tif.aux.xml"works better.

Using ArcGIS to calulate statistics on the same file after deleting .aux.xml reveals the true min/max statistics:

    High : 580.340637
    Low : 213.000000

I'm using GDAL 1.7.0dev, released 2008/11/26 on Windows7 x64 via OSGeo4w.

[maphew]

data set with original elevation model, after gdal stats and after arcgis stats

[Even Rouault]

The reason is that the nodata value wasn't ignored when computing the stats. Nodata value is stored in GeoTIFF files as a text value, but the pixel values are stored as IEEE floats. So strict comparison can fail. The fix is to test that the relative difference between nodata and pixel values is small (1e-10), but not exactly 0.

Fixed in trunk (r19669) and branches/1.7 (r19671). Test added in r19670

After fix, gdalinfo -stats small/orig/dem.tif returns :

Band 1 Block=493x4 Type=Float32, ColorInterp=Gray
  Minimum=213.000, Maximum=578.979, Mean=355.119, StdDev=84.672
  NoData Value=-3.40282346638528898e+38
  Metadata:
    LAYER_TYPE=athematic
    STATISTICS_MINIMUM=213
    STATISTICS_MAXIMUM=578.97894287109
    STATISTICS_MEAN=355.11909149198
    STATISTICS_STDDEV=84.671745438196

[maphew]

wow, that was fast. Thanks Evan :)