[Gdal-dev] GTOPO30 and max value

[Frank Warmerdam]

Markus Neteler wrote:
> Hi,
> 
> should GDAL read the GTOPO30 data properly?
> 
> gdalinfo -mm W100N40.DEM
> Driver: EHdr/ESRI .hdr Labelled
> Size is 4800, 6000
> Coordinate System is:
> GEOGCS["WGS 84",
>     DATUM["WGS_1984",
>         SPHEROID["WGS 84",6378137,298.257223563,
>             AUTHORITY["EPSG","7030"]],
>         TOWGS84[0,0,0,0,0,0,0],
>         AUTHORITY["EPSG","6326"]],
>     PRIMEM["Greenwich",0,
>         AUTHORITY["EPSG","8901"]],
>     UNIT["degree",0.0174532925199433,
>         AUTHORITY["EPSG","9108"]],
>     AXIS["Lat",NORTH],
>     AXIS["Long",EAST],
>     AUTHORITY["EPSG","4326"]]
> Origin = (-100.000000000000000,39.999999999999993)
> Pixel Size = (0.008333333333330,-0.008333333333330)
> Corner Coordinates:
> Upper Left  (-100.0000000,  40.0000000) (100d 0'0.00"W, 40d 0'0.00"N)
> Lower Left  (-100.0000000, -10.0000000) (100d 0'0.00"W, 10d 0'0.00"S)
> Upper Right ( -60.0000000,  40.0000000) ( 60d 0'0.00"W, 40d 0'0.00"N)
> Lower Right ( -60.0000000, -10.0000000) ( 60d 0'0.00"W, 10d 0'0.00"S)
> Center      ( -80.0000000,  15.0000000) ( 80d 0'0.00"W, 15d 0'0.00"N)
> Band 1 Block=4800x1 Type=UInt16, ColorInterp=Undefined
>   Min=-9999.000 Max=6710.000   Computed Min/Max=1.000,55537.000
>   Minimum=-9999.000, Maximum=6710.000, Mean=-6078.800, StdDev=5044.200
>   NoData Value=-9999
> 
> -> Computed Min/Max=1.000,55537.000
> is confusing me. Maybe a bug in the data set?

Markus,

It would be helpful to see the .HDR file.  It seems likely that the
data is actually signed 16bit integer, but being interpreted as
unsigned 16bit integer.  EHDR .HDR files are notoriously underspecified.
I think when used with Arc/Info there was some information (like pixel
type) that was passed on the commandline or something.

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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and watch the world go round - Rush    | President OSGeo, http://osgeo.org