[Geotiff] Linear Units of Projections Parameters

[pw]

If the projection linear units are declared as feet
then the eastings and northings should be in feet.

The information declared in the file is all one
has to work from. As soon as programs begin to
make assumptions regarding things that are not
absolutely declared in the embedded information
you end up with a hole in the specification.

Peter


David Burken wrote:
> All:
> 
> Here is an example of the problem Frank is talking about:
> 
> The geotiff file is keyed as such:
> 
> ProjLinearUnitsGeoKey (Short,1): Linear_Foot
> ProjFalseEastingGeoKey (Double,1): 400000
> 
> So is the false easting 400,000 meters or 400,000 feet?
> 
> Snip from the spec:
> ProjLinearUnitsGeoKey = 3076 /* Section 6.3.1.3 
> <http://www.remotesensing.org/geotiff/spec/geotiff6.html#6.3.1.3> codes
> ProjFalseEastingGeoKey = 3082 /* ProjLinearUnits */
> 
> Frank Warmerdam wrote:
> 
>> Folks,
>>
>> What are peoples opinions on how the units of linear quantities
>> like false easting and northing should be interpreted in a GeoTIFF file?
>>
>> Reviewing the specification (ie. the definition of ProjFalseEastingGeoKey
>> in 2.7.2 of the specification) it seems clear that the false easting and
>> northing should be in the units defined by ProjLinearGeoKey.
>>
>> But listgeo's normalized parameter report, GDAL, and a variety of other
>> commercial software packages all seem to make the assumption that false
>> easting and northing are always in meters.
>>
>> I have created a bug report against the specification about this 
>> topic, though
>> it may just be that libgeotiff, GDAL and various other packages need 
>> to be
>> fixed.  I am worried if alot of packages get it wrong, and no one gets it
>> right (other than OSSIM so far) that we might cause more problems by 
>> changing
>> the common convention.
>>
>>   http://bugzilla.remotesensing.org/show_bug.cgi?id=1094
>>
>> Best regards,
> 
> 
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