[Geotiff] User defined datum: Transformation parameters

noel khan noel.khan at rdmd.ocgov.com
Thu Feb 28 16:10:07 PST 2008


    DISCLAIMER: My perpective if the only the GeoTiff spec. I am not
familiar with the library you are using, which contains the call

    Please refer to the GeoTif specsection 2.4. 

    From my understanding, GeoTiff keys can either 

    [1] store a single SHORT value within the KeyEntry 


    [2] store values in a TIFF-tag, which means that both [a] the
TiffTagLocation must be set to a nonzero value and [b] the Value_Offset must
be set to the location of the TIFF-tag containing your value(s).

Therefore, it appears that your implementation should involve: 

[1] Storing your array of doubles within a TIFF tag that can accomodate
those values. NOTE: Upon storage, I presume you necessarily know the base-0
position of that array within the TIFF file (hereinafter

[2] Set your GeoTiff KeyEntry {KeyID, TiffTagLocation, Count,Value_Offset} =
{<CustomTransformationKeyID>, 34736, 7,

NOTE: The spec has that "all key-values that are not of type SHORT are to be
stored in one of the following two tags, based on their format: ..."
GeoDoubleParamsTag (34736) or GeoAsciiParamsTag (34737).

Finally, regarding "Is it correct that multiple doubles have to be passed as
an array" -- this is my understanding. 

Best Regards,

Christian Zietz wrote:
> Christian Zietz schrieb:
>> And that's exactly where I'm having problems right now. Whenever I try
>> to store multiple double values, an access violation occurs somewhere
>> inside GTIFKeySet.
> And a follow-up question/remark: Is it correct that multiple doubles
> have to be passed as an array? When I do something like
> double d[7];
> d[0] = 1; d[1] = 2; d[2] = 3; d[3] = 4;
> d[4] = 5; d[5] = 6; d[6] = 7.1;
> GTIFKeySet(gtif, MY_KEY, TYPE_DOUBLE, 7, d);
> everything works as expected. No access violation and listgeo confirms
> that the key is correctly set.
> Christian
> -- 
> Christian Zietz  -  CHZ-Soft  -  czietz at gmx.net
> WWW: http://www.chzsoft.com.ar/
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