[gdal-dev] troubles with OVERRIDE_PROJ_DATUM_WITH_TOWGS84

[Frank Warmerdam]

On Tue, May 22, 2012 at 1:47 PM, G. Allegri <giohappy at gmail.com> wrote:
>> Your issue with EPSG:3003 is of a different kind, and related to another change where the new logic now peaks up the EPSG preferred transformation whereas it didn't before
>
> Hi Even,
> I have some difficults to follow the overall flow controlling the
> peaking of the preferred transformation. What is the general rational
> that define a transformation considered "preferred"?
> In the case reported by Maddalena, the chosen transformation
> parameters are (approximately) good for central Italy, while they are
> not well suited for other regions of our country. In our case they're
> "preferred" only by some italians :)

Giovanni,

The logic for deciding which available datum shift to use as
preferred is embedded in:

  http://svn.osgeo.org/metacrs/geotiff/trunk/libgeotiff/csv/build_pcs.py

The logic around setting pref_rec and preferred_op is the key.

Skimming the logic it:
 * Prefers the transformation with the largest area of use.
 * avoids deprecated transformations.
 * avoids records that have been superceeded.

I believe I discuss the topic in:

http://fwarmerdam.blogspot.com/2010/03/in-last-few-weeks-i-believe-i-have-made.html

There is the possibility of pre-setting a preferred datum transformation in:

    http://svn.osgeo.org/metacrs/geotiff/trunk/libgeotiff/csv/datum_shift_pref.csv

If you feel there is a more appropriate set of shift parameters to use
for the datum in question, please file a ticket (ideally against libgeotiff)
with a justification I can reference when I add it in the file.  Hopefully
a reasonably convincing explanation.

Also, please understand that no set of shift values is ideal and I
prefer something broadly reasonable to something that is super
in one local region and very poor in another where the datum is
used.   The "largest area of use" rule of thumb is intended to
select on this basis.

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    | Geospatial Software Developer