Thanks Frank for the clear explanation. I've missed your blog post. I think that the rational behind the sorting logic is the best that can be done if a choice must be made. The point is IF that choice should be made, and you give lot of reasons for it, and I suppose lot of users agree with having a default transformation. In general I think that transformation parameters shouldn't be provided as part of a CRS definition, but I suppose I'm not part of the majority of the users :)
giovanni 2012/5/22 Frank Warmerdam <[email protected]>: > On Tue, May 22, 2012 at 1:47 PM, G. Allegri <[email protected]> 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, > -- > ---------------------------------------+-------------------------------------- > I set the clouds in motion - turn up | Frank Warmerdam, [email protected] > light and sound - activate the windows | http://pobox.com/~warmerdam > and watch the world go round - Rush | Geospatial Software Developer _______________________________________________ gdal-dev mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/gdal-dev
