[QGIS-Developer] [Qgis-user] (how to) register gsb transformation in proj.db?

[Even Rouault]

On vendredi 5 juin 2020 11:13:09 CEST Thomas Schüttenberg wrote:
> Hi all!
> 
> Because I better should not roll out QGIS with the afore mentioned premanent
> ballpark-warning in place, I investigatetd further and stumbled across an
> "allowFallback"-setting:
> 
> By placing a default datum transformation in the settings/options.../CRS you
> get new entries in your QGIS3.ini [find it on win at
> %APPDATA%\QGIS\QGIS3\], which are the operation parameters themselve and a
> setting to allow the approximate “ballpark” transformation [...] between a
> source and destination CRS pair, in the case that the preferred coordinate
> operation fails".
> 
> [Projections]
> EPSG%3A31467\EPSG%3A25832_allowFallback=true
> EPSG%3A31467\EPSG%3A25832_coordinateOp="+proj=pipeline +step +inv
> +proj=tmerc +lat_0=0 +lon_0=9 +k=1 +x_0=3500000 +y_0=0 +ellps=bessel +step
> +proj=hgridshift +grids=Minden.gsb +step +proj=utm +zone=32 +ellps=GRS80"
> 
> This is also writen to any project file containing layers with the crs',
> covered by the default datum transformation. It appears within in the tag
> <transformContext><srcDest [...] allowFallback="1"> and causes the warning
> to show up every time this project will be loaded, although the datum
> transformation is already working an no fallback seems necessary.
> 
> If the _allowFallback=-setting in QGIS3.ini is (manually) edited to 'false',
> newly saved project files also contain <srcDest [...] allowFallback="0">
> and the warning will not be displayed again on project load.
> 
> Reference to my quite hacky findings can be found in the QGIS Python API
> Doc, search for 'allowFallback':
> https://qgis.org/pyqgis/3.10/search.html?q=allowFallback Some of the
> allowFallback stuff is labeled "New in version 3.12", however I currently
> investigate on 3.10.5-LTR. Might this indicate a backporting issue?
> (Otherwise, the implementation of my datum transformation very well might
> still be incomplete, don't know.)
> 
> Just wanted to inform you about my (current) workaround, any further hints
> are certainly appreciated. Greetings!

I'm not super familiar with the QGIS details about the selection and preference settings of 
transformations. I now that Nyall made a lot of efforts to make this as smooth as possible 
(difficult challenge to have user friendliness and accurate behaviour in that field!).
Looking at your record for minden.gsb, it uses a very large area of use. Once integrated into 
EPSG, it will probably use a much smaller one fitted to the grid extent, and I'm not sure how 
QGIS will react when zooming out sufficiently so that the grid doesn't cover the displayed 
extent.
You could simulate that by creating a new record into the area table which uses the real 
extent of your grid

-- 
Spatialys - Geospatial professional services
http://www.spatialys.com
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