Dear qgis devs
In QGIS, there is currently no possibility to choose the type of datum
transform used in otf-reprojecting. Additionally, some national mapping
agencies provide datum shift grids and they cannot be used conveniently
in QGIS ( a user needs to make user defined crs and store the +nadgrids=
parameter there).
Some proprietary systems pop up a dialog box in case there are several
datum transformations available and let the user choose an appropriate
one. Therefore, I've been in charge of implementing a similar solution
for QGIS, and the code is ready for first testing / feedback now:
https://github.com/mhugent/Quantum-GIS/commits/datum_transform
Here a short summary of the changes:
- There is a new table in srs.db created from the gdal file
'datum_shift.csv'. It contains 3/7 parameter towgs84 parameters.
Additionally, there is also the possibility to extend srs.db with ntv2
(grid shift) transformations (with the .gbs filename as first parameter).
- Once otf-reprojection is enabled and QGIS detects more than one
possible datum transformation between the two CRS, the user is asked in
a dialog which one to use. This is stored into the project file, so only
needs to be entered if adding the layer or changing the layer / map CRS.
The user is also asked if saving a layer in a different CRS and the
datum transform is of course also picked up by QGIS server.
possible problems from user perspective:
- The dialogs asking for datum transform could be annoying. Does it need
an option to suppress it (and should it be enabled / disabled by default)?
- The synchronisation of srs.db with datum_shift.csv makes the install
time longer (maybe it can be solved more efficiently?)
technical details for devs:
- QgsCoordinateTransform currently contains srcCRS, destCRS. It is
extended to hold optionally source datum transform / dest datum
transform (int is coord_op_code in srs.db). In
QgsCoordinateTransform::initialise, the datum transform strings are
modified to match the choosen datum transforms.
- QgsCRSCache is extended to search by datum transform too (to avoid
expensive lookups in the sqlite db every time)
- QgsMapRenderer stores the information about layer / datum
transformation and reads / writes to project filename
- To receive the current QgsCoordinateTransformation for a layer, tools
may query QgsMapRenderer::tr (or QgsMapRenderer::mapToLayerCoordinates
etc.). Creating a new QgsCoordinateTransform from layerCRS and mapCRS is
not correct in all cases any more.
What are your opinions / suggestions?
Regards,
Marco
--
Dr. Marco Hugentobler
Sourcepole - Linux & Open Source Solutions
Weberstrasse 5, CH-8004 Zürich, Switzerland
[email protected] http://www.sourcepole.ch
Technical Advisor QGIS Project Steering Committee
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