my 2cts from a user's perspective:
- annoying dialog:
How about an option "Store my choice and do not ask me again"
This choice will go in a user setting where all choices for all datum
shifts (encountered by this user) are user-accessible and can be deleted
or changed.
This probably extends the whole thing a lot but as a user I might always
want to use the same paramter set and therefore need an option to save
my choice.
the dialog should be enabled by default to make users aware of what they
are doing
Bernhard
Am 07.11.2013 16:39, schrieb Marco Hugentobler:
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
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