Hi all Recently I needed transformation between points from high-accuracy GPS to British National Grid (EPSG:27700) which involves datum shift (WGS 84 to OSGB 1936) using OSTN15 grid.
The process to do that with QGIS is currently... well... really clumsy. One needs to: 1. manually download grids from the web and extract .gsb file to Proj data directory 2. manually add an entry to tbl_datum_transform table in srs.db in QGIS installation There are two main problems with that: - both steps need adjustment of QGIS installation - not everyone has admin privileges - doing manual changes to srs.db is just too much to ask from ordinary user I understand that it is hard to ship datum shift grids with QGIS due to various issues (their size and amount, sometimes licensing) and that's fine. But we should think about how to make it easier to use them. There is a QGIS plugin [1] that provides a set of algs for Processing that call ogr2ogr with the right grid files and download them if they are not yet available - however there is no integration with QGIS CRS database, so any other transforms inside QGIS can't make use of that. Note for those not familiar with srs.db: it is a sqlite3 db living in QGIS source code which has been generated at some point from GDAL data files. Over time it has gathered some manual ad-hoc updates of parameters. Then upon QGIS build/installation we run "crssync" utility to parse current GDAL data files again and update/insert CRS and datum shift parameters (see QgsCoordinateReferenceSystem::syncDatabase()). My suggestions for QGIS: 1. any manual additions to srs.db should be listed as SQL commands because it is difficult to track changes to srs.db which is a binary blob (sqlite db). During the build of QGIS we would create final srs.db 2. rather than manually adding entries about datum shift grids to tbl_datum_transform table (which is generated from GDAL's datum_shift.csv [2]) and contains parameters of 3/7 parameter transform between datums, there should be a new table just for grid files, ideally with links where they can be downloaded. (fun fact: names of grid files in tbl_datum_transform table are stored in a numeric column but because this is sqlite it just works) 3. it should be possible to add custom entries about datum shift grids by users (just like it is possible to add custom CRS) - that is, such table should be also in user's profile (in qgis.db) My suggestions for Proj: 1. it should be possible to specify multiple data directories with PROJ_LIB env variable, so that it is possible to use grids also from user's home directory (not just the read-only system directory) 2. there could be a database similar to datum_shift.csv [2] that would list well known datum shift grid files, so that QGIS does not need to maintain its database and just pull that information out of Proj - other projects could benefit from that as well. I am aware of the fact that there is a big refactoring in Proj going on [3] but I am not sure if any of the above suggestions are also in the plans... I would be happy to help implementing some of the above, but I would like to first hear opinion of others - maybe I got something wrong and maybe there are better ways to move forward. I hope gurus like Kristian or Even read this and can offer some of their wisdom too :-) Cheers Martin [1] https://github.com/NaturalGIS/naturalgis_ntv2_transformations [2] https://github.com/OSGeo/gdal/blob/master/gdal/data/datum_shift.csv [3] https://gdalbarn.com/ _______________________________________________ QGIS-Developer mailing list QGIS-Developer@lists.osgeo.org List info: https://lists.osgeo.org/mailman/listinfo/qgis-developer Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-developer
