[QGIS-Developer] QGIS Georeferencer improvements

[Nico Tripcevich]

Hello I saw the discussion of the Georeferencer improvements from April. I
hope you don't mind if I revive it and chime in in case these improvements
are under consideration. I posted this as a  github issue
<https://github.com/qgis/QGIS/issues/35051>  but maybe it'll get seen here. 
Great there are plans to work on the UI of Georeferencer. I did a lot of
georeferencing work in Arcmap 10.x years ago and have recently been doing
quite a bit of work in QGIS3 so I thought based on my experience with both i
would type up some of the aspects of QGIS Georeferencer I that think could
be modified for easier flow. Some of these are pretty basic (more basic than
what is proposed above) but still fall under the Feature Request category.

 1.   Allow rotating the original 90 deg in the Preview window (arcmap
allows this). I have to flip original and GCPs around in my mind every time
i move my eyes between the windows.I saw discussion of a preview mode in
Georeferencer in the main window and I think this would be a great. At the
very least allowing 90deg rotation would allow us to get the original closer
to the realworld orientation.I've tried rotating the originals in image
editor and this sometimes works. Better to support rotating inside
Georeferencer if possible. 
<http://osgeo-org.1560.x6.nabble.com/file/t387047/94373373-f4be6400-00b9-11eb-9d72-f19176dd442a.jpg> 
2. Auto-Filenaming of GCP.points file: In Georeferencer > Transform Settings
dialog the "Save GCP points" checkbox currently results in a GCP file called
output_filename.tif.points. However because the .points GCP file works with
the original scan file not the output_filename.tif. perhaps it is more
intuitive to have the checkbox "Save GCP points" auto-create a points file
called input_filename.tif.points. I guess the one good thing to the current
approach is the output points are linked to the step in georeferencing by
the filename. But the problem is one assumes they should be loaded along
with that file when in fact they were used to produce it.

3. File> Reset Georeferencer removes all the GCP points and the raster as
well. If I reload the raster the GCPs come back. How do i just remove all
the GCPs and start fresh with the same raster? (this may be a bug)
4.    As the original post suggests this work is occurring in the main map
window (good for smaller screens) and not in a separate window. A separate
window is optionally available in Arcmap.
5.  In Arcmap the GCP number is visible (consistently colored green and thin
font no label buffer like in Q), while the target GCP crosshair and label
are consistently red color.
    snapping result hover label showing what layer it is snapping to
    GCP Link table (docked) shows residual with X and Y separate as well as
transform type
<http://osgeo-org.1560.x6.nabble.com/file/t387047/94373677-c772b580-00bb-11eb-9bd6-26384319b900.jpg> 
6. What about Z values? In steep terrain the lack of Z will introduce great
distortions.
7. In Arcmap the georeferencer is doing realtime collinearity and
distribution tests whenever a new GCP is placed (as soon as 3 or more GCPs
have been placed). This warning pops up if all the GCPs are clustered or
along one edge
<http://osgeo-org.1560.x6.nabble.com/file/t387047/94373848-fc333c80-00bc-11eb-9089-fca124a9da83.jpg> 
The result is that Arcmap allows an iterative process. First place 4 GCPs
roughly around the perimeter to get the thing preview-positioned in the main
map window. Keep working carefully placing GCPs # 4-8 or more, then
investigate residuals and remove / relocate the GCPs with a high residual.
Related to this I recently typed up a QGIS Georeferencer + JOSM workflow on
the OSM wiki to help historical mappers to geolocate the excellent Sanborn
maps for OHM contributions. Thanks
Sanborn_Map_geoferencing for OpenHistoricalMap.org
<https://wiki.openstreetmap.org/wiki/Sanborn_Map_geoferencing>  
Thank you for this excellent Georef tool in QGIS so far. I've found that the
QGIS Georeferencer produces often cleaner GeoTIFF files than Arcmap -- kudos
team!



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