[QGIS-Developer] QGIS Georeferencer improvements

[Luigi Pirelli]

Hi Martin

this was one element in my todo list months ago.
In some automatic processing workflow, sometimes is necessary to give a
straightforward interactive tool to facilitate rectification when
polinomial parameters are not enough to reach precision... fortunately,
struggling with gdal code, I reached enough precision 1.5m (for our
application) on WorldView images.

BTW in case wouldn't be enough, I provided a custom and simplified QGIS to
go straightforward to create GCPs.

A better approach I found, but not ported to 3.x, is the use of "J
Rectifier" [1] plugin. This plugin theoretically reorganise de GUI to allo
live georeferencing that facilitating a lot the work. The main problem to
port to 3.x is that have to be redesigned from scratch, having one big
class doing all!
IMHO from this plugin we can get some interesting ideas how to facilitate
the user interaction.

Luigi Pirelli

[1]  https://plugins.qgis.org/plugins/J_Rectifier/

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On Fri, 13 Mar 2020 at 16:13, Martin Dobias <wonder.sk at gmail.com> wrote:

> Hi all
>
> There has been some interest from a client to improve the
> georeferencer in QGIS to better suit their needs, mainly to support
> their workflow where they commonly georeference multiple raster images
> at the same time. So for example you pick a single ground control
> point (GCP) with XY(Z) map coordinates and in multiple images you
> would visually pick where that point is located in image coordinates.
> Ideally you want to see side-by-side the main map canvas and images
> you are georeferencing.
>
> With this kind of change, it is maybe a good time to have a look at
> what are the other issues with the georeferencer and address them as
> well. The budget is limited (as usual), so I cannot promise that
> anything mentioned in this thread would get implemented, but I am sure
> there are people around who have some ideas what could be improved. At
> some point later I would like to prepare a QEP that will be hopefully
> in line with the discussion here.
>
> Let me start with a list of current issues I see with georeferencer in
> addition to the lack of georeferencing of multiple images at once. The
> following is a mixture of UI/UX and technical bits:
>
> 1. Georeferencer opens in a new window. In order to see the main map
> canvas and the georeferenced layer side by side, it is necessary to do
> some resizing of windows which is inconvenient. It would be maybe more
> practical to have the georeferenced image in a dock widget in the main
> window - and all georeferencer related map tools included in the main
> QGIS window as well. (Or maybe the other way around - to keep
> georeferencer as a separate window, but allow the main map canvas to
> be embedded in it?)
>
> 2. Georeferencer is a plugin. Its core/gui functionality cannot be
> reused elsewhere. It would be useful to have some Processing
> algorithms to load/save GCPs, to extract GCPs to raster images, to
> save GCPs to raster images and to warp imagery according to GCPs (and
> chosen transformation method).
>
> 3. Georeferencer does not allow live preview of how the georeferenced
> raster image would end up looking on the map - user is required to
> write warped image to disk as a new file and only then see the
> results. It should be possible to use the estimated transformation
> based on GCPs and show the layer in the main map canvas any time (if
> wanted).
>
> 4. It uses a combination of GDAL-based transformation methods
> (Polynomial of 1st/2nd/3rd order, thin plate spline) and some of its
> own (Helmert, Projective), for which it also requires GSL library as a
> dependency. In my opinion, QGIS georeferencer should leave all the
> math to GDAL which does a great job there. Moreover, I have some
> doubts about the methods that do not come from GDAL: the “Helmert”
> method is simply 2D affine transform (offset+scale+angle), while true
> Helmert transformation is 3D transformation with 7 parameters. Do we
> need it if GDAL’s 1st order polynomial transformation should do just
> that? And the “Projective” method which is a bit of a mystery to me.
> First of all, I am not sure in what situation it is useful and the
> documentation only says it is “a linear rotation and translation of
> coordinates” - which is a bit vague. It looks like a subset of RPC
> method supported by GDAL (although GDAL does not have estimation of
> RPC coefficients). Overall I would be for removal of these methods and
> stick to what GDAL offers.
>
> I would be keen to hear what you think, if you agree/disagree with the
> above or have some ideas how to improve things here.
>
> Regards
> Martin
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