[QGIS-Developer] QGIS Georeferencer improvements

[Aurelio Pires]

I'm +1 for B as well!

Cheers
A.Pires

On 2020-03-31 22:48, Nyall Dawson wrote:
> On Wed, 1 Apr 2020 at 07:27, Martin Dobias <wonder.sk at gmail.com> wrote:
>
>> I have been thinking about this and what are the pros and cons of
>> having a separate window for georeferencing vs doing georeferencing in
>> main map canvas. Actually rather than discussing whether to have one
>> or two windows, the key bit to decide on is something else - what
>> would be the coordinate system of the georeferencer canvas:
>>
>> A. image coordinates - that is what we have in QGIS georeferencer now:
>> the image gets loaded without any distortion and stays that way all
>> the time. Picking of map coordinates for GCPs has to be done in a
>> separate map canvas. (i.e. one click in "georeferencer canvas", and
>> one click in "reference map canvas")
> Well - kind of. We do some stuff in image coordinates, yet confusingly
> require a CRS selection when first loading a raster (which I believe
> is used for nothing at all), and then the map coordinates of GCPs have
> an innate CRS too (which can be changed when you actually save the
> image, causing all sorts of weirdness!)
>
>> B. map coordinates - that is what ArcGIS does. The image is placed at
>> some initial position in the map, picking of image coordinates and map
>> coordinates for GCPs is done in the same map canvas. (i.e. two clicks
>> in single map canvas)
>>
>> The option B has the advantage that it is quite easy/intuitive to have
>> live preview of the image transformation as GCPs are being added. With
>> option A the live preview of the transform is not really possible (we
>> are still in image coordinates) - and the preview would need to be in
>> a separate map canvas.
>>
>> So where I am hesitating is whether to stick to option A which is
>> probably familiar to QGIS users - or with option B that may provide
>> better UX thanks to the live update of the warped raster (or vector).
>> I guess we will need to choose one or the other - supporting both
>> options would be probably too confusing. I am trying to figure out
>> which way to take, because this will also have great impact on the
>> workflow on how GCPs would be picked on the map(s) using map tools...
> I think option B makes much more sense. It may act different to what
> users are used to, but my experience is that users find the current
> approach confusing. I certainly do myself, and that's a bad sign. I've
> also wasted a lot of time over the years fixing mine/others
> georefencing errors caused by the confusing mix of CRS options in the
> current georeferencer.
>
> Reasons I like option B:
> - the live updates you've mentioned above
> - we know exactly what CRS already recorded GCPs are in, so we can
> easily transform them to a different CRS if at time of saving the user
> does decide to pick a different CRS vs what they've done the
> registration in
>
> My one question: where do you initially place the image? Do you just
> throw it in the middle of the current canvas extent as a starting
> point?
>
> Nyall
>
>
>
>> Thanks for your thoughts :-)
>>
>> Cheers
>> Martin
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