Is setting raster CRS in QGIS georeferencer useless? Still have to choose the same for target SRS?

Question

I'm trying to move out of ArcGIS into QGIS permanently for basic operations, so be patient please. Here is the problem. I'm trying to georeference a topo map in EPSG:22523 (Corrego Alegre UTM). I did the regular procedure, which has worked before for WGS84 stuff, and is also shown in QGIS tutorials, including this one: http://www.qgistutorials.com/en/docs/georeferencing_basics.html

However the georeferencing just doesn't work. When I open the layer properties in the project, the CRS for the layer is actually WGS84 (4326). In the link above a user asks the same question, and his conclusion, after not getting an explanation, is that setting the layer CRS in georeferencer correctly is just useless. You still have to put that CRS in target SRS in transformation settings. However there isn't a single georeferencing tutorial where this is mentioned. In fact they all put target SRS different from layer CRS. Please help.

Answer 1

If you load an unreferenced raster into the georeferencer, the question for the CRS is surely useless. The referencing assings values of pixel/line to target coordinates, so the source CRS does not play any role.

The target CRS however is important, it depends on the way you are doing the georeferencing.

1. You have lat/lon grid lines imprinted on the map, and you enter the values manually. If they are WGS84, target CRS is EPSG:4326, but in your case it can be Corrego Allegre (4225), SAD69 (4291) or PSAD56 (4248) or what was used when the map was printed. All have degrees as units, and differ only by a few hundred meters.
2. You have a projected grid imprinted on the map, with meters as units. Mostly same as above, it can be UTM or a locally established CRS.
3. You have a reference map in the background, and click on identical points in both maps. In this case the target CRS should be identical to that of the background map

In any case, I suggest to visit Settings -> Options, CRS tab. Under CRS for new layers, I suggest to switch to Prompt for CRS instead of a default CRS of EPSG:4326. This will prevent you from many errors when QGIS assigns WGS84 if it has not found CRS information.

The Georeference output raster should be in GeoTiff format, because some other formats like png and jpg do not store CRS information inside.

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Note that the georeferencer behaves differently if the raster already has (probably wrong) geolocation information stored inside, as mentioned in QGIS georeferencer - GCP projection problem. In that case, the source CRS might make sense.