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I looking for advice or recommanded methods to georeference scanned maps for which I know the projection and some coordinates with lat/long dat in degrees

In my test case, projection is:

lcc: Lambert Conformal Conic  - 93 / IGNF:LAMB93)

LAMB93 requires coordinates in meters.

The map contains several cross-hairs marking and I have marked some places with GPS coordinates.

I have started to georeference the raster(scanned map) with QGIS Plugin using these known points with lat/long coordinates in degrees on the map. I used the tutorials here: Georeferencing Topo Sheets and Scanned Map and here: 15.2. Lesson: Georeferencing a Map )

My concern is that when importing the map in QGIS and superposing on other maps, I have an offset (and/or due to a slight rotation).

I assume that by giving coordinates in degrees, QGIS imports the whole map as "a WGS84 raster". I did not find a way to specify target SRC with a lcc projection.

I tried to used RGF93 (EPSG 4171), but it ended also with an offset.

My aim is to find a method to import the scanned maps within the original projection (SCR) and correlating the control points with as they have been done

NB: I managed to reduced the offset by using transformation distortion of the imported map and multiplying the correlation points, but I am not satisfied with the result and extra work required.

Notes:

QGIS version 3.4 used

-LAMB93 proj file:

Proj4: +proj=lcc +nadgrids=@null +a=6378137.0000 +rf=298.2572221010000 +lat_0=46.500000000 +lon_0=3.000000000 +lat_1=44.000000000 +lat_2=49.000000000 +x_0=700000.000 +y_0=6600000.000 +units=m +no_defs

-RFG93 proj file:

Proj4: +proj=longlat +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +no_defs

QGis Georeferencer, Transformation Window Plugin Window QGis Georeferencer, Transformation Window Plugin Window

  • Have you tried using another transformation method like thin plate spline? – Gerardo Jimenez Jul 27 at 3:38
  • Yes, correlation is pretty good locally on the control points. But I wanted to find a work flow which avoids "uncontrolled" distorsion. I guess using a ThinPlate transformation is appropriate if you do not know the projection and have many control points. – kFly Jul 29 at 12:22
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I have a note on that tutorial for this very case.

"...If the map you are trying to georeference uses a projected CRS that you know of, but the graticules labels are in a Geographic CRS (latitude/longitude), you may use an alternate workflow to minimize distortion. Instead of using a Geographic CRS like we are using here, you can create a vector grid in QGIS and transform it to the projected CRS to be used as a reference for accurate coordinate capture..."

See this post for step by step guide.

https://raisedbeaches.net/2018/02/01/georeferencing-in-qgis/

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