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I have a topo map which I want to georeference into Deir ez Zor / Levant Stereographic (EPSG:22780) and then transform into WGS 84/UTM zone 36N (EPSG:32636).

My goal is have as little divergence to the BING satellite imagery provided by the QuickmapServices QGIS plugin as possible. The map depicts an area in Lebanon.

So, I use the coordinates which are specified through the topo maps grid, to georeference the map, E.g. x: 355700 y: 72800. From the additional information given by the author. The given information is sparse, as the author did not give any type of identifyier but called the crs he used only the "syrian system" of which there are several in Lebanon accorsing to epsg.io.

After some trying-out, I concluded that he most likely used Deir ez Zor / Levant Stereographic. I also found out that the coordinates are in fact negative: x: -355700 y: -72800.

After georef. the map appears significantly off of the underlying satellite imagery (some 20 - 30 m). However, I thought this might fix itself by reprojecting the map into UTM 36N. But no matter what parameters I choose for reprojection, the transformed map is identical to the originally referenced one, and thus still off of the satellite imagery. The project CRS was also set to UTM 36N.

I have some gps data in WGS84 / UTM 36N from the same area, which aligns perfectly with the satellite imagery. So, I don't think the problem is caused by the inaccuracy of the projected satellite image.

Any thoughts?

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    If the raster isn't correctly georeferenced, reprojecting it won't help. Did you try all the different transformation types available in the Georeferencer (Linear, Helmert, Polynomial 1,2,3, Thin Plate Spline, and Reprojective)?
    – csk
    Commented Aug 7, 2018 at 18:29
  • Also, the more reference points you can add the more accurate the transformation will be. Try manually adding additional points based on objects visible in both the topo map and satellite imagery, such as road intersections and buildings.
    – csk
    Commented Aug 7, 2018 at 18:31
  • I would expect that there is no geographic/datum transformation occurring between Deir ez Zor and WGS84. If you look up the ProjCRS in QGIS to get its proj4 string, is there a +TOWGS84 parameter? What are the values?
    – mkennedy
    Commented Aug 7, 2018 at 18:37

1 Answer 1

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QGIS uses datum shift values of 

 +towgs84=-190.421,8.532,238.69,0,0,0,0

According to Clifford Mugnier you might try a custom CRS based on the shift parameters for Bekaa Valley 

 +towgs84=-182.966,-14.745,-272.936,0,0,0,0

or the values given in epsg.io

 +towgs84=-183,-15,273,0,0,0,0

Don't reproject, but apply that custom CRS to your Levant stereographic layer.

Clifford Mugnier notes some oddity about the stereographic projection that was used in Syria and Lebanon, which might or might not have been solved in PROJ/GDAL.

An alternative is to forget the old coordinates and georeference your map against points visible in your satellite layer. In that case, you set the target CRS to that of the image (I suspect EPSG:3857), and use from map extent with a thin plate spline interpolation.

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