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How can I see (and edit if possible) the transformation method and parameters used by QGIS for "on-the-fly" transformation, and then for saving data in another CRS? Like in ArcGIS?

Here is the similar question but answered differently, I don't want to create a new coordinate system, I want to check the transformation between existing in Qgis coordinate systems.

In ArcGIS I can select between several transformation methods for the pair of CRS, and what about QGIS?

I've changed the default WGS84 CRS to another local CRS. For example, if I set for the project EPSG 3763 (ETRS PT TM 06), described as +proj=tmerc +lat_0=39.66825833333333 +lon_0=-8.133108333333334 +k=1 +x_0=0 +y_0=0 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs, and add data in EPSG 20790 (Datum Lisboa) +proj=tmerc +lat_0=39.66666666666666 +lon_0=1 +k=1 +x_0=200000 +y_0=300000 +ellps=intl +towgs84=-304.046,-60.576,103.64,0,0,0,0 +pm=lisbon +units=m +no_defs, it is reprojected on-the-fly and looks more or less correctly. But if I cannot see is it really correct I want to know the transformation used. There are several reccomended methods of different accuracy, and I want to know which one was used (if it was one of them).

(I am not fluent in QGIS, just learning). Thanks in advance!

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1 Answer

up vote 8 down vote accepted

You can see the proj string used by QGIS to transform from and to WGS84 when you look at

Settings->Project settings, CRS tab, and search for the EPSG code or name of a CRS. All CRS are referenced by their EPSG code, or user defined CRS.

Uusally, all EPSG-codes are bundled with one certain transformation to WGS84 which was decided to be most appropriate. American NAD27 CRS are the only ones which use a ntv2 grid by default instead of three- or seven-parameter Helmert transformation.

If you are unhappy with the accuracy of a defined transformation, you can define a custom CRS with different +towgs84 parameters.


EDIT

For your Lisbon datum example, which is a projection of EPSG:4207, the EPSG database offers several transformation parameters which can be found in http://svn.osgeo.org/gdal/trunk/gdal/data/datum_shift.csv:

214,1656,4207,4326,"Parameter values from Lisbon to ETRS89 (1) (code 1655). Assumes ETRS89 and WGS 84 can be considered the same to within the accuracy of the transformation. Replaced by Lisbon to WGS 84 (4) (code 1988).",For applications to an accuracy of 3 metres.,1294,36.96,42.15,-9.55,-6.19,1,0,9606, -280.9,-89.8,130.2,-1.721,0.355,-0.371,-5.92,0

215,1944,4207,4326,"Parameter values from Lisbon to ETRS89 (2) (code 1790). Assumes ETRS89 and WGS 84 can be considered the same to within the accuracy of the transformation.",For applications to an accuracy of 2 metres.,1294,36.96,42.15,-9.55,-6.19,1,1,9606, -282.1,-72.2,120,-1.592,0.145,-0.89,-4.46,0

216,1984,4207,4326,,For low resolution applications.,1294,36.96,42.15,-9.55,-6.19,1,0,9603, -304.046,-60.576,103.64,,,,,1

217,1988,4207,4326,,For medium resolution applications.,1294,36.96,42.15,-9.55,-6.19,1,0,9607, -288.885,-91.744,126.244,1.691,-0.41,0.211,-4.598,0

So you see, that the low resolution transformation with three parameters is used in your example. It is stored in http://svn.osgeo.org/gdal/trunk/gdal/data/gcs.csv under code 4207 and used by Qgis and other GIS software depending on proj.4.

http://www.epsg-registry.org/ gives an accuracy of 5 metres for EPSG:1984 (note that transformations have their own code numbers). These values were first published by NIMA when WGS84 was introduced. But its not what the local surveying authority recommends. You can also have a look at http://www.fc.up.pt/pessoas/jagoncal/coordenadas/index_en.htm

If you want it more accurate, try the ntv2 grid from http://www.igeo.pt/produtos/geodesia/Grelhas_NTv2.htm

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Thanks for the answer, but it is not completely clear. In project settings, I've already changed the default WGS84 CRS to another one. So if I understand you correctly, QGIS see all coordinate systems as referred to geographic WGS84? And transformation from one local CRS of data to another CRS of the project anyway always goes through WGS84 like CRS1->WGS84->CRS2? (For EPSG 3763 (ETRS) I see +proj=tmerc +lat_0=39.66825833333333 +lon_0=-8.133108333333334 +k=1 +x_0=0 +y_0=0 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs, so 0,0,0,0,0,0,0 are the parameters, right) –  nadya Jan 9 '13 at 3:15
1  
Yes, WGS84 is the basis against all CRS are transferred from and to. ETRS uses (almost) the same ellipsoid as WGS84, therefore null shift parameters are given. The shift between WGS84 and ETRS caused by tectonic moves is adjusted by redifinition of WGS84 from time to time. –  Andre Joost Jan 9 '13 at 5:12
    
Yes, ETRS is just a case of luck for me. I am wondering what if I had two CRS different from WGS84, CRS1->WGS84->CRS2 is a rather crude way :( –  nadya Jan 9 '13 at 5:28
    
Its jsut a bit more complicated: projected CRS1 -> geodetic CRS1 -> WGS84 -> geodetic CRS2 - projected CRS2. We have over 55000 EPSG codes, you can imagine how much transformation parameters are needed if you want to transform directly between all... –  Andre Joost Jan 9 '13 at 15:36
    
@AndreJoost -- "...three-...parameter Helmert transformation." What is that? I know of 4-, 5- or 7-parameter Helmert transformations, but not 3-parameter Helmert ones. With fewer parameters, I find it less useful to mention Helmert, and more useful to name qualities: 2D or 3D, rigid-body or similarity, rotation or translation, etc. OK, you do mention "null shift parameters" in a comment, but 7-3=4 and 5-3=2. –  martin f Jan 13 at 21:13
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