I want to change the Coordinate Reference System of some raster and vector layers which are from different types and origins so that all are in the same CRS. In QGIS when I want to change the CRS of the project, several transformation options are available for each CRS. Here is the text which is prompted by QGIS:
Multiple operations are possible for converting coordinates between these two Coordinate Reference Systems. Please select the appropriate conversion operation, given the desired area of use, origins of your data, and any other constraints which may alter the "fit for purpose" for particular transformation operations.
QGIS shows a little map with the particular region in which these transformations are tuned to and the accuracy in meters that each transformation would provide (usually ranging from 1 to 10 meters, depending on the transformation).
Here is an example with a subset of my data, in this case a vector layer containing points which I want to project from WGS84 (EPSG 4326) to ED50 UTM30 (EPSG 23030) to later extract info from raster layers at these geographical locations:
| name | lat | long |
|---|---|---|
| 1 | 38.190694 | -5.993028 |
| 2 | 38.419139 | -6.100139 |
| 3 | 38.289917 | -5.984778 |
| 4 | 37.974500 | -6.168806 |
| 5 | 38.745500 | -5.767806 |
| 6 | 36.976222 | -5.657944 |
| 7 | 37.317750 | -6.866444 |
| 8 | 40.772194 | -3.779278 |
| 9 | 40.069639 | -5.438222 |
| 10 | 40.955000 | -5.668944 |
When I try to import these points into QGIS (ver. 3.28.5 "Fierze"), I can choose between 13 transformation options. The one which provides the best result in the area of interest (Spain), is this one:
Inverse of ED50 to WGS 84 (28) + UTM zone 30N.
Scope: Approximation assuming equality between plate-fixed static and earth-fixed dynamic CRSs. Remarks: Parameter values from ED50 to ETRS89 (7) (code 1632). Assumes ETRS89 and WGS 84 can be considered the same to within the accuracy of the transformation.
Scope: Engineering survey, topographic mapping.
Area of use: Spain - onshore mainland except northwest (north of 41°30'N and west of 4°30'W)., Between 6°W and 0°W, northern hemisphere between equator and 84°N, onshore and offshore.
Identifiers: INVERSE(EPSG):1633, EPSG:16030 +proj=pipeline +step +proj=unitconvert +xy_in=deg +xy_out=rad +step +proj=push +v_3 +step +proj=cart +ellps=WGS84 +step +inv +proj=helmert +x=-131 +y=-100.3 +z=-163.4 +rx=-1.244 +ry=-0.02 +rz=-1.144 +s=9.39 +convention=position_vector +step +inv +proj=cart +ellps=intl +step +proj=pop +v_3 +step +proj=utm +zone=30 +ellps=intl
But another possible option could be:
Inverse of ED50 to WGS 84 (13) + UTM zone 30N.
Scope: Military survey.
Remarks: Derived at 18 stations. Accuracy 5m, 6m and 3m in X, Y and Z axes. Scope: Engineering survey, topographic mapping. Area of use:
Portugal; Spain - mainland., Between 6°W and 0°W, northern hemisphere between equator and 84°N, onshore and offshore. Identifiers:
INVERSE(EPSG):1145, EPSG:16030
+proj=pipeline +step +proj=unitconvert +xy_in=deg +xy_out=rad +step +proj=push +v_3 +step +proj=cart +ellps=WGS84 +step +proj=helmert +x=84 +y=107 +z=120 +step +inv +proj=cart +ellps=intl +step +proj=pop +v_3 +step +proj=utm +zone=30 +ellps=intl
However, when I change the projection using the terra package in R, it is quite straight forward and I don't have to specify any transformation type. But, should I be worried about this? Is there a way I can specify the exact transformation type in the terra::project function?
Thanks for any insights about these aspects.
