[gdal-dev] Transformation methods & parameters (OGRCoordinateTransformation)

[Andre Joost]

Am 18.03.2016 um 17:34 schrieb César Martínez:

>
> This is relevant because sometimes there are several transformation
> operations available between 2 CRSs, and we'd like our users being
> able to choose which operation they'd like to use. For instance,
> between EPSG:23030 and EPSG:4326, at least the following
> transformations are suitable for Spain: EPSG:15933, EPSG:1633,
> EPSG:1133, EPSG:1145 [2], each using different methods & parameters
> and thus providing different accuracy.
>
> Is there a simple way to get this information?
>

EPSG:23030 is ED50 / UTM zone 30N.

In a first transformation step, the data is transformed from the 
projected CRS to the geographic CRS. In your case a transverse mercator 
projection on EPSG:4230 ED50.

The datum shift (with different accuracy depending on the parameters) is 
from EPSG:4230 to EPSG:4326. Look out for the datum_shift.csv file, e.g. 
under bin/gdal-data in the Gisinternal builds for Windows. There you 
find all datum shifts from 4230 to 4326 in the lines labelled SEQ_KEY 
306 to 340. Remarks, area bounds and parameters follow in the next 
columns, and the last column PREFERRED has just one line with entry 1 
(SEQ_KEY=308). This one has been chosen as default for all projections 
based on ED50.

You are free to use any of the other ones. For 7-parms trnasformations, 
you might need to change the sign of the rotation parameters RX, RY and RZ.

HTH,
André Joost