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
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