Javier,
you can probably hack around BOUNDCRS in a totally PROJ-specific way by
using the "METHOD["PROJ-based operation method: {proj_pipeline_here}"
For example with the following file, 'projinfo -s @/tmp/tmp.wkt -t "WGS
84"' will return the specified pipeline
BOUNDCRS[
SOURCECRS[
GEOGCRS["unknown",
DATUM["Unknown based on GRS 1980 ellipsoid",
ELLIPSOID["GRS 1980",6378137,298.257222101,
LENGTHUNIT["metre",1]]],
PRIMEM["Greenwich",0,
ANGLEUNIT["degree",0.0174532925199433],
ID["EPSG",8901]],
CS[ellipsoidal,2],
AXIS["longitude",east,
ORDER[1],
ANGLEUNIT["degree",0.0174532925199433,
ID["EPSG",9122]]],
AXIS["latitude",north,
ORDER[2],
ANGLEUNIT["degree",0.0174532925199433,
ID["EPSG",9122]]]]],
TARGETCRS[
GEOGCRS["WGS 84",
DATUM["World Geodetic System 1984",
ELLIPSOID["WGS 84",6378137,298.257223563,
LENGTHUNIT["metre",1]]],
PRIMEM["Greenwich",0,
ANGLEUNIT["degree",0.0174532925199433]],
CS[ellipsoidal,2],
AXIS["latitude",north,
ORDER[1],
ANGLEUNIT["degree",0.0174532925199433]],
AXIS["longitude",east,
ORDER[2],
ANGLEUNIT["degree",0.0174532925199433]],
ID["EPSG",4326]]],
ABRIDGEDTRANSFORMATION["Transformation from unknown to WGS84",
METHOD["PROJ-based operation method: +proj=pipeline +step
+proj=axisswap +order=2,1 +step +proj=cart +ellps=GRS80 +step
+proj=helmert +x=1 +step +inv +proj=cart +ellps=WGS84"]]]
Otherwise having records in the DB will certainly be cleaner.
data/sql/transformations_czechia_extra.sql can probably be used as an
example of quite advanced transformations, including using a grid
referenced in a projected CRS.
Even
Le 03/02/2025 à 16:26, Javier Jimenez Shaw via PROJ a écrit :
Hi
I am trying to make this transformation (that is not in EPGS). I do
not know how far I can go with PROJ.
(Apparently this transformation is needed to go from the coordinates
used by HEPOS.gr, the Greek positioning system, to the official -I
think- CRS in Greece, GGRS87.
The best is that the transformation would be in EPSG. Do you know
anybody in Greece with the proper contacts? I am happy to talk with
them about it, and help if needed)
Until it is in EPSG, I would like to use PROJ to do it.
The accurate transformation from ETRS89 to GGRS87 is something like:
- apply a Helmert transformation
- Project into a transverse Mercator
- Apply a correction grid. Yes, in the projected system.
- (optional) go back from the projected to the geographic crs.
I thought about several options:
- First option: create a WKT2 with a BOUNDCRS that defines the
transformation. Can it be done in a single WKT2?
- Second option: add the transformation to proj.db. Is it doable?
- Third option: Create "my own grid" file (in the geographic system)
combining all the steps. Of course, it will loose some accuracy due to
the new grid interpolation.
Any clue?
Thanks.
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