Dear Nicolas,
If you really need a cs2cs string, that converts from grad to Lambert
projection, here it is:
cs2cs -f "%.3f" +proj=eqc +R=6366197.72367581 +to_meter=100000
+lon_0=-2.3372291667E +nadgrids=fr_ign_ntf_r93.tif +geoidgrids=RAF09.gtx
+to +proj=lcc +lat_0=52 +lon_0=10 +lat_1=35 +lat_2=65 +x_0=4000000
+y_0=2800000 +ellps=GRS80
This produces exactly the same result as your cct code!
It simulates the grad to degree transformation with an equidistant
cylindrical projection. (My favorite trick :-)) Input data (that is in
grad) handled internally as meter, and the first part of the cs2cs
command with equidistant cylindrical projection converts it to degree.
As a surprise for me, nadgrids and geoidgrids should be applied before
the +to switch.
Best wishes,
Gabor
On 2024-02-18 14:16, Even Rouault via PROJ wrote:
Nicolas,
I believe the Helmert transformation is used when using IGNF codes,
because the IGNF database has only recorded the use of the ntf_r93
grid for NTF to RGF93, but not for NTF to ETRS89 (by the way, just
reiterating that IGN is more than welcome to take over on maintenance
& updates of the IGNF part of the PROJ database). And when
transforming between 2 IGNF codes, PROJ by default will only consider
transformations in the IGNF domain. But if you use EPSG codes, then
ntf_r93 is available to transforme between NTF and ETRS89.
To get a vertical transformation, you also need to use 3D CRS.
For example
projinfo -s "NTF geographiques Paris (gr) + NGF-IGN69 height" -t
IGNF:ETRS89LCC --3d --spatial-test intersects
reports the following pipeline:
unknown id, axis order change (2D) + NTF (Paris) to NTF (1) + NTF to
RGF93 v1 (1) + Inverse of RGF93 v1 to NGF-IGN69 height (1) + RGF93 v1
to ETRS89 (1) + axis order change (geographic3D horizontal) + ETRS89
LAMBERT CONFORMAL CONIC, 1.6 m, France - mainland onshore., at least
one grid missing
PROJ string:
+proj=pipeline
+step +proj=unitconvert +xy_in=grad +xy_out=rad
+step +inv +proj=longlat +ellps=clrk80ign +pm=paris
+step +proj=push +v_3
+step +proj=cart +ellps=clrk80ign
+step +proj=xyzgridshift +grids=fr_ign_gr3df97a.tif
+grid_ref=output_crs
+ellps=GRS80
+step +inv +proj=cart +ellps=GRS80
+step +proj=pop +v_3
+step +proj=vgridshift +grids=fr_ign_RAF18.tif +multiplier=1
+step +proj=lcc +lat_0=52 +lon_0=10 +lat_1=35 +lat_2=65 +x_0=4000000
+y_0=2800000 +ellps=GRS80
Here, as there's a mix of objects from different authorities (IGNF and
EPSG), then EPSG transformations are considered, hence you get the
gr3df97a grid.
$ echo 0 56.000000 0 | PROJ_NETWORK=ON bin/cs2cs -d 3 "NTF
geographiques Paris (gr) + NGF-IGN69 height" IGNF:ETRS89LCC --3d
3474789.997 2655359.291 43.644
which is the application of the above pipeline:
$ echo 0 56.000000 0 | PROJ_NETWORK=ON bin/cct -d 3 +proj=pipeline
+step +proj=unitconvert +xy_in=grad +xy_out=rad +step +inv
+proj=longlat +ellps=clrk80ign +pm=paris +step +proj=push +v_3 +step
+proj=cart +ellps=clrk80ign +step +proj=xyzgridshift
+grids=fr_ign_gr3df97a.tif +grid_ref=output_crs +ellps=GRS80 +step
+inv +proj=cart +ellps=GRS80 +step +proj=pop +v_3 +step
+proj=vgridshift +grids=fr_ign_RAF18.tif +multiplier=1 +step +proj=lcc
+lat_0=52 +lon_0=10 +lat_1=35 +lat_2=65 +x_0=4000000 +y_0=2800000
+ellps=GRS80
If you want to get exactly the below pipeline, you'll have to tweak
proj.db to create a compound CRS for "NTF geographiques Paris (gr) +
NGF-IGN69 height", and then create a custom record in
other_transformation for the transformation between this compound CRS
and (a 3D projected CRS derived from) IGNF:ETRS89LCC. Cf
data/sql/other_transformation_custom.sql for potential inspiration
Even
Le 15/02/2024 à 19:46, Nicolas Bellaiche via PROJ a écrit :
Hi there,
I try to make a conversion using shiftgrids from IGNF: NTFP to
IGNF:ETRS89LCC. By default, it seems that it uses the helmert
approximation between the 2 crs and i cannot figure out how to use
the grid fr_ign_ntf_r93.
So far i've found how to create a pipeline that works with cct, but
i'd like to have a code that represents the crs and be able to go to
different destination crs with the cs2cs application. Any idea?
(helmert approx + no geoid with the standard codes)
echo 0 56 0 | ./cs2cs + from IGNF:NTFP +to IGNF:ETRS89LCC
result : 3474788.21 2655358.23 0.00
(correct pipeline)
echo 0 56.000000 0 | PROJ_DEBUG=3 ./cct +proj=pipeline +step
+proj=unitconvert +xy_in=grad +xy_out=rad +step +inv +proj=longlat
+ellps=clrk80ign +pm=paris +step +proj=hgridshift
+grids=fr_ign_ntf_r93.tif +step +inv +proj=vgridshift
+grids=RAF09.gtx +step +proj=lcc +lat_0=52 +lon_0=10 +lat_1=35
+lat_2=65 +x_0=4000000 +y_0=2800000 +ellps=GRS80
result : 3474789.9972 2655359.2908 43.6421 inf
reference from THE official IGN/SGN dataset:
3474789.997 2655359.291 43.642
Thanks a lot for your help,
Nicolas Bellaiche
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