[PROJ] PROJ 9 & old syntax +init=

[Even Rouault]

>
>     1.a) NEW SYNTAX: z not correct (grid shift not used)
>
>     * echo 600000.00000000 300097.53400000 0.00000000 |  ./bin/cs2cs
>     IGNF:LAMB1 +to IGNF:UTM31RGF93*
>     452845.19        5583309.40 0.00
>
Which grid shift file are you refering to ? ntf_r93.gsb ? I assume. But 
this is a horizontal datum shift. No vertical correction. This 
horizontal shift is applied, otherwise you'd get "452843.46 5583308.13 
0.00" as a result
>
>
>     1.b) OLD SYNTAX: z correct:
>
>     *echo 600000.00000000 300097.53400000 0.00000000 | ./bin/cs2cs
>     +init=IGNF:LAMB1 +to +init=IGNF:UTM31RGF93*
>     452845.19        5583309.40 43.64
>
I have no idea how you get this result. You should really try stopping 
using the (old) IGNF file with newer PROJ. This is mostly in unsupported 
land. Even when trying, with both PROJ 5.2 and latest PROJ master, and 
with the IGNF file supplied with PROJ 5.2, I still get the "452845.19 
5583309.40 0.00" result. Did you tweak your IGNF file ?

The only way I can get something close to what you get is by composing a 
manual pipeline using the gr3df97a grid with:

echo  600000.00000000 300097.53400000 0.00000000 | PROJ_NETWORK=ON 
PROJ_DEBUG=3 bin/cct  +proj=pipeline +step +inv +proj=lcc +lat_1=49.5 
+lat_0=49.5 +lon_0=0 +k_0=0.99987734 +x_0=600000 +y_0=200000 
+a=6378249.2 +rf=293.466021 +pm=paris +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=utm 
+zone=31 +ellps=GRS80
   452845.1911   5583309.3959       42.9892           inf

But it seems a bit dubious to have a Z correction term for NTF <--> 
RGF93. And newer PROJ when operating with 2D CRS doesn't change the Z 
value. You may sometimes upgrade your CRS to 3D by adding the  --3d 
switch to cs2cs or projinfo, but if the transformations available are 
only 2D ones, that won't make miracles.

What does Circe does under the hood for that transformation?

>
>
>     2) Example where the old syntax gives a wrong result while the new
>     syntax works:
>
>     2.a) OLD SYNTAX from cartographic to geocentric coordinates /
>     result is wrong
>
>     *echo 652760.737 7033791.243 43.642 | PROJ_DEBUG=3 ./bin/cs2cs
>     +init=IGNF:LAMB93 +to +init=IGNF:RGF93*
>     4070355.05        166080.07 4891332.24
>
I do get "4070327.25166078.93 4891298.61". This really smells like your 
IGNF file isn't the one of PROJ 5.2


>
>     2.b) NEW SYNTAX from cartographic to geocentric coordinates /
>     result is correct
>
>     *echo 652760.737 7033791.243 43.642 | PROJ_DEBUG=3 ./bin/cs2cs
>     IGNF:LAMB93 +to IGNF:RGF93*
>     4070327.25166078.93 4891298.61
>
>
>
>
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