[PROJ] Question about the “triangulation-based transformations”

[Franz-Benjamin Mocnik]

Dear PROJ list members,

I hope I may ask here a question about the use of “triangulation-based transformations”, which are implemented in PROJ.  It is my aim to use it with QGIS, but the problem seems to lie with the way I approach PROJ.

I have created a file according to your specifications documented as part of PROJ:
http://datastore.franz-benjamin.net/my-projection-mlB4gBJk.json
It transforms EPSG:4326 coordinates to coordinates in some new coordinate system.  I have now the following questions:

(1) The following command works well:
====================================
> echo 2 1 | src/cct -z0 -t0 +proj=tinshift +file=/Users/franz-benjamin/Downloads/my-projection.json
 225850.4491    123074.6957        0.0000        0.0000
====================================

(2) The following does not work:
====================================
> src/proj +proj=tinshift +file=/Users/franz-benjamin/Downloads/my-projection.json
Rel. 8.1.1, September 1st, 2021
<proj>:
can't initialize operations that take non-angular input coordinates
program abnormally terminated
====================================
I have also tried it with the most current version, and it does not work either.  Any idea?

(3) In QGIS (v3.20.2), I go to Settings->Options to then enter a user defined CRS with the Proj String “+proj=tinshift +file=/Users/franz-benjamin/Downloads/my-projection.json”.  When I press validate, it says:
====================================
This proj projection definition is not valid:
proj_create: invalid value for file
====================================
This QGIS version uses PROJ 8.1.1, hence the choice of the PROJ version in (2).  Any idea why it does not work?

(4) I am unsure about which “output_crs” to specify in the my-projection.json file because I am using my own CRS.  I just set it to “EPSG:3395” to use *some* CRS, but also “My CRS” and “EPSG_4326” do not work.  Any idea what I should enter here?


I tried many different options, parameters, google searches, etc. but could not find any way to make it work under QGIS.  Do you have some idea of what I am doing wrong?  I would really be thankful if you could push me in the right direction.  Thank you!  I have tried for two days but could not find any solution.

Thank you in advance for your appreciated help!

Kind regards,
Franz-Benjamin

--
Dr Franz-Benjamin Mocnik
Assistant Professor

Faculty of Geo-Information Science and Earth Observation (ITC), University of Twente
PO Box 217, Drienerlolaan 5, 7500 AE Enschede, the Netherlands
Gebouw Langezijds 19, Hallenweg 8, 7522 NH Enschede, the Netherlands (visiting)

room 1344
phone +31 53 489-9360
www.mocnik-science.net
franz-benjamin.mocnik at utwente.nl
mail at mocnik-science.net