[PROJ] Make push and pop FIFO?

[Lesparre, Jochem]

Hi list members,

The PROJ operations +proj=push and +proj=pop use "last in, first out" (LIFO) to store and retrieve coordinate values in a stack. It seems that I need "first in, first out" (FIFO) or other new functionality in PROJ.

Problem
The Dutch Caribbean islands use TM-projections based on old datums. We have an exact Helmert transformation per island that transforms the horizontal coordinates to an ITRS realisation at a specific epoch. For some islands we need a geoid (EGM2008 sampled at 45" is the best we have at the moment) for the height transformation from local levelling heights to ITRS. That works fine for a forward transformation with this PROJ pipeline:

cct +proj=pipeline
+step +proj=push +v_3
+step +proj=tmerc +inv +lat_0=12.180658675 +lon_0=-68.251802281 +k_0=1 +x_0=23209.5600 +y_0=21423.9900 +ellps=intl
+step +proj=cart +ellps=intl
+step +proj=helmert +x=-366.1939 +y=-115.0688 +z=-776.7039 +rx=20.963080 +ry=16.462749 +rz=-14.276379 +s=-12.80900 +convention=coordinate_frame +exact
+step +proj=cart +inv +ellps=GRS80
+step +proj=pop +v_3
+step +proj=vgridshift +grids=egm2008_0_45.tif +inv

However, I do not get exactly the same results for the inverse transformation with -I, since I apply the geoid on the ITRS coordinates, but then the physical height is used for the Helmert transformation instead of the correct ellipsoidal height. This gives a small error in the horizontal coordinates in this case, but can easily reach more than 50 metres for other CRSs.

Solution
The most elegant solution seems to me to use FIFO for operations +proj=push and +proj=pop, so I can use this pipeline:

cct +proj=pipeline
+step +proj=push +v_3
+step +proj=tmerc +inv +lat_0=12.180658675 +lon_0=-68.251802281 +k_0=1 +x_0=23209.5600 +y_0=21423.9900 +ellps=intl
+step +proj=cart +ellps=intl
+step +proj=helmert +x=-366.1939 +y=-115.0688 +z=-776.7039 +rx=20.963080 +ry=16.462749 +rz=-14.276379 +s=-12.80900 +convention=coordinate_frame +exact
+step +proj=cart +inv +ellps=GRS80
+step +proj=push +v_3
+step +proj=pop +v_3
+step +proj=vgridshift +grids=egm2008_0_45.tif +inv
+step +proj=pop +v_3 +omit_fwd

Alternative
Another possible solution is a new PROJ operation e.g. +proj=flip to make the values of a coordinate value and the value stored in the stack switch places.

cct -I +proj=pipeline
+step +proj=push +v_3
+step +proj=utm +zone=19 +inv +ellps=intl
+step +proj=cart +ellps=intl
+step +proj=helmert +x=-366.1939 +y=-115.0688 +z=-776.7039 +rx=20.963080 +ry=16.462749 +rz=-14.276379 +s=-12.80900 +convention=coordinate_frame +exact
+step +proj=cart +inv +ellps=GRS80
+step +proj=flip +v_3
+step +proj=vgridshift +grids=egm96_15.tif +inv
+step +proj=pop +v_3 +omit_fwd

Regards, Jochem



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