[PROJ] Make push and pop FIFO?

[Thomas Knudsen]

If your input data does not need time stamps, you may be able to work
around this using +omit_fwd/+omit_inv selectors and a set of
axisswap order=1,2,4,3 steps?

Den tors. 14. dec. 2023 kl. 14.13 skrev Lesparre, Jochem via PROJ <
proj at lists.osgeo.org>:

> 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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