Push coordinate value to pipeline stack¶
New in version 6.0.0.
Save components of a coordinate from one step of a pipeline and make it available for retrieving in another pipeline step.
This operations allows for components of coordinates to be saved for application in a later step. A saved coordinate component is moved, or pushed, to a memory stack that is part of a pipeline. The pipeline coordinate stack is inspired by the stack data structure that is commonly used in computer science. There’s four stacks available: One four each coordinate dimension. The dimensions, or coordinate components, are numbered 1–4. It is only possible to move data to and from the stack within the same coordinate component number. Values can be moved off the stack again by using the pop operation.
If the push operation is used by itself, e.g. not in a pipeline, it will function as a no-operation that passes the coordinate through unchanged.
A common use of the push and pop operations is in 3D Helmert transformations where only the horizontal components are needed. This is often the case when combining heights from a legacy vertical reference with a modern geocentric reference. Below is a an example of such a transformation, where the horizontal part is transformed with a Helmert operation but the vertical part is kept exactly as the input was.
$ echo 12 56 12.3 2020 | cct +proj=pipeline \ +step +proj=push +v_3 \ +step +proj=cart +ellps=GRS80 \ +step +proj=helmert +x=3000 +y=1000 +z=2000 \ +step +proj=cart +ellps=GRS80 +inv \ +step +proj=pop +v_3 \ 12.0056753463 55.9866540552 12.3000 2000.0000
Note that the third coordinate component in the output is the same as the input.
The same transformation without the push and pop operations would look like this:
$ echo 12 56 12.3 2020 | cct +proj=pipeline \ +step +proj=cart +ellps=GRS80 \ +step +proj=helmert +x=3000 +y=1000 +z=2000 \ +step +proj=cart +ellps=GRS80 +inv \ 12.0057 55.9867 3427.7404 2000.0000
Here the vertical component is adjusted significantly.
Stores the first coordinate component on the pipeline stack
Stores the second coordinate component on the pipeline stack
Stores the third coordinate component on the pipeline stack
Stores the fourth coordinate component on the pipeline stack