The pipeline operator¶
Construct complex operations by daisy-chaining operations in a sequential pipeline.
|step||Separate each step in a pipeline.|
|inv||Invert a step in a pipeline.|
See the section on Geodetic transformation for a more thorough introduction to the concept of transformation pipelines in PROJ.
With the pipeline operation it is possible to perform several operations after each other on the same input data. This feature makes it possible to create transformations that are made up of more than one operation, e.g. performing a datum shift and then applying a suitable map projection. Theoretically any transformation between two coordinate reference systems is possible to perform using the pipeline operation, provided that the necessary coordinate operations in each step is available in PROJ.
A pipeline is made up of a number of steps, with each step being a coordinate operation in itself. By connecting these individual steps sequentially we end up with a concatenated coordinate operation. An example of this is a transformation from geodetic coordinates on the GRS80 ellipsoid to a projected system where the east-west and north-east axes has been swapped:
+proj=pipeline +ellps=GRS80 +step +proj=merc +step +proj=axisswap +order=2,1
Here the first step is applying the Mercator projection and the second step is applying the Axis swap conversion. Note that the +ellps=GRS80 is specified before the first occurrence of +step. This means that the GRS80 ellipsoid is used in both steps, since any parameter stated before the first occurrence of +step is treated as a global parameter and is transferred to each individual steps.
Rules for pipelines¶
1. Pipelines must consist of at least one step.
Will result in an error.
2. Pipelines can only be nested if the nested pipeline is defined in an init-file.
+proj=pipeline +step +proj=pipeline +step +proj=merc +step +proj=axisswap +order=2,1 +step +proj=unitconvert +xy_in=m +xy_out=us-ft
Results in an error, while
+proj=pipeline +step +init=predefined_pipelines:projectandswap +step +proj=unitconvert +xy_in=m +xy_out=us-ft
3. Pipelines without a forward path can’t be constructed.
+proj=pipeline +step +inv +proj=urm5
Will result in an error since Urmaev V does not have an inverse operation defined.
4. Parameters added before the first `+step` are global and will be applied to all steps.
In the following the GRS80 ellipsoid will be applied to all steps.
+proj=pipeline +ellps=GRS80 +step +proj=cart +step +proj=helmert +x=10 +y=3 +z=1 +step +proj=cart +inv +step +proj=merc