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diff --git a/docs/source/operations/transformations/vgridshift.rst b/docs/source/operations/transformations/vgridshift.rst new file mode 100644 index 00000000..e9f78310 --- /dev/null +++ b/docs/source/operations/transformations/vgridshift.rst @@ -0,0 +1,41 @@ +.. _vgridshift: + +================================================================================ +Vertical grid shift +================================================================================ + +Change Vertical datum change by grid shift + ++-----------------+--------------------------------------------------------------------+ +| **Input type** | Geodetic coordinates (horizontal), meters (vertical). | ++-----------------+--------------------------------------------------------------------+ +| **Output type** | Geodetic coordinates (horizontal), meters (vertical). | ++-----------------+--------------------------------------------------------------------+ +| **Options** | ++-----------------+--------------------------------------------------------------------+ +| `+grids` | Comma-separated list of grids to load. | ++-----------------+--------------------------------------------------------------------+ + +The vertical grid shift is done by offsetting the vertical input coordinates by +a specific amount determined by the loaded grids. The simplest use case of the +horizontal grid shift is applying a single grid. Here we change the vertical +reference from the ellipsoid to the global geoid model, EGM96:: + + +vgridshift +grids=egm96_16.gtx + + +More than one grid can be loaded at the same time, for instance in the case where +a better geoid model than the global is available for a certain area. Here the +gridshift is set up so that the local DVR90 geoid model takes presedence over +the global model:: + + +vgridshift +grids=@dvr90.gtx,egm96_16.gtx + +The ``@`` in the above example states that the grid is optional, in case the grid +is not found in the PROJ search path. The list of grids is prioritized so that +grids in the start of the list takes presedence over the grids in the back of the +list. + +PROJ supports the GTX file format for vertical grid corrections. Details +about all the format can be found in the GDAL documentation. GDAL both reads and +writes the format. Using GDAL for construction of new grids is recommended. |