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diff --git a/docs/source/operations/transformations/hgridshift.rst b/docs/source/operations/transformations/hgridshift.rst new file mode 100644 index 00000000..7f3c9895 --- /dev/null +++ b/docs/source/operations/transformations/hgridshift.rst @@ -0,0 +1,39 @@ +.. _hgridshift: + +================================================================================ +Horizontal grid shift +================================================================================ + +Change of horizontal datum by grid shift. + ++-----------------+--------------------------------------------------------------------+ +| **Input type** | Geodetic coordinates. | ++-----------------+--------------------------------------------------------------------+ +| **Output type** | Geodetic coordinates. | ++-----------------+--------------------------------------------------------------------+ +| **Options** | ++-----------------+--------------------------------------------------------------------+ +| `+grids` | Comma-separated list of grids to load. | ++-----------------+--------------------------------------------------------------------+ + +The horizontal grid shift is done by offsetting the planar 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:: + + +hgridshift +grids=nzgr2kgrid0005.gsb + + +More than one grid can be loaded at the same time, for instance in case the +dataset needs to be transformed spans several countries. In this example grids +of the continental US, Alaska and Canada is loaded at the same time:: + + +hgridshift +grids=@conus,@alaska,@ntv2_0.gsb,@ntv_can.dat + +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 CTable2, NTv1 and NTv2 files for horizontal grid corrections. Details +about all three formats can be found in the GDAL documentation. GDAL reads and +writes all three formats. Using GDAL for construction of new grids is recommended. |