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Geocentric grid shift

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New in version 7.0.0.

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Geocentric translation using a grid shift

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Alias

xyzgridshift

Domain

3D

Input type

Cartesian coordinates

Output type

Cartesian coordinates

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Perform a geocentric translation by bilinear interpolation of dx, dy, dz +translation values from a grid. The grid is referenced against either the +2D geographic CRS corresponding to the input (or sometimes output) CRS.

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This method is described (in French) in [NTF_88] +and as EPSG operation method code 9655 in [IOGP2018] (§2.4.4.1.1 +France geocentric interpolation).

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The translation in the grids are added to the input coordinates in the forward direction, +and subtracted in the reverse direction. +By default (if grid_ref=input_crs), in the forward direction, the input coordinates +are converted to their geographic equivalent to directly read and interpolate from +the grid. In the reverse direction, an iterative method is used to be able to find +the grid locations to read. +If grid_ref=output_crs is used, then the reverse strategy is applied: iterative +method in the forward direction, and direct read in the reverse direction.

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Example

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NTF to RGF93 transformation using gr3df97a.tif grid

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+proj=pipeline
+    +step +proj=push +v_3
+    +step +proj=cart +ellps=clrk80ign
+    +step +proj=xyzgridshift +grids=gr3df97a.tif +grid_ref=output_crs +ellps=GRS80
+    +step +proj=cart +ellps=GRS80 +inv
+    +step +proj=pop +v_3
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Parameters

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The ellipsoid parameters should be the ones consistent with grid_ref. +They are used to perform a geocentric to geographic conversion to find the +translation parameters.

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Required

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++ellps=<value>
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The name of a built-in ellipsoid definition.

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See Ellipsoids for more information, or execute +proj -le for a list of built-in ellipsoid names.

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Defaults to “GRS80”.

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++grids=<list>
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Comma-separated list of grids to load. If a grid is prefixed by an @ the +grid is considered optional and PROJ will the not complain if the grid is +not available.

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Grids are expected to be in GeoTIFF format. If no metadata is provided, +the first, second and third samples are assumed to be the geocentric +translation along X, Y and Z axis respectively, in metres.

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Optional

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++grid_ref=input_crs/output_crs
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Specify in which CRS the grid is referenced to. The default value is +input_crs. That is the grid is referenced in the geographic CRS corresponding +to the input geocentric CRS.

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If output_crs is specified, the grid is referenced in the geographic CRS corresponding +to the output geocentric CRS. This is for example the case for the French +gr3df97a.tif grid converting from NTF to RGF93, but referenced against RGF93. +Thus in the forward direction (NTF->RGF93), an iterative method is used to find +the appropriate shift.

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++multiplier=<value>
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Specify the multiplier to apply to the grid values. Defaults to 1.0

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