Geocentric grid shift

Added in version 7.0.0.

Geocentric translation using a grid shift

Alias	xyzgridshift
Domain	3D
Input type	Cartesian coordinates
Output type	Cartesian coordinates

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.

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).

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.

Example

NTF to RGF93 transformation using gr3df97a.tif grid

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

Parameters

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.

Required

+ellps=<value>

The name of a built-in ellipsoid definition.

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

Defaults to "GRS80".

+grids=<list>

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.

Grids are expected to be in GeoTIFF format (Geodetic TIFF grids (GTG)). 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.

Optional

+grid_ref=input_crs/output_crs

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.

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.

+multiplier=<value>

Specify the multiplier to apply to the grid values. Defaults to 1.0