# Horizontal grid shift¶

New in version 5.0.0.

Change of horizontal datum by grid shift.

 Domain 2D, 3D and 4D Input type Geodetic coordinates (horizontal), meters (vertical), decimalyear (temporal) Output type Geodetic coordinates (horizontal), meters (vertical), decimalyear (temporal)

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:

+proj=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:

+proj=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 precedence 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 and/or driver source code. GDAL reads and writes all three formats. Using GDAL for construction of new grids is recommended.

## Temporal gridshifting¶

New in version 5.1.0.

By initializing the horizontal gridshift operation with a central epoch, it can be used as a step function applying the grid offsets only if a coordinate is transformed from an epoch before grids central epoch to an epoch after. This is handy in transformations where it is necessary to handle deformations caused by seismic activity.

The central epoch of the grid is controlled with +t_epoch and the final epoch of the coordinate is set with +t_final. The observation epoch of the coordinate is part of the coordinate tuple.

Suppose we want to model the deformation of the 2008 earthquake in Iceland in a transformation of data from 2005 to 2009:

echo 63.992 -21.014 10.0 2005.0 | cct +proj=hgridshift +grids=iceland2008.gsb +t_epoch=2008.4071 +t_final=2009.0
63.9920021 -21.0140013 10.0 2005.0


Note

The timestamp of the resulting coordinate is still 2005.0. The observation time is always kept unchanged as it would otherwise be impossible to do the inverse transformation.

Temporal gridshifting is especially powerful in transformation pipelines where several gridshifts can be chained together, effectively acting as a series of step functions that can be applied to a coordinate that is propagated through time. In the following example we establish a pipeline that allows transformation of coordinates from any given epoch up until the current date, applying only those gridshifts that have central epochs between the observation epoch and the final epoch:

+proj=pipeline +t_final=now
+step +proj=hgridshift +grids=earthquake_1.gsb +t_epoch=2010.421
+step +proj=hgridshift +grids=earthquake_2.gsb +t_epoch=2013.853
+step +proj=hgridshift +grids=earthquake_3.gsb +t_epoch=2017.713


Note

The special epoch now is used when specifying the final epoch with +t_final. This results in coordinates being transformed to the current date. Additionally, +t_final is used as a global pipeline parameter, which means that it is applied to all the steps in the pipeline.

In the above transformation, a coordinate with observation epoch 2009.32 would be subject to all three gridshift steps in the pipeline. A coordinate with observation epoch 2014.12 would only by offset by the last step in the pipeline.

## Parameters¶

### Required¶

+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 CTable2, NTv1 or NTv2 format.

### Optional¶

+t_epoch=<time>

Central epoch of the transformation.

New in version 5.1.0.

+t_final=<time>

Final epoch that the coordinate will be propagated to after transformation. The special epoch now can be used instead of writing a specific period in time. When now is used, it is replaced internally with the epoch of the transformation. This means that the resulting coordinate will be slightly different if carried out again at a later date.

New in version 5.1.0.