Vertical grid shift¶
Added in version 5.0.0.
Change Vertical datum change by grid shift
Alias |
vgridshift |
Domain |
3D and 4D |
Input type |
Geodetic coordinates (horizontal), meters (vertical), decimalyear (temporal) |
Output type |
Geodetic coordinates (horizontal), meters (vertical), decimalyear (temporal) |
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:
+proj=vgridshift +grids=egm96_15.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 precedence over the global model:
+proj=vgridshift +grids=@dvr90.gtx,egm96_15.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 precedence over the grids in the back of the
list.
PROJ supports the GeoTIFF (Geodetic TIFF grids (GTG)) and GTX file formats for vertical grid corrections. Details about all the formats can be found in the GDAL documentation. GDAL both reads and writes the format. Using GDAL for construction of new grids is recommended.
Temporal gridshifting¶
Added in version 5.1.0.
By initializing the vertical 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=vgridshift +grids=iceland2008.gtx +t_epoch=2008.4071 +t_final=2009.0
63.992 -21.014 10.11 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=vgridshift +grids=earthquake_1.gtx +t_epoch=2010.421
+step +proj=vgridshift +grids=earthquake_2.gtx +t_epoch=2013.853
+step +proj=vgridshift +grids=earthquake_3.gtx +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 GeoTIFF (Geodetic TIFF grids (GTG)) or GTX formats.
Optional¶
- +t_epoch=<time>¶
Central epoch of the transformation.
Added 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.
Added in version 5.1.0.
- +multiplier=<value>¶
Specify the multiplier to apply to the grid value in the forward transformation direction, such that:
(1)¶\[Z_{target} = Z_{source} + multiplier \times gridvalue\]The multiplier can be used to control whether the gridvalue should be added or subtracted, and if unit conversion must be done (the multiplied gridvalue must be expressed in metre).
Note that the default is -1.0 for historical reasons.
Added in version 5.2.0.