# RFC 81: Support for coordinate epochs in geospatial formats

Author: | Even Rouault
Contact: | even.rouault @ spatialys.com
Started: | 2021-May-10
Last Updated: | 2021-Jun-18
Status: | Adopted
Target: | GDAL 3.4

## Summary

This RFC describes support for coordinate epochs in a few key geospatial formats and in GDAL/OGR API and utilities

## Motivation

A number of coordinate reference systems (CRS) are called "dynamic CRS", that is the coordinates of a point on the surface of the Earth in those CRS may change with time. To be unambiguous the coordinates must always be qualified with the epoch at which they are valid. The coordinate epoch is not necessarily the epoch at which the observation was collected.

Examples of dynamic CRS are `WGS 84 (G1762)`, `ITRF2014`, `ATRF2014`.

The generic EPSG:4326 WGS 84 CRS is also considered dynamic, although it is not recommended to use it due to being based on a datum ensemble whose positional accuracy is 2 meters, but prefer one of its realizations, such as WGS 84 (G1762)

At time of writing, no formats handled by GDAL/OGR have a standardized way of encoding a coordinate epoch. We consequently have made choices how to encode it, admittedly not always elegant, with the aim of being as much as possible backward compatible with existing readers. Those encodings might change if corresponding official specifications evolve to take this concept into account. But, as this is a bit of a chicken-and-egg problem ("why should we care about storing coordinate epoch if no software can make use of it ?"), let's start with this initial solution.

PROJ can handle a number of time-dependent transformations between static CRS and dynamic CRS, e.g a GDA2020 (static CRS/datum for Australia) to ATRF2014 (dynamic CRS/datum for Australia), taking into account plate motion. Having support to store coordinate epoch will make it easier to have more accurate coordinate transformation.

## Details

See Coordinate epoch support for impacts on the API, encoding of CRS in a number of formats, and impacts on existing utilities.

## Backward compatibility

At the API level, only additions.

Regarding creation of new datasets, no backward incompatibility at all if datasets are created without a coordinate epoch associated to their CRS, as its encoding is only added when needed.

And when it is used, it is done in a way that shouldn't affect existing reader. The only exception would be the FlatGeobuf format if writing a non-EPSG coded CRS, and with a coordinate epoch (a backport to the 3.3 branch will be done to avoid an error in that case).

## Documentation

New methods are documented, and the page mentioned in the Details paragraph will be part of the user documentation.

## Testing

New methods are tested. Formats extended with coordinate epoch support have also received new tests.

## Related PRs:

https://github.com/OSGeo/gdal/pull/4011

## Voting history

+1 from JukkaR and EvenR, -0 from HowardB and +0 from KurtS