[PROJ] Proposal for a geodetic deformation model format

[Chris Crook]

Hi All

This is an invitation to comment on a straw man format for encoding a deformation model such that it can be used directly by transformation software such as PROJ.

To view and comment on the proposal go to:

https://drive.google.com/file/d/1mRr2gPAKGOgzEqnTf07n9YlSYWgioH1x/view?usp=sharing

This is offered for discussion before formalising intohttps://webmail.linz.govt.nz/owa/?ae=Item&t=IPM.Note&a=New# a proposal for development.  This is a capability that we at Land Information New Zealand have long sought - the GeoTIFF development (link below) has provides an essential capability to support this, which is a format that provide nested grids of with multiple parameters at each node.  Previous geodetic formats have either only supported single grids, or have been limited to 2 dimensional displacement.

This also follows from an optimistic PROJ enhancement request I raised a 18 months ago  - https://github.com/OSGeo/PROJ/issues/1001.  That was proposing a custom binary format for encoding a deformation model.  This proposal is a much simpler implementation that looks relatively easy to implement and requires very little additional supporting capability as it is using existing (or under development) formats for which tooling will be available.

This proposal builds on the GeoTIFF format to allow a generic specification of a time dependent coordinate transformation.

When I started writing this I was thinking the best approach was to encode the deformation model into a single GeoTIFF file using additional TIFF or GDAL metadata tags to identify which grids belonged to which deformation model component, and to their time behaviour.  In the course of writing this I changed my mind to favour a multiple grid implementation employing a single CSV file which references one or more GeoTIFF formatted nested grid files.  The rationale for this choice is described in the straw man proposal.

Curiously this is not dissimilar to the grid catalog capability Even identifies for decommissioning in GeoTIFF proposal (link below) .  However this is a more sophisticated capability in its support for different time behaviours of the component grids.

Land Information New Zealand are keen to support development of this capability, but first we want to get input from the community to ensure the capability is as useful across the geodetic community as it can be.

Thanks in advance for looking at this and offering any criticisms/suggestions/recommendations for improvement
Chris Crook

GeoTiff proposal: https://github.com/rouault/PROJ/blob/rfc4_remote_and_geotiff_grid/docs/source/community/rfc/rfc-4.rst
Grid catalog functionality: https://github.com/rouault/PROJ/blob/rfc4_remote_and_geotiff_grid/docs/source/community/rfc/rfc-4.rst#dropping-grid-catalog-functionality

Previous related conversations:
https://lists.osgeo.org/pipermail/proj/2019-November/009005.html
https://lists.osgeo.org/pipermail/proj/2019-November/009026.html

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