Relationship with GGXF

[desruisseaux]

This is not really an issue, but rather a note. The BAG format seems to have some similarities with an OGC standard currently in final vote state: GGXF : Gridded Geodetic data eXchange Format. Like BAG, the GGXF format is based on HDF5 and supports nested grids of different resolutions. GGXF aims to replace the NADCON and NTv2 formats for encoding datum shifts. It supports coordinate transformations between different tidal surfaces (Low Water, Mean Low Water, etc.) defined by IHO. I presume that transformations from sea floor to a tidal surface using bathymetric data could also count as a vertical datum shift.

GGXF contributors include some of the peoples behind ISO 19111 and EPSG geodetic dataset. I think that there is good chances to see GGXF files referenced from the EPSG database in the future, and to see at least some coordinate transformation libraries supporting it natively. In particular, ESRI is working on GGXF support in their products.

I'm not sure that GGXF contains all metadata that BAG provides, so it may not be a replacement. Those two formats may not pursue the exact same goal neither, because GGXF is about coordinate transformations between two Coordinate Reference Systems (CRS). But it may be useful to establish a mapping between BAG and GGXF. If the latter become sufficiently adopted in software, a tool doing the translation from BAG to GGXF may help to integrate those data in existing referencing libraries.