[PROJ] Lowest Astronimical Tide reference surface in Geodetic TIFF Grid

[Lesparre, Jochem]

I wrote:

> To prevent users to make different choices in similar use cases, the documentation should indicate what to do for Lowest Astronomical Tide (LAT) or other Chart Datum grids.



ER wrote:

> Cf https://github.com/OSGeo/PROJ/pull/3668 + https://github.com/OSGeo/PROJ-data/pull/97



Thanks! This means that the Geodetic TIFF Grid for Chart Datum of Norway (and possibly also the one of Denmark that Kristian Evers is working on) wrongly use type VERTICAL_OFFSET_VERTICAL_TO_VERTICAL instead of VERTICAL_OFFSET_GEOGRAPHIC_TO_VERTICAL. But as that works too, it’s not a big deal.



I have two questions:

  *   Are you sure about the tag “hydroid_height”? A Google search (https://www.google.com/search?q=%22hydroid+height%22) gives only 1 relevant hit. I have no opinion on this, as it is not my expertise. I’m just helping the people at our hydrographic service.
  *   Why limit the description to only predefined options? I can imagine that one could want to use a reference surface that is not a geoid nor a hydroid, for example an ellipsoidal DTM grid to transform to/from Above Ground Level heights. So it might be easier to allow any description to prevent future problems.



I will probably use type VERTICAL_OFFSET_GEOGRAPHIC_TO_VERTICAL with a new description (e.g. hydroid_height) for the tif grid. But I’ll also make a gtx grid for users of PROJ versions that don’t accept the new description. I might use the gtx grid for application in EPSG for that reason. That might be easier anyway, since there are not yet tif grids in EPSG as far as I know.



Jochem




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