[Gdal-dev] Recommendations for vertical data in GDAL

[Ray Gardener]

Further to exploring vertical data in GDAL, I went back to basics.

GDAL basically follows the OpenGIS Grid Coverages spec. This spec has 
some shortcomings, however:

- It has a heavy assumption that datasets are color images.
- It talks a lot about color image processing (filtering, etc.) when
   this is outside the domain of data access.
- Elevation data is mentioned but has no explicit band type.
- The CS_Units type is undefined (it's called "CS_Units" but that's it).
   So even if linear measure types indicate elevations, there's no way
   to be sure what to test the unit type ID against.
- GeoTIFF is highlighted as a well known binary georaster
   representation. But OGC does not support GeoTIFF completely
   since GeoTIFF has explicit elevation typing.
- It has offset/scale attributes which would be presumed used
   for conversion of elevation data between real units and pixel values.
   This is erroneous; nothing short of a procedurally definable transform
   is sufficiently flexible to generally support this conversion. Most
   HDRI images, e.g., have nonlinear pixel mappings.

Overall, elevations are mentioned and anticipated, but they are given 
"lip service" half-defined interfaces.

So... if we want GDAL to follow OGC (and that is generally a good 
thing), then OGC needs to be updated (or it needs to finish what it 
starts). But then I wouldn't expect anything to happen anytime soon, you 
know how standards bodies are.

Ray