Dear Jim and Rich Many thanks for your helpful comments. I see a prospect of my understanding things a bit better than before!
Jim says that a vertical datum always has a reference ellipsoid. Sometimes a vertical datum might *be* a reference ellipsoid. Sometimes it is a geoid, and in that case, is it accompanied by a reference ellipsoid as part of the definition of the vertical datum? Rich comments that a vertical datum could be orthometric. If I've understood Jim correctly, orthometric describes how you measure the height wrt the reference surface. It is not a third type of surface, in addition to geoid and reference ellipsoid. Is that right? Tides define a different sort of reference surface from geoid and ellipsoid. Are there also vertical datums which involve tidal levels in their definition? > why can't we just say > "sea_surface_height_above_datum" or just "sea_surface_height" and then > specify the vertical datum, no matter what it is? I don't think we should do so because height wrt geoid and height wrt ellipsoid are rather different quantities. For that reason they have different standard names (altitude and height_above_reference_ellipsoid, and there is also a standard name of geoid_height_above_reference_ellipsoid). They are seriously different in value, aren't they? - by 100s of metres, so you have to know which one you are dealing with. If they had the same standard name, a height wrt geoid from one data source and a height wrt ref ellipsoid from another might be regarded as comparable quantities, which could be a serious error. Of course I recognise that the stdname is not the only metadata one should consult, but it is the first point of call. To make an analogy, suppose we just defined height as "vertical distance above something", with something defined elsewhere. Then altitude and height above sea floor would be synomymous standard names. I don't think that would be as helpful to the data-analyst. I do think, however, that it's acceptable to define the geoid or reference ellipsoid in another place (the grid mapping) from the standard name. This is still a risk, because heights on different vertical datums might be treated as comparable they aren't, but on the other hand there are cases where heights on different vertical datums could be compared e.g. if they come from models with a different shape for the Earth. We can meet Rich's need, I think, if we provide a way for the grid_mapping to specify vertical datums which involve the geoid being implied. Best wishes Jonathan _______________________________________________ CF-metadata mailing list CF-metadata@cgd.ucar.edu http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata