Jonathan,
Thanks for the detailed explanation (and analogy) of why it's useful
to tack on the "_above_geoid" or "_above_ellipsoid" or
"_above_tidal_datum" to the standard-name.
So we do that and then specify the geoid, ellipsoid or tidal datum
elsewhere in the grid_mapping variable, right?
geoid: NAVD88, GEOID93, GEOID96, USGG2009, etc
ellipsoid: WGS84, Airy 1830, Airy Modified 1849, etc
tidal_datum: MLLW, MLW, MTL, MHW, MHHW, etc
Older bathymetry data are almost always reported relative to
tidal_datums. Yes, this is a huge can of worms. It's why vertical
datum software such as
http://vdatum.noaa.gov/
are popular, so that folks can convert from something like MLLW to NAVD88.
-Rich
On Mon, Feb 10, 2014 at 1:30 PM, Jonathan Gregory
<j.m.gregory at reading.ac.uk> wrote:
> 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
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--
Dr. Richard P. Signell (508) 457-2229
USGS, 384 Woods Hole Rd.
Woods Hole, MA 02543-1598
Received on Mon Feb 10 2014 - 11:56:48 GMT
