[CF-metadata] Vertical datums (again)

From: Jonathan Gregory <j.m.gregory>
Date: Wed, 19 Feb 2014 09:16:54 +0000

Dear John and Jim

Thanks for your comments. John, I agree with your classification. I think the
concepts to which we give standard names are really quite simple. They are
distances above or below a reference surface (i.e. a surface which is a 2D
function of geographical position). The distance is signed and we consistently
use "height" to mean positive up and "depth" to mean positive down. It appears
from Jim's email that orthometric, geodetic and geometric indicate which
reference surface is being used. Hence we don't need those words, because
they are redundant if we say explicitly which surface is being used. I think it
is better to be explicit because it's more self-describing, which is a
principle of CF metadata. However, it would be useful to mention these words in
the definitions of the standard names as additional information to relate the
standard names to other terminology.

Therefore I don't think there is anything incorrect in the existing standard
names, actually. However if they are potentially misleading (in case users
don't read the definitions) we can consider clarifying them. That's what I
meant in my previous email. In particular, we could rename altitude as
height_above_geoid, using aliases. There are 14 standard names which use the
word altitude, all meaning height_above_geoid. Is that worth doing or not?

My own opinion is that there is no need to rename plain "height" or "depth"
(without "above_X" or "below_X") which are defined to mean relative to the
surface. There is also no need to rename plain "surface" i.e. the bottom of the
atmosphere, which is used in dozens or hundreds of existing standard names in
that sense.

> in answer to your question about geoids, WKT has a clause ?VERT_CS? where a vertical datum is defined. A geoid is, in most cases, realized as a lat/lon grid in one or more files (often tiled), and is only identified by name.

Thanks. Yes, I appreciate it can only be identified by name, because it is
a non-mathematical 2D dataset, unlike the ellipsoid, which is a 2D function of
position specified by a couple of parameters. So your answer suggests that
WKT does not indicate whether the vertical datum includes a geoid definition or
not. Is that right? It just gives a name, and the user or the software has to
know whether this name implies a particular geoid, or is only a reference
ellipsoid. Correct?

Best wishes and thanks

Jonathan
Received on Wed Feb 19 2014 - 02:16:54 GMT