Hi Jon,
Jon Blower wrote:
> Hi Philip,
>
>> This situation is especially pertinent to the complex realm of coordinate
>> reference systems, where it is difficult to focus on one particular facet -
>> vertical coordinate systems, say - without also having to consider other
>> facets - horizontal coordinate systems, geodetic datums, ellipsoids,
>> projections, and so on. It's an all-or-nothing kind of problem domain.
>>
> This is a very good point - in fact ISO 19111 says that you can't
> separate horizontal and vertical CRSs in some cases. I'm wary of
> making such proposals myself because I frankly don't understand all
> the issues properly (we need some geodesists or GIS experts or
> something). So it's a tricky one.
>
Can you point out where in ISO 19111 it discusses the relation between
horizontal and vertical CRSs?
I see the various subtypes of CRS: geodetic, vertical, engineering, and
image (which all are based on some kind of datum); as well as some more
complex CRS: derived, projected, and compound (which are all based on
one or more of the simpler types). To make a 3D CRS you combine a
horizontal CRS and a vertical CRS with a compound CRS. My impression has
always been that vertical CRSs are very separate from horizontal CRSs.
I did find in Annex B 1.2.1 a discussion of the relation between
vertical datum and ellipsoidal height:
"Vertical CRSs make use of the direction of gravity to define the
concept of height or depth. By implication therefore, ellipsoidal
heights (h) cannot be captured in a vertical CRS: ellipsoidal
heights cannot exist independently, but only as an inseparable part
of the 3D coordinate tuple defined in a geodetic 3D CRS."
I'm not sure I understand since I thought a vertical datum could be
based on an ellipsoidal earth. (Though I only see two based on an
ellipsoid out of over 100 vertical datums defined by EPSG. Maybe they
aren't that useful.)
I'm with you, I really don't have a good grasp of all the issues.
Is there a geodesist in the house?!
Ethan
--
Ethan R. Davis Telephone: (303) 497-8155
Software Engineer Fax: (303) 497-8690
UCAR Unidata Program Center E-mail: edavis at ucar.edu
P.O. Box 3000
Boulder, CO 80307-3000 http://www.unidata.ucar.edu/
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Received on Thu Aug 07 2008 - 14:58:05 BST
