Hi Ethan,
I'm afraid I've never actually read ISO 19111 (I don't read many
standards, I let other people tell me about them then I pretend I know
what I'm talking about ;-)) so I can't point you to the information.
I can however suggest that you look at a recent conversation I had on
this subject with the (very helpful) GeoTools guys on the GeoTools
mailing list where this was discussed in more detail.
(ISO 19111 distinguishes between "compound CRSs", which are 2D
horizontal + 1D vertical CRSs, and "true" 3D CRSs, which are combined.
If your horizontal CRS is geographic and your vertical CRS is "height
above ellipsoid" then ISO19111 doesn't allow you to separate them,
although GeoTools actually relaxes this for practical purposes. I
can't say I understand this fully yet.)
The conversation is here
http://www.nabble.com/Vertical-and-3D-coordinate-reference-systems-td18859312.html,
but I think the main issue is that you can't separate the horizontal
and vertical components if you're doing a coordinate transform that
involves a datum shift (e.g. WGS84 lat-lon to British National Grid).
Clear as mud?
Jon
On Thu, Aug 7, 2008 at 9:58 PM, Ethan Davis <edavis at unidata.ucar.edu> wrote:
> 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 Fri Aug 08 2008 - 01:11:14 BST
