Jon,
Vertical datums were originially going to be handled by CF Trac ticket 18:
http://cf-pcmdi.llnl.gov/trac/ticket/18
but then were excluded in the interest of moving forward on the
horizontal datum issues. I think we would need a new ticket for the
vertical datum issues (and of course, someone to champion the
process).
-Rich
On Wed, Aug 6, 2008 at 5:28 PM, Seth McGinnis <mcginnis at ucar.edu> wrote:
> Hi Jon,
>
> I don't know about the other cases, but I believe that if
> you want to specify that
> you're using a spherical earth or the WGS84 ellipsoid, you
> do it by including
> parameters like earth_radius (for spherical) or
> semi_major_axis and
> inverse_flattening (for WGS84) as attributes in the
> grid_mapping that
> describes your map projection. See Appendix F and examples
> 5.8 and 5.9.
>
> That would implicitly define the vertical coordinate
> system, wouldn't it?
>
> (I'm just an end-user, though, not an expert -- I may well
> be wrong.)
>
> --Seth
>
> ----
> Seth McGinnis
> Associate Scientist
> ISSE / NCAR
> ----
>
>
>
>>Hi all,
>>
>>Looking through the CF conventions document (1.3) there
> seems to be no
>>way to distinguish some of the possible vertical
> coordinate systems.
>>CF distinguishes dimensioned vertical coordinates (e.g.
> height or
>>depth in metres) from dimensionless coordinates (e.g.
> sigma) but does
>>not seem to distinguish between the different possible
> heights/depths.
>> Here are a few possibilities that I think can't be
> distinguished in CF:
>>
>>- Depth below the geoid (also, which geoid?)
>>- Depth below an ellipsoidal approximation of the earth
> (e.g. WGS84)
>>- Depth below instantaneous sea surface (e.g. where depth
> is inferred from pressure)
>>- Depth below a spherical approximation of the earth (used
> in many models, I believe)
>>
>>(the same goes for heights of course) Am I right or have
> I
>>misunderstood something? To provide a case study, a
> colleague of mine
>>is combining ocean model results with other GIS data
> sources in order
>>to drive a high-resolution (50m in the horizontal) model
> of an
>>estuary. The ocean model data files (which I think are
> CF-compliant)
>>don't give a vertical datum, and this leads to an
> uncertainty in
>>*horizontal* positioning of around 100m, which is two
> whole grid cells!
>>
>>This problem is most marked, of course, in very
> high-resolution
>>studies - it's not likely to be too important for
> larger-scale work,
>>although it does seem important to be able to describe
> vertical
>>positioning as accurately as we can describe horizontal
>>and temporal positioning.
>>
>>Any thoughts?
>>Jon
>>
>>--
>>--------------------------------------------------------------
>>Dr Jon Blower Tel: +44 118 378 5213 (direct line)
>>Technical Director Tel: +44 118 378 8741 (ESSC)
>>Reading e-Science Centre Fax: +44 118 378 6413
>>ESSC Email: j.d.blower at reading.ac.uk
>>University of Reading
>>3 Earley Gate
>>Reading RG6 6AL, UK
>>--------------------------------------------------------------
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--
Dr. Richard P. Signell (508) 457-2229
USGS, 384 Woods Hole Rd.
Woods Hole, MA 02543-1598
Received on Wed Aug 06 2008 - 15:37:31 BST
