> Ok, so heres a summary of what I understand so far about this issue:
>
> 1. For geolocation of model data, calculating location using
> geodetic/geographic latitude which uses an ellipsoidal earth,
> versus using geocentric latitude which uses a spherical
> earth, can differ up to 20 km at mid-latitude. Differences of
> location using different ellispoids/datums can be as much as
> 300-500 m.
>
> 2. Global models generally are not accurate enough to worry
> about these differences, and so can use any reasonable Earth
> shape. Since input data probably use geographic lat/lon
> locations, there may be some good reasons to prefer an
> ellisoidal model as default, such as WGS80.
>
> 3. For regional and local models that need to be accurate at
> these resolutions, the assumed earth shape should be
> specified in the file metadata. CF intends to specify a
> standard for this.
>
> Now to switch to more of my own thoughts:
>
> 1. Tentatively, i will probably assume WGS80 if not
> explicitly specified.
>
> 2. We had already talked about adding some FGDC compliant
> "earth shape"
> parameters like semimajor axis and inverse flattening. this
> is worth finishing. These would be attached to the
> "grid-mapping" dummy variable that specifies the projection.
>
> 3. We should allow an attribute that refers to the EPSG codes
> in the same place, whose value is any valid code from the EPSG table.
> Specialized clients will know how to interpret these codes.
> Best practice would be to also include explicitly the
> semimajor axis and inverse flattening, so less specialized
> clients can still get "pretty good" accuracy.
>
John,
This all sounds good, but I think it's "WGS84" not "WGS80".
Should we make any effort in reusing some of the terminology for spatial reference frame and ellipsoid abbreviations that may be in use elsewhere?
We did this for GeoVRML and it has helped with allowing client applications to properly interpret the data. Please see:
http://www.geovrml.org/1.1/doc/defs.html.
-Mike
--
Mike McCann (mccann at mbaridotorg)
GeoSpatial working group, Web 3D Consortium
Monterey Bay Aquarium Research Institute
7700 Sandholdt Road
Moss Landing, CA 95039-9644
Voice: (831) 775-1769 http://www.web3d.org/
Received on Tue Apr 12 2005 - 11:17:59 BST
