Heiko,
All of the heights are measured relative to a surface. I guess the proper type names are orthometric (normal to the geoid), geodetic (normal to the ellipsoid), and geocentric (relative to the ellipsoid, but not normal to it). In my personal on-the-job experience the label ?geometric? was usually equated to geocentric, but it could be equally used of geodetic, since both are geometric in construction.
Distances above the center of the Earth would be in a geocentric coordinate system and are not considered to be heights.
The point to take from all this is that geographic coordinate reference systems (CRSs) are quite complicated, and often under-considered. CF should do a good job of documenting the CRS used, and to be complete, we need to specify:
map projection (if X/Y coordinates are specified)
horizontal datum (specifies the ellipsoid and origin point used for the latitudes and longitudes - may not be valid globally)
vertical datum (specifies the ellipsoid or geoid used for heights - may be different than the horizontal datum!)
height type (if height is reported)
Right now, we don?t have attributes defined for all of these in the grid projection variable. There is a certain amount of redundancy to the above list, since a particular map projection usually implies a particular horizontal datum, and it would be unusual (for example) to report geodetic or geocentric heights if a geoidal vertical datum is used. In addition to all this, there is the case of cartesian coordinate systems such as Earth-Centered Fixed and Earth-Centered Inertial.
So we are dealing with a lack of sufficient vocabulary within CF right now. This has come up in Trac Ticket 107 as well.
Grace and peace,
Jim
Visit us on
Facebook Jim Biard
Research Scholar
Cooperative Institute for Climate and Satellites NC
North Carolina State University
NOAA's National Climatic Data Center
151 Patton Ave, Asheville, NC 28801
e: jbiard at cicsnc.org
o: +1 828 271 4900
On Feb 10, 2014, at 3:30 AM, Heiko Klein <Heiko.Klein at met.no> wrote:
> Dear all,
>
> thanks for your answers. I usually use the standard-name table for parameters than for the reference-system. I really would like to see the coordinate-axis standard-names as part of the CF-documentation, since we have already most of them there (ocean_sigma_coordinate, projection_x_axis, time). I'm not aware of a FAQ for CF?
>
> I've never seen a file with geometrical height. If I understand Jim correctly, values are in the order of magnitude 6371km (=earth radius).
>
> Reading the definition of altitude at wikipedia, it seems to be a equivalent to height_above_reference_ellipsoid. But, as John points out, the CF standard-name descriptions sayt the geoid is defined to be MSL, so it's fine for me, unless we want to have different standard_names for reference-systems than for parameters.
>
> Best regards,
>
> Heiko
>
>
> On 2014-02-07 19:49, Jim Biard wrote:
>> Geometrical height is sometimes used synonymously with geodetic height,
>> but the strict interpretation is height above the ellipsoid surface
>> along a line from the center of the Earth to the surface. Geodetic
>> heights are normal to the ellipsoid surface.
>>
>> CICS-NC <http://www.cicsnc.org/>Visit us on
>> Facebook <http://www.facebook.com/cicsnc> *Jim Biard*
>> *Research Scholar*
>> Cooperative Institute for Climate and Satellites NC <http://cicsnc.org/>
>> North Carolina State University <http://ncsu.edu/>
>> NOAA's National Climatic Data Center <http://ncdc.noaa.gov/>
>> 151 Patton Ave, Asheville, NC 28801
>> e: jbiard at cicsnc.org <mailto:jbiard at cicsnc.org>
>> o: +1 828 271 4900
>>
>>
>>
>>
>>
>> On Feb 7, 2014, at 1:25 PM, Jonathan Gregory <j.m.gregory at reading.ac.uk
>> <mailto:j.m.gregory at reading.ac.uk>> wrote:
>>
>>> Dear Heiko
>>>
>>>> 1. height above ground
>>> has standard_name of height, as you say
>>>
>>>> 2. height above mean sea level
>>> I don't think we have a standard name for this yet, but
>>> height_above_sea_level
>>> would be consistent with existing names. For example, there is a
>>> stdname of
>>> sea_surface_height_above_sea_level.
>>>
>>>> 3. depth below surface
>>> is depth, as you say.
>>>
>>>> 4. geometrical height
>>> What does this mean? i.e. height above what reference level?
>>>
>>> altitude is height above the geoid. Maybe that is geometrical height?
>>>
>>>> And for pressure vertical coordinates: is the correct standard_name
>>>> 'air_pressure'?
>>>
>>> Yes.
>>>
>>>> Could these eventually be mentioned in the Convention besides the
>>>> standard_names for dimensionless vertical coordinates?
>>>
>>> This sounds to me like another possible entry for a FAQ. Would that be
>>> a good
>>> idea?
>>>
>>> Best wishes
>>>
>>> Jonathan
>>> _______________________________________________
>>> CF-metadata mailing list
>>> CF-metadata at cgd.ucar.edu <mailto:CF-metadata at cgd.ucar.edu>
>>> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
>>
>>
>>
>> _______________________________________________
>> CF-metadata mailing list
>> CF-metadata at cgd.ucar.edu
>> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
>>
>
> --
> Dr. Heiko Klein Tel. + 47 22 96 32 58
> Development Section / IT Department Fax. + 47 22 69 63 55
> Norwegian Meteorological Institute http://www.met.no
> P.O. Box 43 Blindern 0313 Oslo NORWAY
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <
http://mailman.cgd.ucar.edu/pipermail/cf-metadata/attachments/20140210/9caa640d/attachment-0001.html>
Received on Mon Feb 10 2014 - 09:04:25 GMT
