[CF-metadata] Clarifications on latitude_longitude

From: Maik Riechert <maik.riechert>
Date: Fri, 30 May 2014 00:30:33 +0200

Hi,

I am confused about the following sentence in CF 1.6 under the
Latitude-Longitude heading in appendix F:

"This grid mapping defines the canonical 2D geographical coordinate
system based upon latitude and longitude coordinates on a spherical Earth."

First, why "spherical"? In example 5.9 it is used with WGS84.

Then, does the first sentence refer to a grid which must be regular? Can
it also be rectilinear? Or even irregular?

I think my main confusion is that latitude_longitude and
rotated_latitude_longitude are probably not projections but the rest of
the grid mappings is. Yet there is no clear differentiation between the
two types in appendix F.

I think the term "grid mapping" also adds to my confusion. To me, it
means that I define a mapping from my data array (the grid) to
latitude/longitude coordinates in a descriptive way, that is, as an
implicit transformation operation described by the parameters of the
grid mapping variable. In the case of the projections it makes sense.
You can completely ignore the latitude/longitude arrays and just use the
grid mapping to recompute the latitudes/longitudes (if you were a
software that draws the data on a map). Right? But for
latitude_longitude and rotated_latitude_longitude, this is not the case.
Here the software would be required to use the latitude/longitude
arrays. Is this right? If yes, then should I consider latitude_longitude
as a datum definition with an unknown projection?

I hope I could explain my confusion while not confusing you too.

Thanks for your time,
Maik
Received on Thu May 29 2014 - 16:30:33 BST