Maik,
You have wandered into an area that has a few hot-button topics
associated with it. I'll see if I can help clarify without riling anyone
else. :)
Here we go, in order of your questions:
The spherical Earth is a default. It harkens back to CF's origins in the
modeling community, where they often use a spherical Earth.
The latitude and longitude coordinates variables, if they are true
coordinate variables (and not auxiliary coordinate variables) must each
be monotonic. If they are auxiliary coordinate variables, I don't think
there is any constraint.
The naming "grid_mapping" is (personally) a less-than-perfect choice.
The variable named by the grid_mapping attribute contains Coordinate
Reference System (CRS) information. Even a latitude-longitude CRS needs
to be declared, as there are a number of these. (Their differences are
largely ignorable if you are working on 2.5 degree centers, but that is
an assumption that may not be warranted with observation data.) The
variable named by the grid_mapping attribute contains the declaration of
the horizontal datum, whether it is for a projected CRS or a
latitude-longitude CRS. Declarations of the vertical datum are still
being ironed out.
Depending on what you were wanting to do, you should be able to use the
contents of the variable named by the grid_mapping attribute, along with
a geographic coordinate system package, to convert the values in
variables containing X-Y coordinates or latitude-longitude coordinates
into any other reachable CRS. (Not all CRSs are reachable from all other
CRSs, but that's a different problem.) If you have both X-Y and
latitude-longitude coordinates in your file, the software can use
whichever it chooses, and can convert to another CRS however it choses.
If you only have latitude-longitude coordinates in your file, the
software can use those straight, or can convert to another CRS, but for
precise conversion you need to know which ellipsoid to use (and where
the origin is located, etc) for the latitudes and longitudes in the file.
Does that help?
Grace and peace,
Jim
On 5/29/14, 6:30 PM, Maik Riechert wrote:
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
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