Hi Jonathan,
John proposed a new object that looks like this:
> int Dutch_CoordSystem;
> :coordinates = "dutch_x dutch_y";
> :grid_mapping = EPSG19914;
That's not using the grid_mapping variable to point to the projection
coordinates. It's defining a new object that contains both the projection
coordinates and the grid mapping. John's terminology for this object is a
"coordinate system". To me the new object is really a "grid", i.e., a set
of coordinate values and a description of the coordinate system (via the
grid_mapping) that allows us to locate the points in space. In the
terminology that you've described below the object is a container for a
"grid" and a "mapping". Regardless of the eventually adopted nomenclature,
I think John's got the syntax right. The example you present of several
sets of coordinates using the same projection would be represented as:
int grid1
:coordinates = "grid1_x grid1_y";
:grid_mapping = EPSG19914;
int grid2
:coordinates = "grid2_x grid2_y";
:grid_mapping = EPSG19914;
> The grid is a property of the
> data variable, because it is the only place the particular combination of
> coordinate variables is specified that constitute the grid.
But the grid is not a property of the data variable. And the objects John
proposes gives us a way to explicitly express the combination of coordinate
variables that constitute a grid rather than rely on the specification that
is implicit in a data variable.
Brian
On Fri, Dec 01, 2006 at 06:35:31PM +0000, Jonathan Gregory wrote:
> Dear John and Brian
>
> Oh dear, I am going to disagree again. Yes, I can see there is an idea of a
> grid which could exist separately from a data variable. However, I don't think
> it really makes sense to point to the projection coordinates from the
> grid_mapping variable, because there could be several sets of projection
> coordinates (e.g. velocity and temperature grids) that used the same
> grid_mapping variable, because they shared the same map projection. I think
> that the ideas of "grid" (a particular set of coordinates) and "mapping" (a
> transformation between one space and another) are distinct. The grid_mapping
> variable presently deals with the mapping. The grid is a property of the
> data variable, because it is the only place the particular combination of
> coordinate variables is specified that constitute the grid.
>
> Brian wrote
> > I routinely create files that contain only grid data. A typical use of
> > these files is as input to an application that does regridding. Such an
> > application may, for example, take an input datafile at some resolution, an
> > input grid file at a different resolution, and produce an output datafile
> > at the resolution contained in the grid file.
>
> I can see that is useful, and that you want a way to specify a grid, which
> means that particular combination of coordinate variables. A CF-compliant
> way to do it would be to have a file with a single data variable in it,
> but presumably you want to avoid that because the data variable would be a
> waste of space as it is of no interest. Is that right?
>
> Cheers
>
> Jonathan
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Received on Fri Dec 01 2006 - 12:42:20 GMT
