[CF-metadata] bounds

From: Brian Eaton <eaton>
Date: Wed, 28 May 2003 14:54:42 -0600

Hi Jonathan,

Here are a few comments on the first of your recent posts.

On Tue, May 27, 2003 at 09:38:34AM +0100, Jonathan Gregory wrote:
>
>
> Here's (yet) another idea. I am in effect *defining* 2D rectilinear to mean
> that there exist continuous 1D x and y coordinates, even if they aren't
> recorded in the file. That excludes the situation where the cells had two
> index dimensions but were not arranged as a 2D rectilinear array in any
> continuous 2D coordinate plane. I would call that non-rectilinear.

I find this terminology confusing. If I understand correctly, the grids
you are describing are the ones that require 2D lat/lon coordinate
variables in CF lingo (section 5.2 in the doc). In this case there is
always an implicit rectilinear coordinate system, i.e., the index
coordinates of the 2D array. But the fact that the grid requires 2D
lat/lon coordinate variables implies that it is a curvilinear grid in
lat/lon coordinates.

>
>
> * If you know it is rectilinear, you *don't* have to check the entire bounds
> variable for contiguousness. By the same argument as above, you have only to
> check one row and one column. For instance, the slabs (0,*,0,*) and (*,0,*,0)
> have shapes (x,2) and (y,2) respectively, like 1D bounds variables. It would
> be sufficient to check these alone for contiguousness. True?

Consider a domain with a hole in it (say an ocean basin that contains an
island which is not part of the computational domain). I could construct a
grid which requires 2D lat/lon coordinate variables, and for which the
associated cells were contiguous around the outer boundary, yet the cells
would not be contiguous around the hole (i.e., the cells around the hole
would only have 3 neighboring cells rather than 4).

Suppose we have a grid defined by lat(ny,nx), lon(ny,nx). I believe that
the only way to reduce the contiguity checking to the 1D case is if the
dimensions nx and ny have coordinate variables, and those coordinate
variables have associated bounds. Otherwise, contiguity checking must
occur on the complete boundary variables associated with lat and lon.

Brian
Received on Wed May 28 2003 - 14:54:42 BST