[CF-metadata] bounds

From: Brian Eaton <eaton>
Date: Mon, 9 Jun 2003 15:22:34 -0600

Hi Jonathan,

On Thu, May 29, 2003 at 08:59:19AM +0100, Jonathan Gregory wrote:
> Dear Bob and Brian
>
> Thanks for your responses. Evidently rectilinear is a confusing term to use, as
> you both indicate. I agree we should avoid it altogether.
>
> To be clearer, the case I wanted to distinguish is the one where there are
> 2D lat and lon coordinates and, in addition, where the 2D grid indicates the
> adjacencies of the cells: for 0<i<x and all j, cell (j,i-1) and cell (j,i)
> are adjacent (not necessarily contiguous, but there is no intervening cell)
> and for 0<j<y and all i, cell (j,i) and cell (j-1,i) are adjacent. In the
> other case, the cells might be in an array of any dimensionality, whose
> indices don't easily tell you about their adjacencies. In many cases they
> may be in a 1D array. However, a 2D array doesn't necessarily tell you which
> cells are adjacent. Suppose, for example, you covered the plane with hexagons
> and gave each hexagon an index, then subdivided the hexagons into six
> equilateral triangles and gave each triangle an index within its hexagon.
> You then have a 2D array but cells (hexagon,triangle) and (hexagon-1,triangle)
> are probably not adjacent. Furthermore the hexagons themselves might be viewed
> as a 2D array in which case the cells are in a 3D array even though they cover
> a 2D plane.
>
> Is that clear? This distinction matters precisely because in the former case
> you can decide contiguousness with no special knowledge about the x-y grid.

I don't think this distinction is well defined. Given a boundary variable
dimensioned (y,x,2,2) you can test for whether the cells are contiguous or
not. I don't believe that in the case of non-contiguity that you can
determine the cells to be "adjacent, but not contiguous". Your test for
being adjacent (no intervening cell) is not well defined because the cells
themselves are not well defined, i.e., we define cell vertices but not cell
edges.

Extending the new cell representation (y,x,2,2) to the "adjacent but not
contiguous" case adds alot of complication and no benefit that I can see.
Why not restrict the new representation to the contiguous case and retain
the current representation (y,x,4) for the non-contiguous case?

> From the current draft of the CF document, I wonder if we can call these cases
> Cartesian and non-Cartesian?

Cartesian has the same drawback as rectilinear. The problem is that you're
trying to classify the grid in terms of some underlying coordinate system
to which the terms Cartesian or rectilinear apply, but the grid may look
neither Cartesian nor rectilinear in lat/lon coordinates which are the
coordinates we require the user to supply. That's confusing. As you
mention below it's also possible that an underlying x-y coordinate system
doesn't exist. It's more than a bit confusing to classify a coordinate
system that doesn't exist as being Cartesian or not.

Brian
Received on Mon Jun 09 2003 - 15:22:34 BST