[CF-metadata] bounds

From: Jonathan Gregory <j.m.gregory>
Date: Thu, 12 Jun 2003 18:27:37 +0100

Dear Brian

I think we must have a terminological confusion, because we are apparently in
agreement.

> 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?

That is what I am proposing, I think, except that contiguous doesn't seem the
right word to me.

Let me try some different words. When there is an underlying x-y arrangement
of cells, the bounds should be (y,x,2,2) and you can use them to test
contiguousness. When there is not, you wouldn't know which cells are adjacent
and so possibly contiguous. In that case we should stick with (y,x,n) (not
necessarily 4), and admit that contiguousness is harder to test. Examples of
the latter case would be hexagons divided into triangles and Reiner's GME grid.
Is that what you meant as well? If so, what term would distinguish these cases?

Cheers

Jonathan


(1) For a 1D coordinate variable with bounds (x,2), define an ordering for the
2nd index, namely that the bounds should be ordered in the same sense as the
coordinates, so that the 0-bound of cell i is on the side that faces cell i-1
and the 1-bound on the side facing cell i+1. [At present, there is no ordering
defined for 1D bounds.]

(2) If 1D cells i and i+1 are contiguous, bound (i,1) should exactly equal
bound (i+1,0). The data-writer should ensure that this is so. [At present, the
standard does not state how contiguousness should be tested.]

(3) In 2D CASE X, the bounds should be dimensioned (y,x,2,2), where the first
trailing index corresponds to the y dimension and the second to the x. The
trailing indexes have the same convention as for 1D points, so that the
0-bound in either x or y is on the side which faces the cell with the next
smallest index in that dimension, the 1-bound on the side facing the cell with
the next largest index. If the coordinate system x-y-upward is right-handed
(like lon-lat-upward), the elements (j,i,0,0), (j,i,0,1), (j,i,1,1), (j,i,1,0)
in that order traverse the vertices of cell (j,i) anticlockwise in the lon-lat
plane as viewed from above. If the coordinate system is left-handed, this
sequence of elements traverse the vertices clockwise. [This dimensioning of
the 2D vertices is new.]

(4) In 2D CASE Y, the bounds should be dimensioned (...,n,p). The vertices
must be traversed anticlockwise in the lon-lat plane as viewed from above. The
starting vertex is not specified by the standard. [At present, the standard
does not state any ordering.]
Received on Thu Jun 12 2003 - 11:27:37 BST