[CF-metadata] bounds

From: Jonathan Gregory <jonathan.gregory>
Date: Wed, 21 May 2003 09:15:23 +0100

Dear Bob

Thank you for raising this again. I agree, we should try to wrap this up and
release CF. The other issue, of ancillary data, has been settled, I believe.

We don't have to introduce the "Z"-ordering i.e. (x,y,2,2), about which you
had reservations. Brian suggested that its advantage over (x,y,4) ordered
anticlockwise is that it specifies which is the starting vertex as well as the
ordering. We could have that advantage and still keep (x,y,4). That seems
preferable to me as it is less complicated for the standard than any other
choice.

Brian proposed that exact equality should be required for contiguous bounds.
That seems reasonable to me too. Exact equality can presumably be guaranteed
when writing a file just by copying the value. However, as an application-writer
I'd probably still allow a tolerance, to be safe!

With those amendments, here are the proposed changes to CF for bounds. Could
we suggest a deadline of the end of this month (May), say, for further
comments? If everyone is happy, I suppose we can release CF 1.0 as amended.

Best wishes

Jonathan


(1) In the 1D case, if no bounds are supplied, assume contiguous cells. [At
present there is no assumption in the standard.]

(2) 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.]

(3) 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.]

(4) In the 2D rectilinear case, the bounds should be dimensioned (y,x,4). The
four vertices for each cell should be given in the order (0,0), (0,1), (1,0),
(1,1), where the first index corresponds to the 1D y-bounds of the cell and
the second to its 1D x-bounds. This defines an anticlockwise ordering in the
lat-lon plane as viewed from above. (Is that right? - please check me!) [At
present, the standard does not state any ordering.]

(5) In the non-rectilinear case, the bounds should be dimensioned
(...,n,p). The vertices must be traversed anticlockwise in the lat-lon 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 Wed May 21 2003 - 02:15:23 BST