regularly-spaced coordinates

```gabor@xxxxxxxxxxxxxxxxxxxxxx writes:
>
> I suggest we
> approach the question carefully and look around before fixing this
> problem locally. Quick fire-fighting work always turns out to be
> painful.

This is certainly my opinion also.  I just thought that I would throw
out a couple of examples to "stir the pot".

< Text deleted >

> Harry Jentner said he would not like to have assumptions. Me either.
> But netCDF already does have one. It is assumed all data are given
> over rectangular grid.

I think that netCDF does not require this assumption to be made.  It is
just more intuitive for storing rectangular or, more generally,
quadrilateral grids, without introducing one's own conventions. (Note:
I stick to the 2-d grid analogy for the rest of this post.  The
arguments can be extended to 3 or more dimensions, but I don't know the
words for "equivalent of quadrilateral" in 3 or more dimensions.)
There seem to be no assumptions regarding grid shape.  There are also
seem to be no conventions regarding grid shape.

I think, however, that the issue of storing non-quadrilateral grids is
a more difficult issue to address than the regularly-spaced-coordinate
issue. In fact, it is, in a sense, the opposite problem.  For
regularly-spaced coordinates, one can reduce the information stored in
a netCDF file relative to the "intuitive" method of storing a 1-d array for
each coordinate, and still can reproduce the grid.  For non-quadrilateral
grids or even quadrilateral-but-not-rectangular grids, one must
increase the amount of information stored in the netCDF file in order
what nodes are connected must be stored.  For curvilinear grids, the
coordinate arrays must have more than one dimension.

< Text deleted >

>   What Harry suggests is a way to indicate if the given grid in one
> dimension (that may happen to be TIME) is equally spaced. But what
> about the other functions? What if a dimension is logarithmical or
> derived with any other function?

This is a good point. I forgot about grids that are stretched according
to a function.

>   And further, data points are not necessarily given over a
> rectangular grid. The grid could be just genarally quadrilateral, or
> may be triangular as often is. Or what about a general polygonal grid?

I do not know much about non-rectangular grids, but I have heard that
the HDF Vset has a mechanism for storing them.  I have also heard that
the DX format for IBM's Data Explorer product has provisions for
storing non-rectangular grids (I think that Lloyd Treinish, a frequent
contributor to netcdfgroup is helping to design this format.).  Does

> Will we introduce more and more assumptions?  What we need is to think
> over the whole issue, introduce a CONCEPT instead of AD-HOC attribute
> or whatever solutions.

You don't have to YELL. I agree with you.  I also think that the
netcdfgroup is an appropriate forum for discussing these ideas.

--
Harry L. Jenter                        hjenter@xxxxxxxxxxxxxxxxxx
U.S. Geological Survey                 COM: (703) 648-5916 FTS: 959-5916
Mailstop 430, National Center          "Sometimes you're the bug.
Reston, Virginia 22092                   Sometimes you're the windshield."

```