Re: Coordinate Systems Proposals

John Sheldon (jps@gfdl.gov)
Tue, 15 Jul 1997 11:58:56 -0400 (EDT) 

Hi Brian-

One last round here...

> > > Your question is really about how to interpret the coordinates.  The same
> > 
> > Actually, my question was more fundamental than an interpretation of
> > the coordinates.  (I can wire in a projection module later if I want to
> > display the data on a globe.)  At first, I usually just render the data
> > in a cartesian "box".  That is, I'm willing to take the lat's and lon's
> > and just treat them as "distances" along 2 axes, but I would like the
> > visualization to reflect the fact that, eg, the lon and lat spacing
> > differ, so that I can get something close to a "correct" aspect ratio.
> > What's the rule that tell me that lat(2,3) is the y-axis coordinate for
> > temp(2,3)?
> 
> When you refer to a "correct" aspect ratio aren't you talking about an
> interpretation issue?  You only know that a one degree lon spacing
> corresponds to a smaller great circle distance the closer you get to the
> pole because you are interpreting the coordinate as a longitude coordinate.
> If you're not interested in that interpretation of the coordinate variable
> then it seems to me that it doesn't matter which one you assign to the
> y-axis.

Forget latitude and longitude!  The issue goes way back to a basic need
for rules by which to interpret metadata/coordinates.  Change the strings
in the original example:

    dimensions:
        i = 5;
        j = 6;

    variables:
        float A(i,j);
            A:coordinates = "abc, def";
        float abc(i,j);
        float def(i,j);

Now suppose I want to create a visualization of "A", say balls whose
colors represent the value of "A".  There are 30 data points in a 6x5
grid, so I get 30 balls in my picture.  Each point needs an "X" and a
"Y" coordinate.  Do I get "X" from "abc" or from "def"?

And, yes, it *does* matter which one I assign to the y-axis.  If the
values in "abc" run from 5 to 15 and the values in "def" run from 22 to
84, you can bet that which one I pick for X and which one I pick for Y
will makes a big difference to the picture I'm going to end up with!
One way it will be correct (ie, the way the originator of the file
intended), and the other way I get the transpose...

John