Regularly-spaced coordinates

Harry Jenter (hjenter@stress.er.usgs.gov)
Mon, 23 Mar 92 14:49:07 EST 

Hi,
A follow-up question to the time thread of a couple of weeks ago:

Is anyone considering a "convention" for implicitly saving time for
variables that are stored at regular time intervals?

As an example, consider N salinity measurements, salt(time):

time(1)		salt(1)
time(2)		salt(2)
      	     .
             .
time(I-1)	salt(I-1)
time(I)		salt(I)
time(I+1)	salt(I+1)
             .
             .
time(N)		salt(N)

where 

time(I+1)-time(I) = deltat,           1 <= I <= N-1

and deltat is a constant.

The time array can be reproduced by storing only three numbers: time(1),
N and deltat.  It would be nice to save space by storing these numbers
instead of all N times.

One possible solution is to store attributes of the array salt that
look something like:

salt:time_base      = time(1);
salt:time_increment = deltat;

This works, but imposes two new attribute "conventions".   Note: N is
is the dimension of salt, and, therefore, is already stored using
standard netCDF means.

Another possibility is to retain time as a variable, but dimension it
to have only 1 element and assign NaN or FloatInf or whatever to it.
This would permit an attribute definition more in-line with the time
conventions that seem to be emerging from the discussions a couple of
weeks ago.  An example would be:

time:units = deltat @ time(1);

No new attribute conventions are imposed, but this requires the
implicit assumption that, if time is a 1-element empty array, the
times are to be reproduced using the units attribute.

Any suggestions, comments?

Note: The problem of saving space by storing only a base and increment
for a coordinate variable can be extended to regularly-spaced (gridded)
data in multiple dimensions. However, the stored grid becomes a much
smaller part of the file as the number of dimensions increases. As an
example, a 4-d salt array might be regularly-spaced in time, x, y and
z, and stored in a KxLxMxN array where time is Kx1, x is Lx1, y is Mx1
and z is Nx1. For large K,L,M and N the sizes of time, x, y, and z
become a non-issue for reducing file size since salt will be
proportionally larger.  Therefore, the 1-d example case is likely to be
the most important one, since storing time as time(1) and deltat
reduces the file to just larger than half its size, as N gets large.
-- 
Harry L. Jenter                        hjenter@stress.er.usgs.gov
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."