[CF-metadata] COARDS name for a time offset

From: John Caron <caron>
Date: Wed, 27 Dec 2006 14:06:56 -0800

Sorry my previous post was incomplete (trying to finish it as I walked
out the door).
Try it again:

heres a Grid with 1D coordinates (list coordinates explicitly for clarity):

1) short sea_surface_temperature(time, lat, lon) ;
           sea_surface_temperature:coordinates = "lon lat time" ;
       float lat(lat) ;
       float lon(lon) ;
       int time(time) ;

heres a Grid with 2D lat/lon coordinates:

2) short sea_surface_temperature( time, nj, ni) ;
            sea_surface_temperature:coordinates = "lon lat time" ;
       float lat(nj, ni) ;
       float lon(nj, ni) ;
       int time(time) ;

we were thinking that this is a Swath:

3) short sea_surface_temperature( nj, ni) ;
               sea_surface_temperature:coordinates = "lon lat time" ;
       float lat(nj, ni) ;
       float lon(nj, ni) ;
       int time(nj, ni) ;

the difference being that Swaths have a 2D time coordinate.

Now consider

       short sea_surface_temperature( timeRef, nj, ni) ;
               sea_surface_temperature:coordinates = "lon lat" ;

       int timeRef(timeRef) ;
                timeRef:long_name = "reference time of sst file" ;

       short sst_dtime(time, nj, ni) ;
                sst_dtime:long_name = "time difference from reference
time" ;

       float lat(nj, ni) ;
       float lon(nj, ni) ;

As it is written, the coordinate system for sea_surface_temperature is
(lat, lon, timeRef). This looks like a Grid with 2D lat/lon coordinates
(case 2 above). There is no reference to sst_dtime

But I suppose its more accurate to see it as a swath:

     short sea_surface_temperature( nj, ni) ;
            sea_surface_temperature:coordinates = "lon lat sst_dtime" ;

       short sst_dtime(time, nj, ni) ;
            sst_dtime:long_name = "time difference from reference time"
            sst_dtime:formula = "timeRef + sst_dtime" ;

where I have imagined that we have come up with some convention for
calculating the time coordinate values.

Im guessing it is useful to see it as a Grid (ignore 2D time) or a Swath
(ignore 1D time). Because the lat/lon coordinates are the same for each
timeRef coordinate, it seems to me to really be a hybrid of the 2.

     short sea_surface_temperature( timeRef, nj, ni) ;
               sea_surface_temperature:coordinates = "lon lat timeRef
sst_dtime" ;

       int timeRef(timeRef) ;
                timeRef:long_name = "reference time of sst file" ;

       short sst_dtime(time, nj, ni) ;
                sst_dtime:long_name = "time difference from reference
time" ;
                sst_dtime:formula = "timeRef + sst_dtime" ;

It would seem that there are 2 time coordinates, an approximate time for
showing the data as a Grid, and an actual "pixel-by-pixel" time.

Ed, can you tell us more about this dataset? Its derived from Swath
data, and resampled? to a fixed lat/lon array? What causes the constant
offsets of different variables? Does each variable have a different offset?
Received on Wed Dec 27 2006 - 15:06:56 GMT