Hi John,
Thanks for the reply. Some comments below.
At 2:06 PM -0800 2006/12/27, John Caron wrote:
>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?
The GHRSST L2P dataset is swath data, not resampled. There is a
unique lat/lon for each pixel, thus a 2D array for both lat and lon.
The offsets in time are caused by non-satellite ancillary fields
(wind, aerosols, solar insolation, sea ice) in the file that have
different observation times (or prediction times if model forecast
fields) from the SST satellite data (which are are referenced to the
sst_dtime array). Each of the ancillary variable could have a
different time offset.
I like the solution proposed by Johanthen with a 'time_offset'
standard name that can then be used with the relative time from array
sst_dtime to calculated a new relative time from the absolute time
specified in array 'time' . Not quite an elegant solution, but within
the framework of the current file format it will work.
--
~ed
Edward M. Armstrong
Physical Oceanography DAAC Tel: 818 393-6710
Jet Propulsion Laboratory Fax: 818 393-2710
edward.armstrong at jpl.nasa.gov
Received on Mon Jan 08 2007 - 17:23:24 GMT
