[CF-metadata] COARDS name for a time offset

From: John Caron <caron>
Date: Mon, 08 Jan 2007 18:33:11 -0700

Ed Armstrong wrote:
>
> 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.

thanks for the additional info. my main point is that there are 2 time variables here. this brings up a number of issues, for example, to which should the time_offset be added?
Received on Mon Jan 08 2007 - 18:33:11 GMT