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Re: [ferret_users] Problem with @AVE gridding on intermitent time series data
Here is the reply i send to Mike.....but forgot to add the email address
for the group....sorry about that...
------------------------------------------------------------------
Hi Mike,
Some points to note :
1. There is no missing values information for the time points over which
measurements does
not exist. Instead, the time axis have only those time points over
which data exists.
This creates the problem. Solution is to regrid the original data to
an equally
spaced (10 minute resolution) time axis first. (The @XACT regridding
is ideal in this
case, but it is not working properly for this dataset. See the point 2
below.)
2. When you define a time axis, take special care to see that the time axis
values remains
reasonably smaller (ideally to get real values which can be precisely
represented in
a 32 bit machine). To achieve this, always choose proper time units
and T0. Also
start your time axis name with the letter "T", which will help other
programs to
identify time axis.
What is wrong with your dataset? Due to the violation of 2 above (try
list/T="01-JUL-2007 19:50":"10-JUL-2007 14:00" t[gt=temperature], also notice
that the time delta is not precisely
10 minutes! all becase of this wierd time axis definition!), the time axis is
not at all
friendly for any operation in Ferret. For the same reason, regridding
operations like @AVE or
@XACT may not result in expected values.
I have some other ideas on a solution for this problem...but they are tedious.
Let me see
whether anybody have any quick fixes.
Your questions are most welcome....
Regards,
Jaison
On Mon, 15 Oct 2007 16:07:36 -0700, McCann, Mike wrote
> Hello,
>
> I'm use the @AVE regridding transform to take in situ mooring data
> and create regular hourly gridded data sets.
>
> I've encountered a problem which I think might be a bug, namely
> because the @AVE and @MAX transforms behave differently. Here is an
> example using an actual data set:
>
> ! An original Conductivity Temperature Depth data "10-minute" time series
> USE
"http://dods.mbari.org/cgi-bin/nph-nc/data/ssdsdata/deployments/m1/200610/m1_ctd0000_20070405_original.nc";
>
> ! Define an hourly axis
>
> DEFINE AXIS/T="12-Oct-2006 18:00:00":"15-Oct-2007
> 18:00:00":3600/T0="01-JAN-1970 00:00:00"/UNITS=seconds HR_TIME
>
> ! Set region for when telemetry was lost for 15 hours
> SET REGION/T="4-JUL-2007 20:00":"5-JUL-2007 14:00"
>
> ! Show the data to see the gap
> LIST TEMPERATURE
>
> ! Grid the data with the @MAX gridding transform This does the
> appropriate thing: ! finds max from input and puts in missing values
> for the time period of the gap. LIST TEMPERATURE[gt=HR_TIME@MAX]
>
> yes? LIST TEMPERATURE[gt=HR_TIME@MAX]
> VARIABLE : Water Temperature (deg C)
> regrid: 3600 sec on T@MAX
> DATA SET : Mooring M1 CTD data from MBARI at original
> sampling intervals FILENAME : m1_ctd0000_20070405_original.nc
> FILEPATH :
http://dods.mbari.org/cgi-bin/nph-nc/data/ssdsdata/deployments/m1/200610/
> SUBSET : 19 points (TIME)
> LONGITUDE: 122W(-122)
> LATITUDE : 36.8N
> DEPTH (m): 0
> 122W
> 1
> 04-JUL-2007 20:00 / 6363: 11.30
> 04-JUL-2007 21:00 / 6364: 11.37
> 04-JUL-2007 22:00 / 6365: ....
> 04-JUL-2007 23:00 / 6366: ....
> 05-JUL-2007 00:00 / 6367: ....
> 05-JUL-2007 01:00 / 6368: ....
> 05-JUL-2007 02:00 / 6369: ....
> 05-JUL-2007 03:00 / 6370: ....
> 05-JUL-2007 04:00 / 6371: ....
> 05-JUL-2007 05:00 / 6372: ....
> 05-JUL-2007 06:00 / 6373: ....
> 05-JUL-2007 07:00 / 6374: ....
> 05-JUL-2007 08:00 / 6375: ....
> 05-JUL-2007 09:00 / 6376: ....
> 05-JUL-2007 10:00 / 6377: ....
> 05-JUL-2007 11:00 / 6378: ....
> 05-JUL-2007 12:00 / 6379: 11.96
> 05-JUL-2007 13:00 / 6380: 12.08
> 05-JUL-2007 14:00 / 6381: 12.13
>
> ! Grid the data with the @AVE gridding transform. This does an
> unexpected thing: ! fills the time period of the gap with numbers
> that are extrapolations of bounding ! good points except for the
> center point which is an average of those two points. LIST
> TEMPERATURE[gt=HR_TIME@AVE]
>
> yes? LIST TEMPERATURE[gt=HR_TIME@AVE]
> VARIABLE : Water Temperature (deg C)
> regrid: 3600 sec on T@AVE
> DATA SET : Mooring M1 CTD data from MBARI at original
> sampling intervals FILENAME : m1_ctd0000_20070405_original.nc
> FILEPATH :
http://dods.mbari.org/cgi-bin/nph-nc/data/ssdsdata/deployments/m1/200610/
> SUBSET : 19 points (TIME)
> LONGITUDE: 122W(-122)
> LATITUDE : 36.8N
> DEPTH (m): 0
> 122W
> 1
> 04-JUL-2007 20:00 / 6363: 11.13
> 04-JUL-2007 21:00 / 6364: 11.33
> 04-JUL-2007 22:00 / 6365: 11.32
> 04-JUL-2007 23:00 / 6366: 11.32
> 05-JUL-2007 00:00 / 6367: 11.32
> 05-JUL-2007 01:00 / 6368: 11.32
> 05-JUL-2007 02:00 / 6369: 11.32
> 05-JUL-2007 03:00 / 6370: 11.32
> 05-JUL-2007 04:00 / 6371: 11.32
> 05-JUL-2007 05:00 / 6372: 11.79
> 05-JUL-2007 06:00 / 6373: 11.96
> 05-JUL-2007 07:00 / 6374: 11.96
> 05-JUL-2007 08:00 / 6375: 11.96
> 05-JUL-2007 09:00 / 6376: 11.96
> 05-JUL-2007 10:00 / 6377: 11.96
> 05-JUL-2007 11:00 / 6378: 11.96
> 05-JUL-2007 12:00 / 6379: 11.96
> 05-JUL-2007 13:00 / 6380: 12.04
> 05-JUL-2007 14:00 / 6381: 12.12
>
> Am I interpreting these transforms properly? Should @AVE have a
> behavior similar to @MAX (and @MIN)? I'd think that if there were
> no data for a time period then that should be reflected with missing
> data values for whatever gridding transform is used.
>
> Thanks,
> Mike
>
> --
> Mike McCann
> Software Engineer
> Monterey Bay Aquarium Research Institute
> 7700 Sandholdt Road
> Moss Landing, CA 95039-9644
> Voice: 831.775.1769 Fax: 831.775.1736 http://www.mbari.org
> <http://www.mbari.org/>
>
> --
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--
Centre for Atmospheric
and
Oceanic Sciences,
Indian Institute of Science
Bangalore-560 012
India
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