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Re: [ferret_users] Selecting groups of data
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- Subject: Re: [ferret_users] Selecting groups of data
- From: "Ansley C. Manke" <ansley.b.manke@xxxxxxxx>
- Date: Tue, 7 Jan 2020 09:53:07 -0800
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Hi Alexander,
No, this kind of striding isn't available in Ferret/PyFerret.
Do you want to compute a seasonal climatology? Or an average to an axis
with 4 points per year? If so then a regridding operation can do it for
you without your having to specify strides yourself
For climatologies, here are some details -- see the note at the bottom
about the "clim_axes" datasets which define a variety of seasonal
calendar axes for both the Gregorian calendar and other calendar definitions
https://ferret.pmel.noaa.gov/Ferret/faq/how-do-i-calculate-climatologies-and-climatological-anomalies
To regrid to a seasonal calendar, you would define your calendar axis
and use a regridding operation.
If the grid is still too large to work with for one of the above
operations, or if you're doing some other kind of analysis you can work
on the grid in pieces, appending to a netCDF file in some dimension
other than time. You need to first set up the file with the full
range, and then append into it, filling the grid piece by piece.
https://ferret.pmel.noaa.gov/Ferret/documentation/users-guide/converting-to-netcdf/SIMPLE-CONVERSIONS-USING-FERRET#_VPINDEXENTRY_1071
-Ansley
On 1/3/2020 7:23 AM, Alexander Audet wrote:
Ferret User Community
I am trying to work with a very
large grided dataset and trying to get as high a resolution
as possible. I am trying to correlate 1 season of data across a number
of years to a time series. Thus, it appears that I need to average
498355200 points of data into four seasons then perform a stride so
that only the winter season is selected (right now defined as JFM,
later I will need to compare to DJF).
The problem is that there are
too many data points for my ferret to average the data into 4 seasons
as it gives me a memory error when I try to work with the resulting
variable. If I first trim down the data (stride over several degrees
Lat and Long) into a variable, the correlation does work. I think that
if I could first isolate JFM monthly data from the rest of the year,
then average those, I will be able to perform @AVE over a larger part
of the dataset of interest to me and create a higher
resolution correlation.
Is there a way to select
chunks of data? Say if I had 24 months over 2 years, so that L=1:24,
somehow select 1:3, then 13:15? I could imagine using a command like
1:24:3:12, (select 3 points at every 12th value). My browsing through
the documentation makes it seem impossible, but it seems like such a
simple request, I have a hard time believing that.
Alexander Audet
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