# Re: annual means, sums

```

>
> Hi,
> I bet a trivial question -
> I have monthly data, say 360 months. I want to make annual data
> out of those and it seems that something like:
>
> xxx[i=3,GL=1:360:12@ave] or xxx[i=3,GL=1:360:12@sum]
>
> works fine. A question about a detail:
> The average runs over months 1-12, 13-24, 25-36 ... ? Or
> should I use
> xxx[i=3,GT=0.5:360.5:12@ave] or xxx[i=3,GT=0.5:360.5:12@sum]
>
> to be on the save side?

Hi Jvrg,

The best way to answer this sort of question is to "ask Ferret".

Immediately after
yes? LIST xxx[i=3,GL=1:360:12@ave]
issue the command
yes? SHOW GRID/L
It will show you the time axis of the result -- the dates listed will
be the midpoints of thge averaging intervals.

For a nice graphical rendering of the answer try a 2-D SHADE plot with
the annual-averaged time as one of the axes:

What you will find is that"GL=1:36:12" implies a regrid to an axis
with **POINTS** located at L=1, 13, 25, ... which means that the
corresponding calendar interval for the L=13 result (e.g.) runs from
L=7 to L=19 (actually, it includes 1/2 of the L=7 and 1/2 of L=19 for
a total of 12 complete time axis "cells").

This is probably not quite what you are after.
Precise control is available using "DEFINE AXIS" to create your target
axis
and then regridding with GT=target@AVE. If the calendar axis you are
starting from has regularly spaced points (equal length "months") then
using DEFINE AXIS is straightforeward. If it is irregular, then see
http://ferret.wrc.noaa.gov/HOMEPAGE/Documentation/Users_Guide/v50/partII.html#V5.0_climatological_axes\$defining
(DEFINE AXIS/EDGES will be available in Ferret V5)

> Now, my real question is:
> I get the mean plotted on the 13'th month. Is there a way
> to talk ferret into plotting the means in the middle of the
> years? I.e. at the end of the months 6, 18, ...?

... as described above.

- steve

> Thanks for your time and thoughts,
> Cheers,
> Joerg
>
> --
> Jvrg Kaduk                              Tel.: 1 650 325 1521 x 416
> Carnegie Institution of Washington      FAX: 1 650 325 6857
> Dept. of Plant Biology
> 260 Panama Street                       joerg@jasper.stanford.edu
> Stanford, CA 94305-1297
> http://Jasper.Stanford.EDU/joerg/

--
Steve Hankin
NOAA/PMEL, 7600 Sand Point Way NE, Seattle, WA 98115-0070
ph. (206) 526-6080 -- FAX (206) 526-6744

```