Re: [ferret_users] extra point after monthly average

[Ansley Manke <ansley.b.manke@xxxxxxxx>]

Hi Zhen,
The problem is in getting the start and end points exactly right for the 
equally-spaced Julian axis.  You've put the first and last coordinates 
in the middle of January and December, but for equally-spaced months, 
each of length `365.25/12` days, the start and end coordinate should be 
located at a coordinate that is half of the length of the month from the 
start of 1984 and the same amount of time from the end of 1992.

Here is the definition you used,

yes? define 
axis/calendar=julian/t="15-jan-1984:12:00:00":"15-dec-1992:12:00:00":`365.25/12`/units=days 
monthlytax
yes? list tax_datestring(t[gt=monthlytax], t[gt=monthlytax], "minutes")
            VARIABLE : TAX_DATESTRING(T[GT=MONTHLYTAX], 
T[GT=MONTHLYTAX], "minutes")
            SUBSET   : 109 points (TIME)
            CALENDAR : JULIAN
15-JAN-1984 /   1:"15-JAN-1984 12:00"
14-FEB-1984 /   2:"14-FEB-1984 22:30"
16-MAR-1984 /   3:"16-MAR-1984 09:02"
15-APR-1984 /   4:"15-APR-1984 19:32"
16-MAY-1984 /   5:"16-MAY-1984 06:02"
...
15-SEP-1992 / 105:"15-SEP-1992 01:13"
15-OCT-1992 / 106:"15-OCT-1992 11:43"
14-NOV-1992 / 107:"14-NOV-1992 22:18"
15-DEC-1992 / 108:"15-DEC-1992 08:48"
14-JAN-1993 / 109:"14-JAN-1993 19:18"

To get the equally-spaced axis that you want, it's probably simpler and 
more accurate to use DEFINE AXIS/EDGES/NPOINTS

yes? define 
axis/calendar=julian/edges/t="1-jan-1984:00:00:00":"31-dec-1992:23:59:59"/npoints=108/units=days 
monthlytax

zhen.li@xxxxxxxxxxxxx wrote:
> Hi Ferret users:
>
> I have a script (below) to average daily data from 1984 to 1992 to 
> monthly mean output.
> I should have 9(yr)*12 =108 data points after averaging. However, I 
> got 109 points.
> I wonder why there is an extra point?  My daily data has 3288 data 
> points, and 3288/30.4375
> gives me 108 points on monthly basis.
>
> >>>>>
> use "salt_ave_daily.cdf"
>
> define 
> axis/calendar=julian/t="15-jan-1984:12:00:00":"15-dec-1992:12:00:00":`365.25/12`/units=days 
> monthlytax
> define grid/x=xdeg/y=ydeg/t=monthlytax monthlygrd
> let salt_mon=salt_ave[gt=monthlygrd@ave]
> set mem/size=500
> save/file=salt_mon.cdf salt_mon
> <<<<<
>
> Thanks very much for your help!
>
> Zhen

--

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Ansley Manke, NOAA/PMEL   ansley.b.manke@xxxxxxxx
7600 Sand Point Way NE, Seattle WA
Phone 206-526-6246,  FAX 206-526-6744