[R] change frequency of wind data correctly

[Jeff Newmiller]

Beware of missing or extra records with these approaches. Also may be tricky to get the time aligned to the hour properly.

On December 6, 2020 9:12:01 PM PST, Richard O'Keefe <raoknz using gmail.com> wrote:
>To be honest, I would do this one of two ways.
>
>(1) Use ?decimate from library(signal),
>    decimating by a factor of three.
>
>(2) Convert the variable to an (n/3)*3 matrix using
>    as.matrix then use rowMeans or apply.
>
>On Thu, 3 Dec 2020 at 06:55, Stefano Sofia
><stefano.sofia using regione.marche.it>
>wrote:
>
>> Dear list users,
>> I have wind data with frequency of 10 minutes (three years data). For
>> simplicity let me use only max wind speed.
>> I need to reduce the frequency to 30 minutes,  at  00 (taking the
>mean of
>> data at 40, 50 and 00 minutes) and at 30 (taking the mean of data at
>10, 20
>> and 30 minutes) of each hour.
>>
>> The simple code here reported works well, but the column "interval"
>groups
>> data forward, not backward:
>>
>> init_day <- as.POSIXct("2018-02-01-00-00", format="%Y-%m-%d-%H-%M",
>> tz="Etc/GMT-1")
>> fin_day <- as.POSIXct("2018-02-01-02-00", format="%Y-%m-%d-%H-%M",
>> tz="Etc/GMT-1")
>> mydf <- data.frame(data_POSIX=seq(init_day, fin_day, by="10 mins"))
>> mydf$vmax <- round(rnorm(13, 35, 10))
>> mydf$interval <- cut(mydf$data_POSIX, , breaks="30 min")
>> means <- aggregate(vmax ~ interval, mydf, mean)
>>
>>     data_POSIX                  vmax  interval
>> 1  2018-02-01 00:00:00     27     2018-02-01 00:00:00
>> 2  2018-02-01 00:10:00     41     2018-02-01 00:00:00
>> 3  2018-02-01 00:20:00     46     2018-02-01 00:00:00
>> 4  2018-02-01 00:30:00     39     2018-02-01 00:30:00
>> 5  2018-02-01 00:40:00     34     2018-02-01 00:30:00
>> 6  2018-02-01 00:50:00     32     2018-02-01 00:30:00
>> ...
>>
>> I should work with
>>
>>     data_POSIX                  vmax  interval
>> 1  2018-02-01 00:00:00     27     2018-02-01 00:00:00
>> 2  2018-02-01 00:10:00     41     2018-02-01 00:30:00
>> 3  2018-02-01 00:20:00     46     2018-02-01 00:30:00
>> 4  2018-02-01 00:30:00     39     2018-02-01 00:30:00
>> 5  2018-02-01 00:40:00     34     2018-02-01 00:00:00
>> 6  2018-02-01 00:50:00     32     2018-02-01 00:00:00
>> ...
>>
>>
>> Is there a way to modify this code to groupp data correctly? (I would
>> prefer using only the base package)
>>
>> Thank you for your help
>> Stefano
>>
>>
>>
>>          (oo)
>> --oOO--( )--OOo----------------
>> Stefano Sofia PhD
>> Civil Protection - Marche Region
>> Meteo Section
>> Snow Section
>> Via del Colle Ameno 5
>> 60126 Torrette di Ancona, Ancona
>> Uff: 071 806 7743
>> E-mail: stefano.sofia using regione.marche.it
>> ---Oo---------oO----------------
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-- 
Sent from my phone. Please excuse my brevity.