# [R] change frequency of wind data correctly

Richard O'Keefe r@oknz @end|ng |rom gm@||@com
Mon Dec 7 06:12:01 CET 2020

```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----------------
>
> ________________________________
>
> AVVISO IMPORTANTE: Questo messaggio di posta elettronica può contenere
> informazioni confidenziali, pertanto è destinato solo a persone autorizzate
> alla ricezione. I messaggi di posta elettronica per i client di Regione
> Marche possono contenere informazioni confidenziali e con privilegi legali.
> Se non si è il destinatario specificato, non leggere, copiare, inoltrare o
> archiviare questo messaggio. Se si è ricevuto questo messaggio per errore,
> inoltrarlo al mittente ed eliminarlo completamente dal sistema del proprio
> computer. Ai sensi dell’art. 6 della DGR n. 1394/2008 si segnala che, in
> caso di necessità ed urgenza, la risposta al presente messaggio di posta
> elettronica può essere visionata da persone estranee al destinatario.
> IMPORTANT NOTICE: This e-mail message is intended to be received only by
> persons entitled to receive the confidential information it may contain.
> E-mail messages to clients of Regione Marche may contain information that
> is confidential and legally privileged. Please do not read, copy, forward,
> or store this message unless you are an intended recipient of it. If you
> have received this message in error, please forward it to the sender and
> delete it completely from your computer system.
>
> --
> Questo messaggio  stato analizzato da Libra ESVA ed  risultato non infetto.
> This message was scanned by Libra ESVA and is believed to be clean.
>
>
>         [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help using r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help