[R] change frequency of wind data correctly

[Richard O'Keefe]

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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