[R] change frequency of wind data correctly

[Stefano Sofia]

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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