Creating regular 15-minute time-series from irregular time-series

Question

I have an irregular time-series (with DateTime and RainfallValue) in a csv file C:\\SampleData.csv:

DateTime,RainInches
1/6/2000 11:59,0
1/6/20         


        

Answer 1

Here is a data.table solution, this can be neatly done using a rolling join:

library(data.table)
library(xts)

lu <- data.table(index=as.POSIXct("2012-05-02") + (0:7)*15*60)

observation <- xts(1:10,
                   order.by=lu[1,index +cumsum(runif(10)*60*10)])

observation.dt <- as.data.table(observation)
observation.dt[lu,on="index",roll=T]

Answer 2

xts extends zoo, and zoo has extensive examples for this in its vignettes and documentation.
Here is a worked example. I think I have done that more elegantly in the past, but this is all I am coming up with now:

R> twohours <- ISOdatetime(2012,05,02,9,0,0) + seq(0:7)*15*60
R> twohours
[1] "2012-05-02 09:15:00 GMT" "2012-05-02 09:30:00 GMT" 
[3] "2012-05-02 09:45:00 GMT" "2012-05-02 10:00:00 GMT" 
[5] "2012-05-02 10:15:00 GMT" "2012-05-02 10:30:00 GMT" 
[7] "2012-05-02 10:45:00 GMT" "2012-05-02 11:00:00 GMT"
R> set.seed(42)
R> observation <- xts(1:10, order.by=twohours[1]+cumsum(runif(10)*60*10))
R> observation
                           [,1]
2012-05-02 09:24:08.883625    1
2012-05-02 09:33:31.128874    2
2012-05-02 09:36:22.812594    3
2012-05-02 09:44:41.081170    4
2012-05-02 09:51:06.128481    5
2012-05-02 09:56:17.586051    6
2012-05-02 10:03:39.539040    7
2012-05-02 10:05:00.338998    8
2012-05-02 10:11:34.534372    9
2012-05-02 10:18:37.573243   10

A two hour time grid, and some random observations leaving some cells empty and some filled.

R> to.minutes15(observation)[,4]
                           observation.Close
2012-05-02 09:24:08.883625                 1
2012-05-02 09:44:41.081170                 4
2012-05-02 09:56:17.586051                 6
2012-05-02 10:11:34.534372                 9
2012-05-02 10:18:37.573243                10

That is a 15 minutes grid aggregation but not on our time grid.

R> twoh <- xts(rep(NA,8), order.by=twohours)
R> twoh
                    [,1]
2012-05-02 09:15:00   NA
2012-05-02 09:30:00   NA
2012-05-02 09:45:00   NA
2012-05-02 10:00:00   NA
2012-05-02 10:15:00   NA
2012-05-02 10:30:00   NA
2012-05-02 10:45:00   NA
2012-05-02 11:00:00   NA

R> merge(twoh, observation)
                           twoh observation
2012-05-02 09:15:00.000000   NA          NA
2012-05-02 09:24:08.883625   NA           1
2012-05-02 09:30:00.000000   NA          NA
2012-05-02 09:33:31.128874   NA           2
2012-05-02 09:36:22.812594   NA           3
2012-05-02 09:44:41.081170   NA           4
2012-05-02 09:45:00.000000   NA          NA
2012-05-02 09:51:06.128481   NA           5
2012-05-02 09:56:17.586051   NA           6
2012-05-02 10:00:00.000000   NA          NA
2012-05-02 10:03:39.539040   NA           7
2012-05-02 10:05:00.338998   NA           8
2012-05-02 10:11:34.534372   NA           9
2012-05-02 10:15:00.000000   NA          NA
2012-05-02 10:18:37.573243   NA          10
2012-05-02 10:30:00.000000   NA          NA
2012-05-02 10:45:00.000000   NA          NA
2012-05-02 11:00:00.000000   NA          NA

New xts object, and merged object. Now use na.locf() to carry the observations forward:

R> na.locf(merge(twoh, observation)[,2])
                           observation
2012-05-02 09:15:00.000000          NA
2012-05-02 09:24:08.883625           1
2012-05-02 09:30:00.000000           1
2012-05-02 09:33:31.128874           2
2012-05-02 09:36:22.812594           3
2012-05-02 09:44:41.081170           4
2012-05-02 09:45:00.000000           4
2012-05-02 09:51:06.128481           5
2012-05-02 09:56:17.586051           6
2012-05-02 10:00:00.000000           6
2012-05-02 10:03:39.539040           7
2012-05-02 10:05:00.338998           8
2012-05-02 10:11:34.534372           9
2012-05-02 10:15:00.000000           9
2012-05-02 10:18:37.573243          10
2012-05-02 10:30:00.000000          10
2012-05-02 10:45:00.000000          10
2012-05-02 11:00:00.000000          10

And then we can merge again as an inner join on the time-grid xts twoh:

R> merge(twoh, na.locf(merge(twoh, observation)[,2]), join="inner")[,2]
                    observation
2012-05-02 09:15:00          NA
2012-05-02 09:30:00           1
2012-05-02 09:45:00           4
2012-05-02 10:00:00           6
2012-05-02 10:15:00           9
2012-05-02 10:30:00          10
2012-05-02 10:45:00          10
2012-05-02 11:00:00          10
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