Function to flexibly aggregate or expand data frames by different time periods, calculating vector-averaged wind direction where appropriate. The averaged periods can also take account of data capture rates.
timeAverage(
mydata,
avg.time = "day",
data.thresh = 0,
statistic = "mean",
type = "default",
percentile = NA,
start.date = NA,
end.date = NA,
interval = NA,
vector.ws = FALSE,
fill = FALSE,
...
)Returns a data frame with date in class POSIXct.
A data frame containing a date field . Can be class
POSIXct or Date.
This defines the time period to average to. Can be
“sec”, “min”, “hour”, “day”, “DSTday”,
“week”, “month”, “quarter” or “year”. For much
increased flexibility a number can precede these options followed by a
space. For example, a timeAverage of 2 months would be period = "2
month". In addition, avg.time can equal “season”, in which
case 3-month seasonal values are calculated with spring defined as March,
April, May and so on.
Note that avg.time can be less than the time interval of the
original series, in which case the series is expanded to the new time
interval. This is useful, for example, for calculating a 15-minute time
series from an hourly one where an hourly value is repeated for each new
15-minute period. Note that when expanding data in this way it is
necessary to ensure that the time interval of the original series is an
exact multiple of avg.time e.g. hour to 10 minutes, day to hour.
Also, the input time series must have consistent time gaps between
successive intervals so that timeAverage can work out how much
‘padding’ to apply. To pad-out data in this way choose fill =
TRUE.
The data capture threshold to use (%). A value of zero
means that all available data will be used in a particular period
regardless if of the number of values available. Conversely, a value of
100 will mean that all data will need to be present for the average to be
calculated, else it is recorded as NA. See also interval,
start.date and end.date to see whether it is advisable to
set these other options.
The statistic to apply when aggregating the data; default
is the mean. Can be one of “mean”, “max”, “min”,
“median”, “frequency”, “sum”, “sd”,
“percentile”. Note that “sd” is the standard deviation,
“frequency” is the number (frequency) of valid records in the
period and “data.cap” is the percentage data capture.
“percentile” is the percentile level (%) between 0-100, which can
be set using the “percentile” option --- see below. Not used if
avg.time = "default".
type allows timeAverage to be applied to cases
where there are groups of data that need to be split and the function
applied to each group. The most common example is data with multiple sites
identified with a column representing site name e.g. type = "site".
More generally, type should be used where the date repeats for a
particular grouping variable. However, if type is not supplied the data
will still be averaged but the grouping variables (character or factor)
will be dropped.
The percentile level in % used when statistic =
"percentile". The default is 95.
A string giving a start date to use. This is sometimes
useful if a time series starts between obvious intervals. For example, for
a 1-minute time series that starts “2009-11-29 12:07:00” that needs
to be averaged up to 15-minute means, the intervals would be
“2009-11-29 12:07:00”, “2009-11-29 12:22:00” etc. Often,
however, it is better to round down to a more obvious start point e.g.
“2009-11-29 12:00:00” such that the sequence is then
“2009-11-29 12:00:00”, “2009-11-29 12:15:00” ...
start.date is therefore used to force this type of sequence.
A string giving an end date to use. This is sometimes useful
to make sure a time series extends to a known end point and is useful when
data.thresh > 0 but the input time series does not extend up to the
final full interval. For example, if a time series ends sometime in
October but annual means are required with a data capture of >75% then it
is necessary to extend the time series up until the end of the year. Input
in the format yyyy-mm-dd HH:MM.
The timeAverage function tries to determine the
interval of the original time series (e.g. hourly) by calculating the most
common interval between time steps. The interval is needed for
calculations where the data.thresh >0. For the vast majority of
regular time series this works fine. However, for data with very poor data
capture or irregular time series the automatic detection may not work.
Also, for time series such as monthly time series where there is a
variable difference in time between months users should specify the time
interval explicitly e.g. interval = "month". Users can also supply
a time interval to force on the time series. See avg.time
for the format.
This option can sometimes be useful with start.date and
end.date to ensure full periods are considered e.g. a full year
when avg.time = "year".
Should vector averaging be carried out on wind speed if
available? The default is FALSE and scalar averages are calculated.
Vector averaging of the wind speed is carried out on the u and v wind
components. For example, consider the average of two hours where the wind
direction and speed of the first hour is 0 degrees and 2m/s and 180
degrees and 2m/s for the second hour. The scalar average of the wind speed
is simply the arithmetic average = 2m/s and the vector average is 0m/s.
Vector-averaged wind speeds will always be lower than scalar-averaged
values.
When time series are expanded i.e. when a time interval is less
than the original time series, data are ‘padded out’ with
NA. To ‘pad-out’ the additional data with the first row in
each original time interval, choose fill = TRUE.
Additional arguments for other functions calling
timeAverage.
David Carslaw
This function calculates time averages for a data frame. It also treats wind direction correctly through vector-averaging. For example, the average of 350 degrees and 10 degrees is either 0 or 360 - not 180. The calculations therefore average the wind components.
When a data capture threshold is set through data.thresh it is
necessary for timeAverage to know what the original time interval of
the input time series is. The function will try and calculate this interval
based on the most common time gap (and will print the assumed time gap to
the screen). This works fine most of the time but there are occasions where
it may not e.g. when very few data exist in a data frame or the data are
monthly (i.e. non-regular time interval between months). In this case the
user can explicitly specify the interval through interval in the same
format as avg.time e.g. interval = "month". It may also be
useful to set start.date and end.date if the time series do
not span the entire period of interest. For example, if a time series ended
in October and annual means are required, setting end.date to the end
of the year will ensure that the whole period is covered and that
data.thresh is correctly calculated. The same also goes for a time
series that starts later in the year where start.date should be set
to the beginning of the year.
timeAverage should be useful in many circumstances where it is
necessary to work with different time average data. For example, hourly air
pollution data and 15-minute meteorological data. To merge the two data sets
timeAverage can be used to make the meteorological data 1-hour means
first. Alternatively, timeAverage can be used to expand the hourly
data to 15 minute data - see example below.
For the research community timeAverage should be useful for dealing
with outputs from instruments where there are a range of time periods used.
It is also very useful for plotting data using timePlot.
Often the data are too dense to see patterns and setting different averaging
periods easily helps with interpretation.
See timePlot that plots time series data and uses
timeAverage to aggregate data where necessary.
## daily average values
daily <- timeAverage(mydata, avg.time = "day")
## daily average values ensuring at least 75 % data capture
## i.e. at least 18 valid hours
if (FALSE) daily <- timeAverage(mydata, avg.time = "day", data.thresh = 75)
## 2-weekly averages
if (FALSE) fortnight <- timeAverage(mydata, avg.time = "2 week")
## make a 15-minute time series from an hourly one
if (FALSE) {
min15 <- timeAverage(mydata, avg.time = "15 min", fill = TRUE)
}
# average by grouping variable
if (FALSE) {
dat <- importAURN(c("kc1", "my1"), year = 2011:2013)
timeAverage(dat, avg.time = "year", type = "site")
# can also retain site code
timeAverage(dat, avg.time = "year", type = c("site", "code"))
# or just average all the data, dropping site/code
timeAverage(dat, avg.time = "year")
}
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