seas (version 0.3-3)

mkann: Make annum from a date

Description

Discretizes a date into an annum, using a starting day to specify the start of a season, and ends in the next year.

Usage

# normal usage, where 'x' is a data.frame with a date column
mkann(x)
# all options
mkann(x, start.day, year.length)

Arguments

x

A data.frame with a date column (of Date or POSIXct class)

It may also be a vector of Date or POSIXct<

start.day

This is the starting day of the annum, and can be specified as either a Date, where year is ignored (e.g., as.Date("2000-08-01") for August 1st of any year); or it can be a day of the year, from 1--365

year.length

required if a non-Gregorian calendar is used, or the attribute (using attr) is not set in x or x$date

Value

• Returns factors for each date given in x, grouped by each annum.

synopsis

mkann(x, start.day, year.length)

Details

This date function finds the annual-breaks between seasons, using a start.day. Often, the start.day is 1, or January 1st, in which case simply the year is returned, since the season starts on January 1 and ends on December 31st. Otherwise, each annual break is set using start.day, and the annum is identified by the range of years, for example 1991_1992, identifying a season starting on start.day in 1991, and ending in the day before start.day in 1992.

If a Gregorian calendar is used, the lengths of each annum are 365 or 366, depending where the leap year occurs, and also whether start.day is before or after March 1st. For other calendars (such as used by various numerical models), the annum length is same as the year.length.

A choice of start.day can influence annual totals using seas.sum, such as annual precipitation. For instance, if a particular winter in the Northern hemisphere has snow before and after the new year, these would be divided counting annual sums based on the year, whereas if start.day were before the winter season, the annual sum would be calculated throughout the winter season.

References

http://en.wikipedia.org/wiki/Gregorian_calendar

See Also

mkseas, seas.sum

Examples

data(mscdata)
dat <- mksub(mscdata,id=1108447)
dat$ann1 <- mkann(dat,start.day=1)
dat$ann2 <- mkann(dat,start.day=as.Date("2000-02-01"))
dat$ann3 <- mkann(dat,start.day=as.Date("2000-08-01"))
table(dat$ann1)
table(dat$ann2)
table(dat$ann3)
dat[26:36,c("date",paste("ann",1:3,sep=""))]