This function compensates for drifting instrument clocks, according to
\(t'=t + a + b (t-t0)\), where \(t'\) is the
true time and \(t\) is the time stored in the object. A single check
on time mismatch can be described by a simple time offset, with a non-zero
value of a, a zero value of b, and an arbitrary value of
t0. Checking the mismatch before and after an experiment yields
sufficient information to specify a linear drift, via a, b,
and t0. Note that t0 is just a convenience parameter, which
avoids the user having to know the "zero time" used in R and clarifies the
values of the other two parameters. It makes sense for t0 to have
the same timezone as the time within x.
Usage
retime(x, a, b, t0, debug = getOption("oceDebug"))Arguments
- x
an oce object.
- a
intercept (in seconds) in linear model of time drift (see “Details”).
- b
slope (unitless) in linear model of time drift (unitless) (see “Details”).
- t0
reference time (in
POSIXct()format) used in linear model of time drift (see “Details”).- debug
a flag that, if nonzero, turns on debugging.
Details
The returned object is computed by linear interpolation, using
approx() with rule=2, to avoid NA values at the
start or end. The new time will be as given by the formula above. Note that
if the drift is large enough, the sampling rate will be changed. It is a
good idea to start with an object that has an extended time range, so that,
after this is called, subset() can be used to trim to a desired
time range.
Examples
library(oce)
data(adv)
adv2 <- retime(adv, 0, 1e-4, as.POSIXct("2008-07-01 00:00:00", tz = "UTC"))
plot(adv[["time"]], adv2[["time"]] - adv[["time"]], type = "l")
