oce (version 1.8-1)

as.tidem: Create tidem object from fitted harmonic data

Description

This function is intended to provide a bridge to predict.tidem(), enabling tidal predictions based on published tables of harmonic fits.

Usage

as.tidem(tRef, latitude, name, amplitude, phase, debug = getOption("oceDebug"))

Value

An object of tidem, with only minimal contents.

Arguments

tRef

a POSIXt value indicating the mean time of the observations used to develop the harmonic model. This is rounded to the nearest hour in as.tidem(), to match tidem().

latitude

numerical value indicating the latitude of the observations that were used to create the harmonic model. This is needed for nodal-correction procedures carried out by tidemVuf().

name

character vector holding names of constituents.

amplitude

Numeric vector of constituent amplitudes.

phase

Numeric vector of constituent Greenwich phases.

debug

an integer specifying whether debugging information is to be printed during the processing. This is a general parameter that is used by many oce functions. Generally, setting debug=0 turns off the printing, while higher values suggest that more information be printed. If one function calls another, it usually reduces the value of debug first, so that a user can often obtain deeper debugging by specifying higher debug values.

Known issues

There are two known differences between tidem() and the Matlab T_TIDE package, as listed in references 3 and 4. Work on these issues is planned for the summer of 2020.

Details

Note that only constituent names known to tidem() are handled. The permitted names are those listed in Foreman (1978), and tabulated with

data(tidedata)
data.frame(name=tidedata$const$name, freq=tidedata$const$freq)

Warnings are issued for any constituent name that is not in this list; as of the late summer of 2019, the only example seen in practice is M1, which according to Wikipedia (2019) has frequency 0.0402557, which is very close to that of NO1, i.e. 0.04026859, perhaps explaining why Foreman (1978) did not handle this constituent. A warning is issued if this or any other unhandled constituent is provided in the name argument to as.tidem().

References

1. Foreman, M. G. G., 1978. Manual for Tidal Currents Analysis and Prediction. Pacific Marine Science Report. British Columbia, Canada: Institute of Ocean Sciences, Patricia Bay.

2. Wikipedia, "Theory of Tides." https://en.wikipedia.org/wiki/Theory_of_tides Downloaded Aug 17, 2019.

3. Github issue 1653: tidem() and t_tide do not produce identical results https://github.com/dankelley/oce/issues/1653

4. Github issue 1654: predict(tidem()) uses all constituents, unlike T_TIDE https://github.com/dankelley/oce/issues/1654

See Also

Other things related to tides: [[,tidem-method, [[<-,tidem-method, plot,tidem-method, predict.tidem(), summary,tidem-method, tidalCurrent, tidedata, tidem-class, tidemAstron(), tidemVuf(), tidem, webtide()

Examples

# Simulate a tide table with output from tidem().
data(sealevelTuktoyaktuk)
# 'm0' is model fitted by tidem()
m0 <- tidem(sealevelTuktoyaktuk)
p0 <- predict(m0, sealevelTuktoyaktuk[["time"]])
m1 <- as.tidem(mean(sealevelTuktoyaktuk[["time"]]), sealevelTuktoyaktuk[["latitude"]],
               m0[["name"]], m0[["amplitude"]], m0[["phase"]])
# Test agreement with tidem() result, by comparing predicted sealevels.
p1 <- predict(m1, sealevelTuktoyaktuk[["time"]])
stopifnot(max(abs(p1 - p0), na.rm=TRUE) < 1e-10)
# Simplified harmonic model, using large constituents
# > m0[["name"]][which(m[["amplitude"]]>0.05)]
# [1] "Z0"  "MM"  "MSF" "O1"  "K1"  "OO1" "N2"  "M2"  "S2"
h <- "
name  amplitude      phase
  Z0 1.98061875   0.000000
  MM 0.21213065 263.344739
 MSF 0.15605629 133.795004
  O1 0.07641438  74.233130
  K1 0.13473817  81.093134
 OO1 0.05309911 235.749693
  N2 0.08377108  44.521462
  M2 0.49041340  77.703594
  S2 0.22023705 137.475767"
coef <- read.table(text=h, header=TRUE)
m2 <- as.tidem(mean(sealevelTuktoyaktuk[["time"]]),
               sealevelTuktoyaktuk[["latitude"]],
               coef$name, coef$amplitude, coef$phase)
p2 <- predict(m2, sealevelTuktoyaktuk[["time"]])
stopifnot(max(abs(p2 - p0), na.rm=TRUE) < 1)
par(mfrow=c(3, 1))
oce.plot.ts(sealevelTuktoyaktuk[["time"]], p0)
ylim <- par("usr")[3:4] # to match scales in other panels
oce.plot.ts(sealevelTuktoyaktuk[["time"]], p1, ylim=ylim)
oce.plot.ts(sealevelTuktoyaktuk[["time"]], p2, ylim=ylim)