A drifting buoy
Data from buoy number 4201703 were downloaded from
http://www.coriolis.eu.org/Data-Products/Data-selection on
2022-01-09, by deselecting all data types except for “Drifting buoy”,
selecting a data window south of Nova Scotia and of area similar to that
province, expanding the tarball created by the website and then
focussing on the downloaded file named
"drifting-buoys-4201703.csv“.
There are many data in the source data file, but only a subset is
used here: starting with 300 samples of just five of the data fields:
time (called t for brevity), longitude (lon),
latitude (lat), temperature (Tatm) and
atmospheric pressure (Patm). This set was further trimmed
by removing data ensembles in which temperature was not reported. The
result was 146 observations of 5 variables. (Note that they are
not spaced uniformly in time, a feature that is of importance
only at the end of this document.)
The data were combined into a data frame named db and
packaged in an RDA file, an overview of which is provided as
follows.
library(oce)
load(system.file("extdata", "drifter.rda", package = "oce"))
str(db)
#> 'data.frame': 146 obs. of 5 variables:
#> $ t : POSIXct, format: "2021-11-01 21:04:00" "2021-11-01 23:04:00" ...
#> $ lon : num -62.7 -62.8 -62.8 -62.8 -62.8 ...
#> $ lat : num 42.5 42.5 42.5 42.6 42.6 ...
#> $ Tatm: num 19.2 19.2 19.2 19.2 19.2 ...
#> $ Patm: num 1013 1014 1015 1016 1017 ...
Before showing how to create a new subclass, it will first be shown
that such data could also be stored in the foundational class of the
package.
Using the base oce
class
The base class is named "oce", and so a new object of
that type can be created with
At this point, the summary() method does not reveal
much:
summary(o)
#> * Processing Log
#>
#> - 2024-08-17 09:38:33 UTC: `Create oce object`
but if we populate the object with some data and metadata
o <- oceSetData(o, "time", db$t)
o <- oceSetData(o, "longitude", db$lon)
o <- oceSetData(o, "latitude", db$lat)
o <- oceSetMetadata(o, "ID", 4201703)
then the summary is more informative
summary(o)
#> * Time: 2021-11-01 21:04:00 to 2021-11-08 06:00:00 (146 samples, mean increment 1.054713 hour)
#> * Data Overview
#>
#> Min. Mean Max. Dim. NAs
#> longitude -63.011 -62.775 -62.636 146 0
#> latitude 41.932 42.305 42.572 146 0
#>
#> * Processing Log
#>
#> - 2024-08-17 09:38:33 UTC: `Create oce object`
#> - 2024-08-17 09:38:34 UTC: `oceSetData(object = o, name = "time", value = db$t)`
#> - 2024-08-17 09:38:34 UTC: `oceSetData(object = o, name = "longitude", value = db$lon)`
#> - 2024-08-17 09:38:34 UTC: `oceSetData(object = o, name = "latitude", value = db$lat)`
#> - 2024-08-17 09:38:34 UTC: `oceSetMetadata(object = o, name = "ID", value = 4201703)`
Note, however, that the ID is not listed by
summary(). This is because the general form of that
function is set up to focus on the contents of the data
slot, not the metadata slot. (Extending
summary() will described later.)
Objects of oce-class automatically have other methods,
besides summary(). For example, the accessing operator
[[ works for the object that was just created:
str(o[["latitude"]])
#> num [1:146] 42.5 42.5 42.5 42.6 42.6 ...
We can also use the default plot() function to show the
interdependence of entries in the data slot.
plot(o, pch = 20, cex = 0.5)
Figure 1. Demonstration of
base-level plot().
Although this plot provides a useful overview of the data, it is not
something an analyst would consider publishing. To get more useful plot
types, we must create a new subclass that has a specialized
plot() method.
Creating a new
subclass
A reasonable name for our new subclass might be
"drifter". With this choice, the class can be defined in a
single line of code:
drifter <- setClass(Class = "drifter", contains = "oce")
Once this has been executed, the user can create a new object of this
subclass with
Although it is simple to add data and metadata as in the previous
section, it is more convenient to supply this information when
new() is used. To accomplish this, we must set up an
initialization function, as follows.
setMethod(
f = "initialize",
signature = "drifter",
definition = function(.Object, time, longitude, latitude, ID = "unknown") {
if (missing(time)) {
stop("In new(drifter) : must provide 'time'", call. = FALSE)
}
if (missing(longitude)) {
stop("In new(drifter) : must provide 'longitude'", call. = FALSE)
}
if (missing(latitude)) {
stop("In new(drifter) : must provide 'latitude'", call. = FALSE)
}
.Object@data$time <- time
.Object@data$longitude <- longitude
.Object@data$latitude <- latitude
.Object@metadata$ID <- ID
.Object@processingLog$time <- presentTime()
.Object@processingLog$value <- "create 'drifter' object"
return(.Object)
}
)
This method takes away the choice of where to put things (in
data or in metadata), saving users from having
to make that decision, possibly breaking other code. Also, note that
some error checking has been used here, so that an attempt to use
new("drifter") without providing time, etc., produces an
error message, as shown below.
d <- new("drifter")
Error: In new(drifter) : must provide 'time'
With this setup, we may now create an object that we can use for the
rest of this vignette:
d <- new("drifter", time = db$t, longitude = db$lon, latitude = db$lat, ID = 4201703)
and this will be used in the next three subsections, showing how to
specialize the plot() method, the summary()
method, and the [[ accessor. Note the use of
setMethod() in each instance.
Specializing the
plot() function
The f, signature and
definition arguments follow the same pattern as in the
previous example, and readers ought to focus on last of these. Here,
three plot types are provided, as selected by an argument called
which. The first two give time-series plots of drifter
longitude and latitude, and the third shows the trajectory.
setMethod(
f = "plot",
signature = signature("drifter"),
definition = function(x, which = 1, ...) {
lonlab <- expression("Longitude [" * degree * "E]")
latlab <- expression("Latitude [" * degree * "N]")
if (which == 1) {
oce.plot.ts(x[["time"]], x[["longitude"]],
ylab = lonlab, ...
)
} else if (which == 2) {
oce.plot.ts(x[["time"]], x[["latitude"]],
ylab = latlab, ...
)
} else if (which == 3) {
asp <- 1 / cos(mean(range(x[["latitude"]]) * pi / 180))
plot(x[["longitude"]], x[["latitude"]],
asp = asp, xlab = lonlab, ylab = latlab, ...
)
} else {
stop("In plot,drifter-method : try which=1, 2 or 3", call. = FALSE)
}
}
)
The three plot types are fairly rudimentary, but the ...
argument can be used for customization, as in the following example.
par(mar = c(3.3, 3.3, 1, 1), mgp = c(2, 0.7, 0))
layout(matrix(c(1, 3, 2, 3), nrow = 2, byrow = TRUE))
plot(d, which = 1, drawTimeRange = FALSE)
plot(d, which = 2, drawTimeRange = FALSE)
plot(d, which = 3)
Figure 2. Demonstration of
specialized plot().
Specializing the
summary() function
Here, our sole purpose is to add an indication of the ID of the
drifter. The rest of the information provided by the base-level
summary() function will suffice to summarize the rest.
So, with
setMethod(
f = "summary",
signature = "drifter",
definition = function(object, ...) {
cat("CTD Summary\n-----------\n\n")
cat("* ID: ", object[["ID"]], "\n", sep = "")
invisible(callNextMethod())
}
)
An example follows.
summary(d)
#> CTD Summary
#> -----------
#>
#> * ID: 4201703
#> * Time: 2021-11-01 21:04:00 to 2021-11-08 06:00:00 (146 samples, mean increment 1.054713 hour)
#> * Data Overview
#>
#> Min. Mean Max. Dim. NAs
#> longitude -63.011 -62.775 -62.636 146 0
#> latitude 41.932 42.305 42.572 146 0
#>
#> * Processing Log
#>
#> - 2024-08-17 09:38:34 UTC: `create 'drifter' object`
Specializing the
[[ accessor
The variation of drifter position provides information on ocean
velocity, and this is of sufficient interest that it might be worth
adding to [[. This may be done as follows.
setMethod(
f = "[[",
signature(x = "drifter", i = "ANY", j = "ANY"),
definition = function(x, i, j, ...) {
if (i == "velocity") {
D <- function(x) {
dx <- diff(x)
c(dx[1], dx)
}
lat <- x[["latitude"]]
lon <- x[["longitude"]]
dt <- D(as.numeric(x[["time"]])) # seconds
scalex <- 111.12e3 # m per degree latitude
scaley <- scalex * cos(lat * pi / 180)
u <- scalex * D(lon) / dt
v <- scaley * D(lat) / dt
list(u = u, v = v)
} else {
callNextMethod()
}
}
)
As a test, we can plot the velocity components.
uv <- d[["velocity"]]
par(mfrow = c(2, 1))
oce.plot.ts(d[["time"]], uv$u, ylab = "Eastward velo. [m/s]", grid = TRUE)
oce.plot.ts(d[["time"]], uv$v, ylab = "Northward vel. [m/s]", grid = TRUE)
Figure 3. Velocities inferred
from drifter motion.
The acf() function, which computes lagged
autocorrelation, can shed some light on the periodicity of the velocity
signals (but see Exercise 3), as follows.
data(tidedata)
M2period <- 1 / with(tidedata$const, freq[[which(name == "M2")]])
uv <- d[["velocity"]]
par(mfrow = c(2, 1), mar = c(3, 3, 1, 1), mgp = c(2, 0.7, 0))
acf(uv$u, main = "", ylab = "u ACF")
abline(v = M2period, col = 2)
acf(uv$v, main = "", ylab = "v ACF")
abline(v = M2period, col = 2)
Figure 4. Autocorrelation
analysis of drifter velocities, showing also the M2 period.