The [[ method works for all oce objects, i.e.
objects inheriting from oce-class. The purpose
is to insulate users from the internal details of oce
objects, by looking for items within the various storage
slots of the object. Items not actually stored can also be
extracted, including derived data, units of measurement,
and data-quality flags.
# S4 method for ctd
[[(x, i, j, ...)An ctd object, i.e. one inheriting from ctd-class.
Character string indicating the name of item to extract.
Optional additional information on the i item.
Optional additional information (ignored).
If the specialized method produces no matches, the following generalized
method is applied. As with the specialized method, the procedure hinges
first on the value of i.
First, a check is made as to whether i names one of the standard
oce slots, and returns the slot contents if so. Thus,
x[["metadata"]] will retrieve the metadata slot,
while x[["data"]] and x[["processingLog"]] return
those slots.
Next, if i is a string ending in the
"Unit", then the characters preceding that string
are taken to be the name of an item in the data object, and a list
containing the unit is returned. This list consists of an item
named unit, which is an expression, and
an item named scale, which is a string describing the
measurement scale. If the string ends in " unit", e.g.
x[["temperature unit"]], then just the expression is returned,
and if it ends in " scale", then just the scale is returned.
Next, if i is a string ending in "Flag", then the corresponding
data-quality flag is returned (or NULL if there is no such flag).
For example, x[["salinityFlag"]] returns a vector of salinity
flags if x is a ctd object.
If none of the preceding conditions are met, a check is done
to see if the metadata slot contains an item with
the provided name, and that is returned, if so. A direct match
is required for this condition.
Finally, the data slot is checked to see if it contains
an item with the name indicated by i. In this case, a partial
match will work; this is accomplished by using pmatch.
If none of the above-listed conditions holds, then NULL is returned.
Some uses of [[,ctd-method involve direct retrieval of
items within the data slot of the ctd object,
while other uses involve calculations based on items in that
data slot. For an example, all ctd objects
should hold an item named temperature in the data
slot, so for example x[["temperature"]] will retrieve that
item. By contrast, x[["sigmaTheta"]] is taken to be a
request to compute \(\sigma_\theta\), and so
it yields a call to swTheta(x) even if
the data slot of x might happen to contain an item
named theta. This can be confusing at first, but it tends
to lead to fewer surprises in everyday work, for otherwise the
user would be forced to check the contents of any ctd
object under analysis, to determine whether that item will be looked
up or computed. Nothing is lost in this scheme, since the data
within the object are always accessible with oceGetData.
It should be noted that the accessor is set up to retrieve quantities
in conventional units. For example, read.ctd.sbe is
used on a .cnv file that stores pressure in psi, it will
be stored in the same unit within the ctd object, but
x[["pressure"]] will return a value that has been converted
to decibars. (Users who need the pressure in PSI can
use x@data$pressure.)
Similarly, temperature is
returned in the ITS-90 scale, with a conversion having been performed with
T90fromT68, if the object holds temperature in
IPTS-68. Again, temperature on the IPTS-68
scale is returned with x@data$temperature.
This preference for computed over stored quantities is accomplished
by first checking for computed quantities, and then falling
back to the general [[ method if no match is found.
Some quantities are optionally computed. For example, some data files
(e.g. the one upon which the section dataset is based)
store nitrite along with the sum of nitrite and nitrate, the
latter with name `NO2+NO3`. In this case, e.g. x[["nitrate"]]
will detect the setup, and subtract nitrite from the sum to yield
nitrate.
The list given below provides notes on some quantities that are computed.
conductivity without a second argument (e.g. a[["conductivity"]])
returns the value stored in the object. However, if a second argument is given,
and it is string specifying a unit, then conversion is made to that unit. The
permitted units are: either "" or "ratio" (for ratio),
"uS/cm", "mS/cm" and "S/m". The calculations are based on
the definition of conductivity ratio as the ratio between measured conductivity
and the standard value 4.2914 S/m.
CT or Conservative Temperature: Conservative Temperature,
computed with gsw_CT_from_t in the gsw package.
density: seawater density, computed with swRho(x).
(Note that it may be better to call that function directly, to gain
control of the choice of equation of state, etc.)
depth: Depth in metres below the surface, computed
with swDepth(x).
N2: Square of Brunt-Vaisala frequency, computed with
swN2(x).
potential temperature: Potential temperature in the
UNESCO formulation, computed with swTheta(x).
This is a synonym for theta.
Rrho: Density ratio, computed with swRrho(x).
SA or Absolute Salinity: Absolute Salinity,
computed with gsw_SA_from_SP in the gsw package.
The calculation involves location as well as measured water properties.
If the object x does not containing information on the location,
then 30N and 60W is used for the calculation, and a warning is generated.
sigmaTheta: A form of potential density anomaly, computed with
swSigmaTheta(x).
sigma0 Equal to sigmaTheta, i.e. potential density anomaly
referenced to a pressure of 0dbar, computed with swSigma0(x).
sigma1: Potential density anomaly
referenced to a pressure of 1000dbar, computed with swSigma1(x).
sigma2: Potential density anomaly
referenced to a pressure of 2000dbar, computed with swSigma2(x).
sigma3: Potential density anomaly
referenced to a pressure of 3000dbar, computed with swSigma3(x).
sigma4: potential density anomaly
referenced to a pressure of 4000dbar, computed with swSigma4(x).
SP: Salinity on the Practical Salinity Scale, which is
salinity in the data slot.
spice: a variable that is in some sense orthogonal to density,
calculated with swSpice(x).
SR: Reference Salinity computed with
gsw_SR_from_SP in the gsw package.
Sstar: Preformed Salinity computed with
gsw_SR_from_SP in the gsw package.
See SA for a note on longitude and latitude.
theta: potential temperature in the UNESCO formulation,
computed with swTheta(x). This is a synonym for
potential temperature.
z: Vertical coordinate in metres above the surface, computed with
swZ(x).
A two-step process is used to try to find the
requested information. First, a class-specific function
tries to find it (see
“Details of the specialized ... method”), but
if that fails, then a general function is used
(see ‘Details of the general method’). If both
fail, NULL is returned.
Other functions that extract parts of oce objects: [[,adp-method,
[[,adv-method,
[[,amsr-method,
[[,argo-method,
[[,bremen-method,
[[,cm-method,
[[,coastline-method,
[[,echosounder-method,
[[,g1sst-method,
[[,gps-method,
[[,ladp-method,
[[,landsat-method,
[[,lisst-method,
[[,lobo-method,
[[,met-method, [[,odf-method,
[[,rsk-method,
[[,sealevel-method,
[[,section-method,
[[,tidem-method,
[[,topo-method,
[[,windrose-method,
[[<-,adv-method
Other things related to ctd data: [[<-,ctd-method,
as.ctd, cnvName2oceName,
ctd-class, ctdDecimate,
ctdFindProfiles, ctdRaw,
ctdTrim, ctd,
handleFlags,ctd-method,
initialize,ctd-method,
initializeFlagScheme,ctd-method,
oceNames2whpNames,
oceUnits2whpUnits,
plot,ctd-method, plotProfile,
plotScan, plotTS,
read.ctd.itp, read.ctd.odf,
read.ctd.sbe,
read.ctd.woce.other,
read.ctd.woce, read.ctd,
setFlags,ctd-method,
subset,ctd-method,
summary,ctd-method,
woceNames2oceNames,
woceUnit2oceUnit, write.ctd
# NOT RUN {
data(ctd)
head(ctd[["temperature"]])
# }
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