Generally, the [[ method lets users extract information from oce
objects, without having to know the details of the internal storage. For
many oce sub-classes, [[ can also return quantities that are computed
from the object's contents.
# S4 method for ctd
[[(x, i, j, ...)a ctd object.
character value indicating the name of an item to extract.
optional additional information on the i item.
ignored.
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 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. (To get pressure in PSI, use x[["pressurePSI"]].)
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
available using e.g. ctd[[i]].
If i is "?", then the return value is a list
containing four items, each of which is a character vector
holding the names of things that can be accessed with [[.
The data and metadata items hold the names of
entries in the object's data and metadata
slots, respectively. The dataDerived
and metadataDerived items hold the names of things
that can be inferred from the object's contents, e.g.
"SA" is named in dataDerived, indicating that
argo[["SA"]] is permitted (to compute Absolute Salinity).
If i is "conductivity" without a second argument (e.g. a[["conductivity"]])
then the return value is the seawater electrical conductivity (if available
or computable). 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.
If i is "CT" or "Conservative Temperature" then Conservative
Temperature, computed with gsw::gsw_CT_from_t(), is returned.
If i is "density" then seawater density, computed with swRho(x),
is returned. (Note that it may be better to call that function directly, to gain
control of the choice of equation of state, etc.)
If i is "depth" then the depth in metres below the surface, computed
with swDepth(x), is returned.
If i is "N2" then the square of Brunt-Vaisala frequency, computed with
swN2(x), is returned.
If i is "potential temperature" or "theta", then potential temperature in the
UNESCO formulation, computed with swTheta(x), is returned.
If i is "Rrho" then density ratio, computed with swRrho(x), is
returned.
If i is "SA" or "Absolute Salinity" then Absolute Salinity,
computed with gsw::gsw_SA_from_SP(), is returned.
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.
If i is "sigmaTheta" then a form of potential density anomaly, computed with
swSigmaTheta(x), is returned.
If i is "sigma0" then potential density anomaly
referenced to a sea pressure of 0dbar (the surface), computed with swSigma0(x),
is returned.
If i is "sigma2" then potential density anomaly
referenced to a sea pressure of 1000dbar, computed with swSigma1(x),
is returned.
If i is "sigma2" then potential density anomaly
referenced to a sea pressure of 2000dbar, computed with swSigma2(x),
is returned.
If i is "sigma3" then potential density anomaly
referenced to a sea pressure of 3000dbar, computed with swSigma3(x),
is returned.
If i is "sigma4" then potential density anomaly
referenced to a sea pressure of 4000dbar, computed with swSigma4(x),
is returned.
If i is "SP" then salinity on the Practical Salinity Scale, which is
salinity in the data slot, is returned.
If i is "spice" then swSpice() is called to compute a quantity that
is in some sense orthogonal to density on a T-S diagram. This is done by
calling swSpice() with the eos argument set to "unesco". In an earlier
version of oce, [[ could be provided with a second argument to yield a
return value for "spiciness", a variable that is described in the next item.
On 2024-02-14, this possibility was removed because it could lead to user
confusion and non-reproducible code. To get spiciness, use
[["spiciness0"]].
If i is "spiciness0", "spiciness1" or "spiciness2", then the return
value comes from the Gibbs SeaWater formulation of a variable that is in some
sense orthogonal to density on a T-S diagram. The numbers refer to the
reference pressure, in units of 1000 dbar. These results are computed with
gsw::gsw_spiciness0(), etc.
If i is "SR" then Reference Salinity, computed with
gsw::gsw_SR_from_SP(), is returned.
If i is "Sstar" then Preformed Salinity, computed with
gsw::gsw_SR_from_SP(), is returned.
See SA for a note on longitude and latitude.
If i is "time" then either vector of times or a single
time, is returned, if available. A vector is returned if time
is present in the data slot, or if a time can be
inferred from other entries in the data slot (some of which,
such as the common timeS, also employ
startTime within the metadata slot). A single
value is returned if the dataset only has information on the start
time (which is stored as startTime within the metadata
slot. If it is impossible to determine the sampling time, then
NULL is returned. These time variants occur, in the
present version of oce, only for data read by read.ctd.sbe(),
the documentation of which explains how times are computed.
If i is "z" then vertical coordinate in metres
above the surface, computed with swZ(x), is returned.
Note: the text of this section is identical for all oce subclasses, and so
some of what you read here may not be relevant to the class being described
in this help page.
If the specialized method produces no matches, the following generalized
method is applied. As with the specialized method, the procedure hinges first
on the values of i and, optionally, j. The work proceeds in steps, by
testing a sequence of possible conditions in sequence.
A check is made as to whether i names one of the standard oce slots.
If so, [[ returns the slot contents of that slot. Thus, x[["metadata"]]
will retrieve the metadata slot, while x[["data"]] and
x[["processingLog"]] return those slots.
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 (or NULL if there is no such unit).
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"]] (note the
space), then just the expression is returned, and if it ends in " scale",
then just the scale is returned.
If i is a string ending in "Flag", then the corresponding data-quality
flag is returned (or NULL if there is no such flag).
If the object holds hydrographic information (pressure, salinity,
temperature, longitude and latitude) then another set of possibilities
arises. If i is "sigmaTheta", then the value of swSigmaTheta() is
called with x as the sole argument, and the results are returned.
Similarly, swSigma0() is used if i="sigma0", and swSpice() is used if
i="spice". Of course, these actions only make sense for objects that
contain the relevant items within their data slot.
After these possibilities are eliminated, the action depends on whether
j has been provided. If j is not provided, or is the string "", then
i is sought in the metadata slot, and then in the data slot, returning
whichever is found first. In other words, if j is not provided, the
metadata slot takes preference over the data slot. However, if j is
provided, then it must be either the string "metadata" or "data", and it
directs where to look.
If none of the above-listed conditions holds, then NULL is returned.
Dan Kelley
A two-step process is used to try to find the requested information. First, a
class-specific function is used (see “Details of the Specialized
Method”). If this yields nothing, then a general method is used (see
“Details of the General Method”). If both methods fail, then [[
returns NULL.
Some understanding of the subclass is required to know what can be retrieved
with [[. When dealing with an unfamiliar subclass, it can be useful to
first use x[["?"]] to get a listing of the retrievable items. See
“Details of the Specialized Method” for more information.
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,
[[,oce-method,
[[,odf-method,
[[,rsk-method,
[[,sealevel-method,
[[,section-method,
[[,tidem-method,
[[,topo-method,
[[,windrose-method,
[[,xbt-method,
[[<-,adv-method
Other things related to ctd data:
CTD_BCD2014666_008_1_DN.ODF.gz,
[[<-,ctd-method,
as.ctd(),
cnvName2oceName(),
ctd,
ctd-class,
ctd.cnv.gz,
ctdDecimate(),
ctdFindProfiles(),
ctdFindProfilesRBR(),
ctdRaw,
ctdRepair(),
ctdTrim(),
ctd_aml.csv.gz,
d200321-001.ctd.gz,
d201211_0011.cnv.gz,
handleFlags,ctd-method,
initialize,ctd-method,
initializeFlagScheme,ctd-method,
oceNames2whpNames(),
oceUnits2whpUnits(),
plot,ctd-method,
plotProfile(),
plotScan(),
plotTS(),
read.ctd(),
read.ctd.aml(),
read.ctd.itp(),
read.ctd.odf(),
read.ctd.odv(),
read.ctd.saiv(),
read.ctd.sbe(),
read.ctd.ssda(),
read.ctd.woce(),
read.ctd.woce.other(),
setFlags,ctd-method,
subset,ctd-method,
summary,ctd-method,
woceNames2oceNames(),
woceUnit2oceUnit(),
write.ctd()
data(ctd)
head(ctd[["temperature"]])
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