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 amsr
[[(x, i, j, ...)In all cases, the returned value is a matrix with
with NA values inserted at locations where
the raw data equal as.raw(251:255), as explained
in “Details”.
an amsr object.
character value indicating the name of an item to extract.
optional additional information on the i item.
ignored.
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 are each NULL, because
no derived values are defined by cm objects.
Data within the data slot may be found directly, e.g.
i="SSTDay" will yield sea-surface temperature in the daytime
satellite, and i="SSTNight" is used to access the nighttime data. In
addition, i="SST" yields a computed average of the night and day values
(using just one of these, if the other is missing). This scheme of
providing computed averages works for
all the data stored in amsr objects, namely:
time, SST, LFwind, MFwind,
vapor, cloud and rain. In each case, the default
is to calculate values in scientific units, unless j="raw", in
which case the raw data are returned.
The conversion from raw to scientific units is done with formulae
found at http://www.remss.com/missions/amsre, e.g. SST is
computed by converting the raw value to an integer (between 0 and 255),
multiplying by 0.15C, and subtracting 3C.
The "raw" mode can be useful
in decoding the various types of missing value that are used by amsr
data, namely as.raw(255) for land, as.raw(254) for
a missing observation, as.raw(253) for a bad observation,
as.raw(252) for sea ice, or as.raw(251) for missing SST
due to rain or missing water vapour due to heavy rain. Note that
something special has to be done for e.g. d[["SST","raw"]]
because the idea is that this syntax (as opposed to specifying
"SSTDay") is a request to try to find good
data by looking at both the Day and Night measurements. The scheme
employed is quite detailed. Denote by "A" the raw value of the desired field
in the daytime pass, and by "B" the corresponding value in the
nighttime pass. If either A or B is 255, the code for land, then the
result will be 255. If A is 254 (i.e. there is no observation),
then B is returned, and the reverse holds also. Similarly, if either
A or B equals 253 (bad observation), then the other is returned.
The same is done for code 252 (ice) and code 251 (rain).
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,
[[,argo-method,
[[,bremen-method,
[[,cm-method,
[[,coastline-method,
[[,ctd-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 amsr data:
[[<-,amsr-method,
amsr-class,
amsr,
composite,amsr-method,
download.amsr(),
plot,amsr-method,
read.amsr(),
subset,amsr-method,
summary,amsr-method
# Histogram of SST values
library(oce)
data(amsr)
hist(amsr[["SST"]])
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