[[,landsat-method: Extract Something From a landsat Object

Description

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.

Usage

# S4 method for landsat
[[(x, i, j, ...)

Arguments

x: a landsat object.
i: character value indicating the name of an item to extract.
j: optional additional information on the i item.
...: ignored.

Details of the Specialized Method

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 data entries are difficult to deal with directly, and so users are advised to use dataDerived instead.

Accessing band data. The data may be accessed with e.g. landsat[["panchromatic"]], for the panchromatic band. If a new ``band'' is added with landsatAdd(), it may be referred by name. In all cases, a second argument can be provided, to govern decimation. If this is missing, all the relevant data are returned. If this is present and equal to TRUE, then the data will be automatically decimated (subsampled) to give approximately 800 elements in the longest side of the matrix. If this is present and numerical, then its value governs decimation. For example, landsat[["panchromatic",TRUE]] will auto-decimate, typically reducing the grid width and height from 16000 to about 800. Similarly, landsat[["panchromatic",10]] will reduce width and height to about 1600. On machines with limited RAM (e.g. under about 6GB), decimation is a good idea in almost all processing steps. It also makes sense for plotting, and in fact is done through the `decimate` argument of plot,landsat-method().

Accessing derived data. One may retrieve several derived quantities that are calculated from data stored in the object: landsat[["longitude"]] and landsat[["latitude"]] give pixel locations. Accessing landsat[["temperature"]] creates an estimate of ground temperature as follows (see reference 4). First, the ``count value'' in band 10, denoted $b_{10}$ say, is scaled with coefficients stored in the image metadata using $\lambda_L=b_{10}M_L+A_L$ where $M_L$ and $A_L$ are values stored in the metadata (e.g. the first in landsat@metadata$header$radiance_mult_band_10) Then the result is used, again with coefficients in the metadata, to compute Celcius temperature $T=K_2/ln(\epsilon K_1/\lambda_L+1)-273.15$. The value of the emissivity $\epsilon$ is set to unity by read.landsat(), although it can be changed easily later, by assigning a new value to `landsat@metadata$emissivity`. The default emissivity value set by read.landsat() is from reference 11, and is within the oceanic range suggested by reference 5. Adjustment is as simple as altering `landsat@metadata$emissivity`. This value can be a single number meant to apply for the whole image, or a matrix with dimensions matching those of band 10. The matrix case is probably more useful for images of land, where one might wish to account for the different emissivities of soil and vegetation, etc.; for example, Table 4 of reference 9 lists 0.9668 for soil and 0.9863 for vegetation, while Table 5 of reference 10 lists 0.971 and 0.987 for the same quantities.

Accessing metadata. Anything in the metadata can be accessed by name, e.g. landsat[["time"]]. Note that some items are simply copied over from the source data file and are not altered by e.g. decimation. An exception is the lat-lon box, which is altered by landsatTrim().

Details of the General Method

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.

Author

Dan Kelley

Details

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.