read.landsat: Read a landsat File Directory

Description

Read a landsat data file, producing an object of landsat-class. The actual reading is done with readTIFF in the tiff package, so that package must be installed for read.landsat to work.

Usage

read.landsat(file, band = "all", emissivity = 0.984, decimate,
  debug = getOption("oceDebug"))

Arguments

file

A connection or a character string giving the name of the file to load. This is a directory name containing the data.

band

The bands to be read, by default all of the bands. Use `band=NULL` to skip the reading of bands, instead reading only the image metadata, which is often enough to check if the image is of interest in a given study. See ‘Details’ for the names of the bands, some of which are pseudo-bands, computed from the actual data.

emissivity

Value of the emissivity of the surface, stored as emissivity in the metadata slot of the resultant object. This is used in the calculation of surface temperature, as explained in the discussion of accessor functions for landsat-class. The default value is from Konda et al. (1994). These authors suggest an uncertainty of 0.04, but a wider range of values can be found in the literature. The value of metadata$emissivity is easy to alter, either as a single value or as a matrix, yielding flexibility of calculation.

decimate

optional positive integer indicating the degree to which the data should be subsampled after reading and before storage. Setting this to 10 can speed up reading by a factor of 3 or more, but higher values have diminishing effect. In exploratory work, it is useful to set decimate=10, to plot the image to determine a subregion of interest, and then to use landsatTrim to trim the image.

debug

a flag that turns on debugging. Set to 1 to get a moderate amount of debugging information, or to 2 to get more.

Value

An object of landsat-class, with the conventional Oce slots metadata, data and processingLog. The metadata is mainly intended for use by Oce functions, but for generality it also contains an entry named header that represents the full image header in a list (with names made lower-case). The data slot holds matrices of the data in the requested bands, and users may add extra matrices if desired, e.g. to store calculated quantities.

Storage requirements

Landsat data files (directories, really) are large, accounting for approximately 1 gigabyte each. The storage of the Oce object is similar (see landsat-class). In R, many operations involving copying data, so that dealing with full-scale landsat images can overwhelm computers with storage under 8GB. For this reason, it is typical to read just the bands that are of interest. It is also helpful to use landsatTrim to trim the data to a geographical range, or to use decimate to get a coarse view of the domain, especially early in an analysis.

Details

Landsat data are provided in directories that contain TIFF files and header information, and read.landsat relies on a strict convention for the names of the files in those directories. Those file names were found by inspection of some data, on the assumption that similar patterns will hold for other datasets for any given satellite. This is a brittle approach and it should be born in mind if read.landsat fails for a given dataset.

For Landsat 8, there are 11 bands, with names "aerosol" (band 1), "blue" (band 2), "green" (band 3), "red" (band 4), "nir" (band 5), "swir1" (band 6), "swir2" (band 7), "panchromatic" (band 8), "cirrus" (band 9), "tirs1" (band 10), and "tirs2" (band 11). In addition to the above, setting band="terralook" may be used as an abbreviation for band=c("red", "green", "nir").

For Landsat 7, there 8 bands, with names "blue" (band 1), "green" (band 2), "red" (band 3), "nir" (band 4), "swir1" (band 5), "tir1" (band 6A), "tir2" (band 6B), "swir2" (band 7) and "panchromatic" (band 8).

For Landsat 4 and 5, the bands similar to Landsat 7 but without "panchromatic" (band 8).

References

1. Konda, M. Imasato N., Nishi, K., and T. Toda, 1994. Measurement of the Sea Surface Emissivity. Journal of Oceanography, 50, 17:30. Available at http://www.terrapub.co.jp/journals/JO/pdf/5001/50010017.pdf as of February 2015.