i.gensigsetgrass - Online in the Cloud

PROGRAM:

NAME

i.gensigset - Generates statistics for i.smap from raster map.

KEYWORDS

imagery, classification, supervised classification, SMAP, signatures

SYNOPSIS

i.gensigset
i.gensigset --help
i.gensigset trainingmap=name group=name subgroup=name signaturefile=name [maxsig=integer]
[--help] [--verbose] [--quiet] [--ui]

Flags:
--help
Print usage summary

--verbose
Verbose module output

--quiet
Quiet module output

--ui
Force launching GUI dialog

Parameters:
trainingmap=name [required]
Ground truth training map

group=name [required]
Name of input imagery group

subgroup=name [required]
Name of input imagery subgroup

signaturefile=name [required]
Name for output file containing result signatures

maxsig=integer
Maximum number of sub-signatures in any class
Default: 5

DESCRIPTION

i.gensigset is a non-interactive method for generating input into i.smap. It is used as
the first pass in the a two-pass classification process. It reads a raster map layer,
called the training map, which has some of the pixels or regions already classified.
i.gensigset will then extract spectral signatures from an image based on the
classification of the pixels in the training map and make these signatures available to
i.smap.

The user would then execute the GRASS program i.smap to create the final classified map.

OPTIONS

Parameters
trainingmap=name
ground truth training map

This raster layer, supplied as input by the user, has some of its pixels already
classified, and the rest (probably most) of the pixels unclassified. Classified means
that the pixel has a non-zero value and unclassified means that the pixel has a zero
value.

This map must be prepared by the user in advance by using a combination of wxGUI vector
digitizer and v.to.rast, or some other import/developement process (e.g., v.transects) to
define the areas representative of the classes in the image.

At present, there is no fully-interactive tool specifically designed for producing this
layer.

group=name
imagery group

This is the name of the group that contains the band files which comprise the image to be
analyzed. The i.group command is used to construct groups of raster layers which comprise
an image.

subgroup=name
subgroup containing image files

This names the subgroup within the group that selects a subset of the bands to be
analyzed. The i.group command is also used to prepare this subgroup. The subgroup
mechanism allows the user to select a subset of all the band files that form an image.

signaturefile=name
resultant signature file

This is the resultant signature file (containing the means and covariance matrices) for
each class in the training map that is associated with the band files in the subgroup
selected.

maxsig=value
maximum number of sub-signatures in any class
default: 5

The spectral signatures which are produced by this program are "mixed" signatures (see
NOTES). Each signature contains one or more subsignatures (represeting subclasses). The
algorithm in this program starts with a maximum number of subclasses and reduces this
number to a minimal number of subclasses which are spectrally distinct. The user has the
option to set this starting value with this option.

INTERACTIVE MODE

If none of the arguments are specified on the command line, i.gensigset will interactively
prompt for the names of these maps and files.

It should be noted that interactive mode here only means interactive prompting for maps
and files. It does not mean visualization of the signatures that result from the process.

NOTES

The algorithm in i.gensigset determines the parameters of a spectral class model known as
a Gaussian mixture distribution. The parameters are estimated using multispectral image
data and a training map which labels the class of a subset of the image pixels. The
mixture class parameters are stored as a class signature which can be used for subsequent
segmentation (i.e., classification) of the multispectral image.

The Gaussian mixture class is a useful model because it can be used to describe the
behavior of an information class which contains pixels with a variety of distinct spectral
characteristics. For example, forest, grasslands or urban areas are examples of
information classes that a user may wish to separate in an image. However, each of these
information classes may contain subclasses each with its own distinctive spectral
characteristic. For example, a forest may contain a variety of different tree species
each with its own spectral behavior.

The objective of mixture classes is to improve segmentation performance by modeling each
information class as a probabilistic mixture with a variety of subclasses. The mixture
class model also removes the need to perform an initial unsupervised segmentation for the
purposes of identifying these subclasses. However, if misclassified samples are used in
the training process, these erroneous samples may be grouped as a separate undesired
subclass. Therefore, care should be taken to provided accurate training data.

This clustering algorithm estimates both the number of distinct subclasses in each class,
and the spectral mean and covariance for each subclass. The number of subclasses is
estimated using Rissanen’s minimum description length (MDL) criteria [1]. This criteria
attempts to determine the number of subclasses which "best" describe the data. The
approximate maximum likelihood estimates of the mean and covariance of the subclasses are
computed using the expectation maximization (EM) algorithm [2,3].

WARNINGS

If warnings like this occur, reducing the remaining classes to 0:
...
WARNING: Removed a singular subsignature number 1 (4 remain)
WARNING: Removed a singular subsignature number 1 (3 remain)
WARNING: Removed a singular subsignature number 1 (2 remain)
WARNING: Removed a singular subsignature number 1 (1 remain)
WARNING: Unreliable clustering. Try a smaller initial number of clusters
WARNING: Removed a singular subsignature number 1 (-1 remain)
WARNING: Unreliable clustering. Try a smaller initial number of clusters
Number of subclasses is 0
then the user should check for:

· the range of the input data should be between 0 and 100 or 255 but not between 0.0
and 1.0 (r.info and r.univar show the range)

· the training areas need to contain a sufficient amount of pixels

REFERENCES

· J. Rissanen, "A Universal Prior for Integers and Estimation by Minimum Description
Length," Annals of Statistics, vol. 11, no. 2, pp. 417-431, 1983.

· A. Dempster, N. Laird and D. Rubin, "Maximum Likelihood from Incomplete Data via
the EM Algorithm," J. Roy. Statist. Soc. B, vol. 39, no. 1, pp. 1-38, 1977.

· E. Redner and H. Walker, "Mixture Densities, Maximum Likelihood and the EM
Algorithm," SIAM Review, vol. 26, no. 2, April 1984.

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