trusty (1) i.cluster.1grass.gz
NAME
i.cluster - Generates spectral signatures for land cover types in an image using a clustering algorithm.
The resulting signature file is used as input for i.maxlik, to generate an unsupervised image
classification.
KEYWORDS
imagery, classification, signatures
SYNOPSIS
i.cluster
i.cluster help
i.cluster [-q] group=name subgroup=name sigfile=name classes=integer [seed=name]
[sample=row_interval,col_interval] [iterations=integer] [convergence=float] [separation=float]
[min_size=integer] [reportfile=name] [--verbose] [--quiet]
Flags:
-q
Quiet
--verbose
Verbose module output
--quiet
Quiet module output
Parameters:
group=name
Name of input imagery group
subgroup=name
Name of input imagery subgroup
sigfile=name
Name for output file containing result signatures
classes=integer
Initial number of classes
Options: 1-255
seed=name
Name of file containing initial signatures
sample=row_interval,col_interval
Sampling intervals (by row and col); default: ~10,000 pixels
iterations=integer
Maximum number of iterations
Default: 30
convergence=float
Percent convergence
Options: 0-100
Default: 98.0
separation=float
Cluster separation
Default: 0.0
min_size=integer
Minimum number of pixels in a class
Default: 17
reportfile=name
Name for output file containing final report
DESCRIPTION
i.cluster performs the first pass in the GRASS two-pass unsupervised classification of imagery, while the
GRASS program i.maxlik executes the second pass. Both programs must be run to complete the unsupervised
classification.
i.cluster is a clustering algorithm that reads through the (raster) imagery data and builds pixel
clusters based on the spectral reflectances of the pixels (see Figure). The pixel clusters are imagery
categories that can be related to land cover types on the ground. The spectral distributions of the
clusters (which will be the land cover spectral signatures) are influenced by six parameters set by the
user. The first parameter set by the user is the initial number of clusters to be discriminated.
| Fig.: Land use/land cover clustering of LANDSAT scene (simplified)
i.cluster starts by generating spectral signatures for this number of clusters and "attempts" to end up
with this number of clusters during the clustering process. The resulting number of clusters and their
spectral distributions, however, are also influenced by the range of the spectral values (category
values) in the image files and the other parameters set by the user. These parameters are: the minimum
cluster size, minimum cluster separation, the percent convergence, the maximum number of iterations, and
the row and column sampling intervals.
The cluster spectral signatures that result are composed of cluster means and covariance matrices. These
cluster means and covariance matrices are used in the second pass (i.maxlik) to classify the image. The
clusters or spectral classes result can be related to land cover types on the ground. The user has to
specify the name of group file, the name of subgroup file, the name of a file to contain result
signatures, the initial number of clusters to be discriminated, and optionally other parameters (see
below) where the group should contain the imagery files that the user wishes to classify. The subgroup
is a subset of this group. The user must create a group and subgroup by running the GRASS program
i.group before running i.cluster. The subgroup should contain only the imagery band files that the user
wishes to classify. Note that this subgroup must contain more than one band file. The purpose of the
group and subgroup is to collect map layers for classification or analysis. The sigfile is the file to
contain result signatures which can be used as input for i.maxlik. The classes value is the initial
number of clusters to be discriminated; any parameter values left unspecified are set to their default
values.
Flags:
-q
Run quietly. Suppresses output of program percent-complete messages and the time elapsed from the
beginning of the program. If this flag is not used, these messages are printed out.
Parameters:
group=name
The name of the group file which contains the imagery files that the user wishes to classify.
subgroup=name
The name of the subset of the group specified in group option, which must contain only imagery
band files and more than one band file. The user must create a group and a subgroup by running the
GRASS program i.group before running i.cluster.
sigfile=name
The name assigned to output signature file which contains signatures of classes and can be used as
the input file for the GRASS program i.maxlik for an unsupervised classification.
classes=value
The number of clusters that will initially be identified in the clustering process before the
iterations begin.
seed=name
The name of a seed signature file is optional. The seed signatures are signatures that contain
cluster means and covariance matrices which were calculated prior to the current run of i.cluster.
They may be acquired from a previously run of i.cluster or from a supervised classification
signature training site section (e.g., using the signature file output by i.class). The purpose
of seed signatures is to optimize the cluster decision boundaries (means) for the number of
clusters specified.
sample=row_interval,col_interval
These numbers are optional with default values based on the size of the data set such that the
total pixels to be processed is approximately 10,000 (consider round up).
iterations=value
This parameter determines the maximum number of iterations which is greater than the number of
iterations predicted to achieve the optimum percent convergence. The default value is 30. If the
number of iterations reaches the maximum designated by the user; the user may want to rerun
i.cluster with a higher number of iterations (see reportfile).
Default: 30
convergence=value
A high percent convergence is the point at which cluster means become stable during the iteration
process. The default value is 98.0 percent. When clusters are being created, their means
constantly change as pixels are assigned to them and the means are recalculated to include the new
pixel. After all clusters have been created, i.cluster begins iterations that change cluster
means by maximizing the distances between them. As these means shift, a higher and higher
convergence is approached. Because means will never become totally static, a percent convergence
and a maximum number of iterations are supplied to stop the iterative process. The percent
convergence should be reached before the maximum number of iterations. If the maximum number of
iterations is reached, it is probable that the desired percent convergence was not reached. The
number of iterations is reported in the cluster statistics in the report file (see reportfile).
Default: 98.0
separation=value
This is the minimum separation below which clusters will be merged in the iteration process. The
default value is 0.0. This is an image-specific number (a "magic" number) that depends on the
image data being classified and the number of final clusters that are acceptable. Its
determination requires experimentation. Note that as the minimum class (or cluster) separation is
increased, the maximum number of iterations should also be increased to achieve this separation
with a high percentage of convergence (see convergence).
Default: 0.0
min_size=value
This is the minimum number of pixels that will be used to define a cluster, and is therefore the
minimum number of pixels for which means and covariance matrices will be calculated.
Default: 17
reportfile=name
The reportfile is an optional parameter which contains the result, i.e., the statistics for each
cluster. Also included are the resulting percent convergence for the clusters, the number of
iterations that was required to achieve the convergence, and the separability matrix.
NOTES
Running in command line mode, i.cluster will overwrite the output signature file and reportfile (if
required by the user) without prompting if the files existed.
EXAMPLE
Preparing the statistics for unsupervised classification of a LANDSAT subscene in North Carolina:
g.region rast=lsat7_2002_10 -p
# store VIZ, NIR, MIR into group/subgroup
i.group group=my_lsat7_2002 subgroup=my_lsat7_2002 \
input=lsat7_2002_10,lsat7_2002_20,lsat7_2002_30,lsat7_2002_40,lsat7_2002_50,lsat7_2002_70
i.cluster group=my_lsat7_2002 subgroup=my_lsat7_2002 sigfile=sig_clust_lsat2002 \
classes=10 report=rep_clust_lsat2002.txt
To complete the unsupervised classification, i.maxlik is subsequently used.
SEE ALSO
The GRASS 4 Image Processing manual
i.class, i.group, i.gensig, i.maxlik
AUTHORS
Michael Shapiro, U.S.Army Construction Engineering Research Laboratory
Tao Wen, University of Illinois at Urbana-Champaign, Illinois
Last changed: $Date: 2012-12-16 04:47:36 -0800 (Sun, 16 Dec 2012) $
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