NAME¶
i.class - Generates spectral signatures for an image by allowing
the user to outline regions of interest.
The resulting signature file can be used as input for i.maxlik or as a seed
signature file for i.cluster.
KEYWORDS¶
imagery, classification
SYNOPSIS¶
i.class
i.class help
i.class map=
name group=
name
subgroup=
name outsig=
name
[
insig=
name] [--
verbose] [--
quiet]
Parameters:¶
- map=name
-
Name of raster map to be displayed
- group=name
-
Name of input imagery group
- subgroup=name
-
Name of input imagery subgroup
- outsig=name
-
File to contain result signatures
- insig=name
-
File containing input signatures (seed)
DESCRIPTION¶
i.class performs the first pass in the GRASS two-pass supervised image
classification process; the GRASS program
i.maxlik executes the second
pass. Both programs must be run to generate a classified map in GRASS raster
format.
i.class is an interactive program that allows the user to outline a
region on the screen and calculate the spectral signature based on the cells
that are within that region. During this process the user will be shown a
histogram of the region for each image band. The user can also display the
cells of the image bands which fall within a user-specified number of standard
deviations from the means in the spectral signature. By doing this, the user
can see how much of the image is likely to be put into the class associated
with the current signature.
The spectral signatures that result are composed of region means and covariance
matrices. These region means and covariance matrices are used in the second
pass (
i.maxlik) to classify the image.
Alternatively, the spectral signatures generated by
i.class can be used
for seed means for the clusters in the
i.cluster program.
At the command line the user is asked to enter the name of the raster map to be
displayed during the process of outlining regions. Typically, the user will
want to enter the name of a color composite previously created by
r.composite. However, the user can enter the name of any existing
raster map. This leaves the potential for using a raster map not directly
derived from the image as a backdrop on which the user can outline the classes
of interest.
The first screen in the program
i.class asks the user for the imagery
group and
subgroup to be analyzed:
LOCATION: location SUPERVISED CLASSIFIER MAPSET: demo
Please select the group and subgroup to be analyzed
GROUP: spot_______ (list will show available groups)
SUBGROUP: 123________ (list will show available subgroups)
AFTER COMPLETING ALL ANSWERS, HIT <ESC> TO CONTINUE
(OR <Ctrl-C> TO CANCEL)
The
group should contain the imagery bands that the user wishes to
classify. The
subgroup is a subset of this group. The user must create
a group and a subgroup by running the GRASS program
i.group before
running
i.class. The subgroup should contain only the image bands that
the user wishes to classify. Note that this subgroup must contain more than
one band.
After the first screen, the program asks the user for the name of the resulting
signature file. The signature file is both the output file for
i.class
and the required input file for the GRASS
i.maxlik module. It contains
the region means and covariance matrices that are used to classify an image in
i.maxlik. The signature file will be saved in the
$MAPSET/group/$GROUP/subgroup/$SUBGROUP/sig/ directory.
After entering the resulting signature file name, the user is asked to enter the
name of a seed signature file. This is optional. A "seed" signature
file is a previously created signature file. Such a seed signature file may be
the result of an earlier run of
i.class. The seed signature file is
copied into the new resulting signature file before any new signatures are
added by
i.class. In this way, you can collect the work from several
sessions with
i.class into one signature file.
At this point the
i.class graphics screen will be drawn on the graphics
monitor and the user will be directed to use the mouse. From this point on the
user will primarily work with the mouse, selecting options from the menus and
outlining regions on the screen. The only time that the user will need to
return to the text terminal is to enter names for the signatures created.
THE DISPLAY FRAMES¶
The display frame layout that
i.class uses is represented below for
reference.
+-----------------------+---------------------------+
| | |
| | Map Display Frame |
| | |
| | |
| Histogram Display | |
| Frame | |
| | |
| | |
| | |
| +---------------------------+
| | |
| | Zoom Display Frame |
| | |
| | |
| | |
| | |
| | |
| | |
+-----------------------+---------------------------+
| Menu Frame |
+---------------------------------------------------+
All of the menus in the
i.class program are displayed across the bottom
of the graphics monitor in the Menu Frame. To select an option from one of
these menus, simply place the cursor over your selection and press any button
on the mouse. Each of the menus is discussed in the following paragraphs.
The Command Menu¶
The Command Menu includes the following selections:
- Zoom
-
This command allows the user to outline a rectangular region in either the
Map or Zoom Display Frames and the region is displayed, magnified, to fit
in the Zoom Display Frame. A red rectangle is drawn in the Map Display
Frame, indicating what area the Zoom Display Frame shows.
To outline the rectangular region simply use any mouse button to anchor the
first corner of the border and then use any button to choose the other corner.
- Define region
-
This selection takes the user to the Region Menu. This menu includes the
options that allow the user to outline a region of interest on the
displayed raster map.
- Redisplay map
-
This selection takes the user to the Redisplay Menu. The Redisplay Menu
allows the user to redraw map display frames.
- Analyze region
-
This selection starts the process of analyzing the currently defined region.
A histogram of the defined region will be displayed for each band. On the
histogram for each band, the mean, standard deviation, minimum cell value
and maximum cell value are marked. The histograms are automatically scaled
in an attempt to fit the data into the space available, but it is possible
that all of the data will not fit. In this case, as much of the data as
possible, centered around the mean, will be displayed. After the
histograms are displayed, the user will be given the Signature Menu.
- Quit
-
The user should make this selection to end the session with
i.class.
The Region Menu contains the following selections:
- Erase region
-
This selection erases any currently defined region.
- Draw region
-
This selection allows the user to use the mouse to draw a region on either
the Map or Zoom Display Frame. An explanation of which mouse buttons to
use is displayed in the Menu Frame. The user does not need to try to
complete the region boundary. The last line of the region will be added
when the user selects the Complete region option on the Region Menu.
- Restore last region
-
This selection restores the last region that was drawn. After a region is
completed, it will be saved to be restored later. Only one previous region
is saved.
- Complete region
-
This selection completes the region that is currently being drawn. As noted
above, it saves the complete region to be restored later, if needed. Once
the user has made a complete region, it can be analyzed with the Analyze
Region selection on the Command Menu.
- Done
-
Use this selection to return to the Command Menu.
The Redisplay Map Menu has the following selections, which are useful to redraw
the raster maps displayed in the Map and Zoom Display Frames.
- Map geographic region
-
This selection causes the raster map in the Map Display Frame to be
redrawn.
- Zoom region
-
This selection causes the Zoom Display Frame to be redrawn.
- Both
-
This selection causes both the Map and Zoom Display Frames to be
redrawn.
- Cancel
-
Use this selection if you do not want to redisplay either of the above
regions. The user will be returned to the Command Menu.
The Analyze Region Menu contains the Signature Menu, which allows the user to
set the number of standard deviations and the display color, and then to
display (as an overlay) the cells that match the signature within the number
of standard deviations specified. Note that once the matching cells are
displayed, the Map Display Frame must be redisplayed to see only the original
raster map again. The following selections are available on the Signature
Menu:
- Set std dev's
-
This selection allows the user to set the number of standard deviations from
the mean for the maximum and minimum range. The maximum and minimum range
is used when finding the cells that "match" the signature. The
user is presented with a menu of typical choices and an "Other"
option. If the "Other" option is selected, enter the number of
standard deviations from the keyboard on the text terminal. Otherwise, the
selected option will be used. When the number of standard deviations is
set, the histograms for each band will be redrawn with the maximum and
minimum range marked.
Note that the number in parentheses on this selection is the current number of
standard deviations.
- Set color
-
This selection allows the user to set the color for the display of cells
that "match" the current signature. The user is presented with a
menu of color choices. The color selected will be used when the Display
Matches Menu selection is made.
Note that the color in parentheses on this selection is the current color for
display.
- Display matches
-
This selection displays the cells that "match" the current
signature in the current color. A cell "matches" the current
signature if the cell value in each band is between the minimum range and
maximum range for that band defined by the number of standard deviations
currently set.
- Done
-
When this selection is chosen, the user will be asked whether or not he/she
would like to save the current signature. If the user answers with the
"Yes" selection, he/she will be asked to enter a description for
the resultant signature file on the text terminal keyboard. The saved
signature file description will be used by i.maxlik to name
the category that is created from the current signature. After either a
"No" answer or the signature description is entered, the user is
returned to the Command Menu.
NOTES¶
i.class uses the current MASK to generate the overlay for cells that
match a signature. As a result, if a MASK already exists it will be removed
during the execution of this program.
The cell values in the image bands cannot fall outside of the range of 0 to 255.
i.class will report an error if they do.
i.class, like some of the other imagery programs, does not use the
standard GRASS display frames. After running
i.class, you will need to
create a display frame (e.g., using
d.frame or
d.erase) before
you can use most of the GRASS display (d.) commands.
i.group must be run before
i.class to create an imagery group and
a subgroup containing the image bands to be classified.
The user can perform a supervised image classification by running
i.class
followed by
i.maxlik. The user can perform an unsupervised
classification by running
i.cluster followed by
i.maxlik.
i.class is interactive and requires the user to be running a graphics
display monitor (see
d.mon) to run this program.
SEE ALSO¶
The GRASS 4
Image Processing manual
d.frame, d.mon, g.region, i.cca, i.cluster,
r.composite, i.group, i.maxlik, r.mapcalc
AUTHOR¶
David Satnik, Central Washington University
Last changed: $Date: 2011-11-08 12:29:50 +0100 (Tue, 08 Nov 2011) $
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