Image Processing Reference
• ROI collection : You will now generate a number of training samples to supervise
the classification. These training areas are called regions of interest (ROIs).
From Basic Tools (on the main horizontal tool bar at the top of your screen
choose) choose Region of Interest and then ROI Tool . The new window will
record all of your training areas as you collect them from the image (because of
the size of the subset you may want to use the zoom window to navigate). Click
on the * next to Region #1 to highlight it, then begin drawing a polygon by mov-
ing the cursor around the area of interest, clicking the left mouse once to draw
an arc and clicking the right mouse once to finish the polygon, clicking right
again will record the ROI permanently. Collect training areas for four types of
residential configurations in Figs. 8.5 and 8.6 as well as for the commercial area
labeled as E and the school in the north-west of the subset. Every time a new
training area is collected you will need to click on New Region . An erroneous
ROI can be deleted.
• Classification : You are now ready to classify. Again, from the main horizontal
tool bar at the top of your screen choose Classification and Supervised and then
Maximum Likelihood . Select the belfast_subset image . Click OK . Another win-
dow will list the Maximum Likelihood Parameters . Select All Items . Click on
Multiple Values and then Assign Multiple Values the following prior probabili-
ties to each Region by highlighting it and typing in the Edit Selected Item :
Assign probability threshold
A: Very high density, strong linear residential
B: Low density, weak linear residential
C: Medium density, strong linear residential
D: Very low density, weak linear residential
E: Low density, weak linear commercial
X: Single commercial
Click OK when done. Do not click OK on the main Maximum Likelihood
Parameters window, yet.
In order to appreciate the power of variable prior probabilities you
can Preview the classification every time you change the probabilities. Experiment
with combinations of prior probabilities to see the effects. When ready, you may
generate a full classification by choosing an Output Result File name. In the
Available Bands List select your classified file and Load Band in New Display .
From Tools , Color Mapping and Class Color Mapping you can change the col-
ors of the classified image. Hopefully, you will have finished within the 7 min!
Acknowledgements ADDRESS-POINT TM and COMPAS TM are Crown Copyright (www.ordsvy.
gov.uk and www.osni.gov.uk respectively). Cities Revealed is the copyright of the GeoInformation
Group (www.crworld.co.uk). IKONOS image was provided by Space Imaging. The author would
like to thank Paul McKenzie for the production of Fig. 8.4 .