orresponding
acts in the
employed.
and 3/1 (in
checked if
composite
en color in
istinct from
sations are
trix-overlay
mbine the
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lated to the
; were then
supervied
je obtained
the training
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)ssans and
ve two GIS
d files were
sediments
ly and the
of 28% for
mages.
The next step was to recode the different classes using an
offset of 1, the alteration bodies were recoded as 1 whereas
all the remaining classes as 0. The obtained two recoded
files were then joined using a matrix overlay technique with
an “intersection” function, whereby those pixels having a
value of 0 and 0, 0 and 1, or 1 and 0 in the GIS files obtained
form band-ratioing and Feature-oriented PC transformation,
respectively were regrouped accordingly as classes 1, 2, and
3 whereby they were given a new value of 0. Those pixels
having a value of 1 in both GIS files, and thus correspond to
the alteration bodies, were grouped as class 4 and obtained
a new value of 1. Further rifining was carried out to omit
isolated random pixels or group of pixels less than about
100mX100m size by runing a 3X3 low-pass filter. The
resulting binary image appears to have succeeded in
excluding the undesired Quaternary sediments and has
substantialy mapped the gossaniferous bodies (Kenea,
1996). Interestingly the image has also managed to exclude
basic metavolcanics that did exhibit a similar feature as the
altered rocks on the ratio image produced by List, et. al.
(1992, personal comm.) for the same area.
4. CONCLUSION
The applied standardized PC transformation has proven
useful for lithologic discrimination among rocks with subtle
chemical difference where commonly used band composites
failed to give good results. Furthermore, in addition to data
compression, it provided images with higher signal-to-noise
ratio that allowed more band combinations thereby enabling
improved mapping of the rock units in the area. Known
hydrothermally altered areas and gossaniferous bodies have
been successfully mapped by using Feature-oriented PCA
followed by low-pass filtering, making the method
recommendable for exploration in similar occurrances.
Computation of band-ratio composites followed by a GIS
matrix-overlay technique have substantially improved the
result by excluding undesired objects. The latter operations
appear least useful for lithologic discrimination, mainly for
lack of morphological features useful for geologic image
interpretation, longer processing steps and involvement of
noise.
5. ACKNOWLEDGEMENTS
Field works were conducted under the Special Research
Project-69 (SFB-69), sub-project F5/E7, financed by the
Deutsche Forschungsgemeinschaft (German Research
Foundation). The stay of the first author in Germany was
sponsored by the German Academic Exchange Service
(DAAD). The research has been supervised by Prof. F. K.
List. All colleagues at the remote sensing section (FU) are
thanked for their assistance at several stages of the work,
particularly R. Schóler for keeping the computers running.
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