International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B7. Istanbul 2004
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Figure 2. Location Map
3. METHODOLOGY
For lineament extraction, PCI Geomatica Version 8.2 is used.
The most important factor for using Geomatica is the ability to
extract lineaments from images automatically with the LINE
option. For testing the reliability of the software, the lineaments
are extracted manually by directional filtering. Furthermore, the
lineaments detected is compared with those appear in the the
slope face of the mines in the area. The face discontinuities are
determined again by directional filtering and followed by
manual extraction.
3.1 Automatic Lineament Extraction
LINE option of Geomatica extracts linear features from an
image and records the polylines in a vector segment.
LINE is controlled by the following global parameters:
Name Description
RADI Radius of filter in pixels
GTHR Threshold for edge gradient
LTHR Threshold for curve length
FTHR Threshold for line fitting error
ATHR Threshold for angular difference
DTHR Threshold for linking distance
The LINE module takes a single image channel as input. If it is
16-bit or 32-bit, the image is first scaled to 8 bit using a
nonlinear scaling routine. The output of the program is a vector
segment which contains linear features as extracted from the
image. If database output channel is specified, a binary edge
image (which is the result of thresholding the gradient) will be
saved in the specified channel.
RADI specifies the size of Gaussian kernel which is used as a
filter during edge detection. The larger the RADI value, the less
noise and less details in the edge detection result.
The thresholding value of the gradient image is given by the
parameter GTHR. This value should be in the range 0 to 255.
The user can experiment with different GTHR values and
choose one which produces a suitable binary image. If the ON
pixels in the image appear to be too sparse, the GTHR value
should be decreased. On the other hand, if the ON pixels are
dense and noisy, the GTHR value should be increased. Note that
it is important to have sufficient in formation in the edge image
as the subsequent lineament extraction process is based on this
input edge image.
Various other parameters control the line extraction process.
FTHR is the tolerance for fitting line segments to a (curved)
lineament. It is specified in number of pixels. LTHR is the
minimum length of a curve (in pixels) to be considered as
lineament for further consideration. ATHR is the maximum
angle (in degrees) between two vectors for them to be linked.
DTHR is the maximum distance (in pixels) between two vectors
for them to be linked. (PCI Geomatica Manual, 2001)
In this study, the suitable parameter of LINE for rock
discontinuity extraction are determined.
3.1.2. Algorithm of LINE
The algorithm of LINE consists of three stages: edge detection,
thresholding, and curve extraction.
In the first stage, the Canny edge detection algorithm is applied
to produce an edge strength image. The Canny edge detection
algorithm has three substeps. First, the input image is filtered
with a Gaussian function whose radius is given by the RADI
parameter. Then gradient is computed from the filtered image.
Finally, those pixels whose gradient are not local maximum are
suppressed (by setting the edge strength to 0).
In the second stage, the edge strength image is thresholded to
obtain a binary image. Each ON pixel of the binary image
represents an edge element. The threshold value is given by the
GTHR parameter.
In the third stage, curves are extracted from the binary edge
image. This step consists of several substeps. First, a thinning
algorithm is applied to the binary edge image to produce pixel-
wide skeleton curves. Then a sequence of pixels for each curve
is extracted from the image. Any curve with the number of
pixels less than the parameter value LTHR is discarded from
further processing. An extracted pixel curve is converted to
vector form by fitting piecewise line segments to it. The
resulting polyline is an approximation to the original pixel curve
where the maximum fitting error (distance between the two) is
specified by the FTHR parameter. Finally, the algorithm links
pairs of polylines which satisfy the following criteria:
(1) two end-segments of the two polylines face each other and
have similar orientation (the angle between the two segment is
less than the parameter ATHR);
(2) the two end-segments are close to each other (the distance
between the end points is less than the parameter DTHR). (PCI
Geomatica Manual, 2001)
3.1. Manual Lineament Extraction
In order to evaluate the performance of discontinuity map
produced by line module, a reference map is required. The
reference map for performance evaluation is determined based
on manual extraction. of lineaments, as suggested in the
literature (Suzen et al., 1998, Koike et al., 1995, Novak et al.,
Mah et al., 1995). The main advantage of manual extraction is
that it is easy to detect the non-geological lineaments such as
roads, fences, field boundaries with human eye.
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