The threshold
θ
is set as 7° empirically, indicating the
minimum slope for forming Martian gullies. This operation
removes the linear structures distributed in flat terrains,
which are usually non-gully features. The non-gully feature in
the bottom of Figure 5 is easily removed by relief calculation.
Results and Discussion
We have carried out gully detection experiments in other five
sites, and the detection results are shown in Figure 6.
Three quality factors are used to evaluate the performance
of our method, including detection percentage D = 100TP/
(
TP
+
FN
), branching factor B =
FP
/
TP
, and quality percentage
Q = 100TP/(
TP
+
FP
+
FN
), which were proposed by Shufelt
(1999) and have been widely used in evaluation of crater
detection algorithms (Barata
et al
., 2004; Kim
et al
., 2005; Bue
and Stepinski, 2007; Bandeira
et al
., 2012). Here,
TP
stands
for the number of true positive detections,
FP
the number of
false positive detections, and
FN
the number of false negative
detections (Swets, 1988). The parameter D can be treated as
a measurement of the detection performance (the larger, the
better), parameter B can be treated as a measurement of de-
lineation performance (the smaller, the better), and parameter
Q can be treated as a measurement of the overall algorithm
performance (the larger, the better). The detected gullies are
counted and compared with the manually delineated gullies
to calculate the three factors, in which the manually delin-
eated gullies are considered as the reference data. Since the
detected gullies may not coincide exactly with the manually
delineated ones, the assessment factors are computed accord-
ing to the following principles: a detection result which cov-
ers at least half length of the corresponding manually delin-
eated gully is considered to be a true positive detection (
TP
);
otherwise, it is a false positive detection result (
FP
); for those
manually delineated gullies which are not covered by detec-
tion results are counted as false negative detection results
Figure 6. Gully detection results in other five test sites (corresponding to Sites B through F in Table 1 from top to bottom).
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December 2015
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