Figure 9. HiRISE image TRA_000828_2495 annotated with analysis
Areas A and B as measured in Fig. 8. This image is an example of typical
terrain in the northern lowlands: low relief, moderate craters, and
polygonal terrain covering most of the surface.
In area A (high boulder density) MBARS and the G-H method produce
similar results. Both predict an RA of ~220% suggesting
an overabundance of boulders within the 1-2.5 m boulder range sampled
here. However, both methods also overpredict the CFA compared to manual
results in area A (Fig. 8). In both cases this overestimate is most
obvious at the higher boulder diameters. In area B (medium boulder
density), there is greater disparity between the MBARS and G-H results.
MBARS results are much closer to the manual results
(~37% RA (Fig. 8)), though the same tendency of
overestimation at higher boulder diameters is present. In this area, the
G-H method underpredicts the boulder abundance (~20%
RA) compared to manual measurements.
We also use a second image, PSP_001391_2465, to compare the
performance of MBARS and the G-H method. In this image, the derived
boulder densities and the Cumulative Number of Rocks (CNR) per
m2 are compared between the two results (Fig. 10). The
maps demonstrate an agreement in boulder density variations, but MBARS
detects a lower boulder density in all locations. The accompanying CNR
plot (Fig. 10) shows that G-H results underpredict boulder abundances at
diameters above 1.5 m compared to manual and MBARS results, despite
having a higher boulder density. Because boulder density is totaled
across all boulder sizes, the higher boulder density is likely caused by
a higher abundance of low-diameter boulders predicted by the G-H method
(Fig. 10).