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).