Mapped estimates of forest aboveground biomass (AGB) at regular intervals are important in carbon cycle studies. In the southwestern United States, there have been extensive changes to forests over the last decade, due to wildfire, climate-driven insect outbreaks and disease, increasing forest-human interaction, resource exploitation, and increasing aridity [1]. Earlier estimates of AGB based on MISR geometric-optical model cover and height retrievals were found to be highly compatible with US Forest Service maps constructed using empirical relationships and MODIS vegetation index data [2][3]. However, these maps were not validated against field data. This paper reports on efforts to assess mapped estimates of forest AGB in the Sierra National Forest, California, against field inventory data.