Wilson's Creek National Battlefield in Republic, Missouri, protects a large concentration of the rare, glade-endemic, winter annual Lesquerella filiformis Rollins (Missouri bladderpod; scientific nomenclature follows USDA PLANTS Database). A sustained monitoring effort provided the opportunity to assess a grid-based survey approach featuring the use of visual density class estimates. Population size intervals, which include the uncertainty associated with the use of density classes, were compared to sample-based population size estimates from a computer simulation of the L. filiformis population. A 100% population size interval was similar to a sample-based 95% confidence interval and much narrower than a 99.9% confidence interval. If the distributions of observations within density classes are not symmetrical, however, a sample-based approach will yield more accurate point estimates. Observers frequently underestimated plant densities when using density classes, although counts of plants provided correction factors. Following recalibration of the density class scale, the grid-based survey approach supported the calculation of population metrics based on historical occupancy and abundance in individual grid cells. For some rare plant populations, conservation practitioners may determine that the advantages of such cell-based metrics outweigh the disadvantages of uncertainty and observer error stemming from the grid-based survey approach.