Agreement Between Measurements of Shrub Cover Using Ground-Based Methods and Very Large Scale Aerial Imagery

Abstract

Very large scale aerial (VLSA) photography is a remote sensing method, which is collected and analyzed more efficiently than ground-based measurement methods, but agreement with ground-based measurements needs to be quantified. In this study, agreement between ground- and image-measured cover and precision, and accuracy of image locations and scale, were assessed. True image locations were determined by georeferencing images and conducting a ground search. Accuracy and precision of planned, aircraft, and georeferenced locations were evaluated by comparison with true image locations. Shrub cover was measured at true image locations using ground-based line-intercept and on the image using point-intercept. Sagebrush (Artemisia spp. L.), antelope bitterbrush (Purshia tridentata [Pursh] DC.), and spineless horsebrush (Tetradymia canescens DC.) were distinguished in the imagery. Agreement between ground- and image-based measurements was quantified using limit-of- agreement analysis. True ground locations of the VLSA images were within a 41-m radius of the aircraft location at the time of image acquisition, with 95% confidence. Using a panchromatic image from the QuickBird satellite (0.6-m pixel resolution) as a base map, 90% of true ground locations were within a 5-m radius of the location estimated from georeferencing the VLSA image to the base map. VLSA image-measured cover was, in general, unbiased with mean absolute differences between VLSA- and ground-based methods less than 1.3%. The degree of agreement and absence of bias between VLSA image-measured and ground-measured cover is sufficient to recommend using VLSA imagery to measure shrub cover.