The effects of global positioning system receiver accuracy on airborne laser scanning-assisted estimates of aboveground biomass

Abstract

National forest inventories routinely report estimates of parameters related to aboveground biomass (AGB), but sample sizes are often insufficient to satisfy precision guidelines and reporting requirements. Aerial photography, satellite imagery, and increasingly airborne laser scanning (ALS) data are all used as sources of auxiliary information to address this challenge.Combining inventory ground plot and ALS data requires that the data be co-registered to a common coordinate system. When measuring plots, inventory field crews typically obtain estimates of plot coordinates using global positioning system (GPS) receivers of varying degrees of accuracy. GPS-related errors in plot coordinates contribute to a sequence of adverse effects: (i) plot data are associated with erroneous ALS metrics, (ii) statistical models fit to such data may not adequately represent the true relationship between the plot data and the ALS metrics; and (iii) bias may be induced into model-assisted statistical estimators of population parameters.The primary objectives of the study focused on assessing the effects of GPS receiver inaccuracies on the estimated bias and precision of model-assisted estimators of mean AGB per unit area. The underlying motivation was to determine if the advantages of using ALS data as auxiliary information can be achieved apart from the substantial additional expense of purchasing GPS receivers with sub-meter accuracy. The analyses focused on comparing estimates based on three variations of plot coordinates obtained using field crew GPS receivers with maximum location errors on the order of 5–10m to estimates based on plot coordinates obtained using survey grade GPS receivers with sub-meter accuracy. The study area was in north central Minnesota in the USA and is characterized by both upland and lowland forest areas interspersed with lakes and wetlands. The primary results were twofold. First, estimates of mean AGB per unit area based on plot coordinates obtained using the less accurate field crew GPS receivers varied little from estimates based on the much more accurate survey grade receivers. Second, standard errors were greater by as much as 20% when using field crew GPS receivers than when using survey grade GPS receivers. However, even though the ALS-assisted standard errors obtained using field crew GPS receivers were greater than when using survey grade receivers, they were still substantially smaller than satellite image-assisted standard errors. Thus, the operational conclusion is that avoiding the substantial additional cost of providing a survey grade GPS receiver for each of more than 100 field crews likely outweighs the adverse consequences of somewhat larger standard errors.