Elevation errors in a LIDAR digital elevation model of West Seattle and their effects on slope-stability calculations

A comparison of 1719 differential global positioning system (GPS) measurements with a 1 m LIDAR (light detection and ranging) digital elevation model (DEM) covering West Seattle shows that DEM elevation errors range from 24.88 m to +3.32 m. The errors are spatially correlated with a semivariogram range of 40 m, an unclustered mean of 20.11 m, and an unclustered standard deviation of ±0.75 m. Although there are statistically significant correlations between elevation error and elevation, slope angle, and topographic roughness, the relationships are weak and have little explanatory power. Monte Carlo simulations of slope angle, static factor of safety, Newmark yield acceleration, and log Newmark displacement show that elevation errors of the magnitude reported here can have significant effects on derivative calculations. The standard deviation of simulated slope angles increases from ,∼2° to±3° as the true slope angle approaches zero. Errors in slope angles calculated from the LIDAR DEM are smaller than those previously reported for a conventional 10 m DEM covering the same area, but the decrease is not proportional to the decrease in DEM grid spacing. The influence of elevation errors on static factor of safety errors is strongly dependent upon the slope angle and decreases significantly as the slope angle increases. Effects on Newmark yield acceleration and log Newmark displacement are not as profound but are still large enough to impart significant uncertainty into calculated results. Therefore, slope angle errors should be considered to be as significant as geotechnical parameter and pore-water pressure uncertainties when performing slope-stability calculations based on high-resolution LIDAR DEMs.