An Evaluation of Lidar-derived Elevation and Terrain Slope in Leaf-off Conditions

The effects of land cover and surface slope on lidar-derived elevation data were examined for a watershed in the piedmont of North Carolina. Lidar data were collected over the study area in a winter (leaf-off) overflight. Survey-grade elevation points (1,225) for six different land cover classes were used as reference points. Root mean squared error (RMSE) for land cover classes ranged from 14.5 cm to 36.1 cm. Land cover with taller canopy vegetation exhibited the largest errors. The largest mean error (36.1 cm RMSE) was in the scrub-shrub cover class. Over the small slope range (0° to 10°) in this study area, there was little evidence for an increase in elevation error with increased slopes. However, for low grass land cover, elevation errors do increase in a consistent manner with increasing slope. Slope errors increased with increasing surface slope, under-predicting true slope on surface slopes 2°. On average, the lidarderived elevation under-predicted true elevation regardless of land cover category. The under-prediction was significant, and ranged up to 23.6 cm under pine land cover. Introduction Large-scale digital elevation models (DEMs) are widely used in research, education, and management of public resources. Absolute elevation is required for mapping floodplains or conducting visibility studies while surface form (e.g., slope and aspect) is used for hydrologic applications. Prior to the late 1940s, topographic maps were created from field surveys and “artistic sketching” of contour lines (Hodgson and Alexander, 1990). Stereoscopic aerial photography for topographic mapping was developed in the 1940s and became the primary source for large-scale mapping (Jensen, 2000). Interferometric Synthetic Aperture Radar (IFSAR) was An Evaluation of Lidar-derived Elevation and Terrain Slope in Leaf-off Conditions Michael E. Hodgson, John Jensen, George Raber, Jason Tullis, Bruce A. Davis, Gary Thompson, and Karen Schuckman developed in the 1960s for medium-scale mapping and more recently, for large-scale mapping. Lidar (Light Detection and Ranging) was developed in the 1980s and is rapidly becoming the preferred method of creating DEMs by counties, states, and some governmental agencies. While the use of lidar for mapping vegetation characteristics is becoming more frequent (Means, et al., 2000), mapping terrain elevation has been the primary focus of most lidar collections. It is generally known that the leaves and branches can reduce the effectiveness of obtaining elevation information from lidar data (Hodgson et al., 2003). The sensitivity of lidar data collection to system parameters (e.g., height above ground level (AGL), laser instrument) and geography (e.g., relative ruggedness, land cover, seasonality) has not been rigorously researched. Even fewer studies have attempted to assess the accuracy or sensitivity of mapping topographic slope with lidar-data sources. Therefore, this study focused on mapping elevation and topographic slope in the winter (leaf-off) season for a mid-latitude geographic area. The research questions examined in this study include: 1) What is the elevation accuracy of a lidar-derived DEM in Leaf-Off Conditions? 2) What is the slope accuracy of a lidar-derived DEM in Leaf-Off