USING WEST HILLS, PORTLAND TO CHARACTERIZE THRESHOLDS FOR LIDAR MAPPING

Mapping landslides in western Oregon is problematic due to the heavy vegetation. The West Hills, Portland is prone to landslides as there is Loess overlying basalt on steep slopes. The geology coupled with high levels of precipitation during winter and parts of spring lead to high volume of slope failures. Landslide indicators are hard to distinguish, especially in aerial photographs or satellite imagery, which would be used for inventory maps in areas that aren't populated. LIDAR (light detection and ranging) penetrates the vegetation for a view of the ground surface. The LIDAR data was acquired by Portland State University from the Puget Sound LIDAR Consortium (PSLC) which flew the West Hills area in winter of 2004. In this project we attempted to define large-medium-small landslides in the West Hills, Portland using LIDAR. We created contour lines with different intervals (5, 15, 50 meters) and compared the number of landslides identified in each. Within the different contour intervals we also measured the area of landslide that we were able to delineate. The intervals set at 5 and 15 meters allowed us to see small landslides, as well as the large ones. We also compared the hill shade to contour lines and their output. We found hill shading wasn't as accurate as contour lines, and it sometimes presented a false image of the area. To maximize the image, we draped contour lines over the hill shade, which seemed to give a better view of what the ground morphology was really like and was a better image to map from.