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Comparing Landslide Inventories: The Map Depends on the Method

¹ California Geological Survey, 801 K Street, MS 12-32, Sacramento, CA 95814-3531

Landslide inventory maps are generally prepared by interpreting the geomorphic expression of landsliding on aerial photos, topographic maps, or on the ground. Distinctive landslide geomorphology allows the recognition and mapping of landslides, although there are always landslides that have very subtle expression and are not identified. The difficulties of mapping landslides based on their geomorphic expression are amplified in heavily forested terrain. The ground surface is obscured by tree cover on aerial photographs, and landslide-related features are often hidden. This limitation affects not only aerial photo interpretation, but also interpretation of topographic maps, which are based on aerial photographs.

We compared five maps showing landslides in the Laurel Quadrangle in the Santa Cruz Mountains, California. These include a geologic map, a map prepared for the county based on interpretation of aerial photographs, a map prepared by us based on aerial photographs and compilation of previous work, a map of features interpreted from the U.S. Geological Survey 7.5-minute topographic map, and a detailed field-based landslide map.

Comparison of these maps shows that the geologic map identifies few landslides, but most landslides on the geologic map are also shown on the other maps. The two maps based mainly on aerial photo interpretation tend to show the larger slides, but there is only about 60 percent correspondence of landslide areas between the two. Comparing the reconnaissance techniques with the much more detailed field mapping shows that the reconnaissance maps emphasize the large slides of bedrock and identify a lower percentage of shallow debris slides and debris flows.

Key Words: Landslide • Geomorphology • California • Field Mapping • Aerial Photograph • Topographic Interpretation