Landslide Mapping and Hazard Assessment by the California Geological Survey

During the past forty years, the California Geological Survey (CGS) has prepared hundreds of landslide maps across the state at a variety of map scales. To promote consistency in mapping and data attribution, CGS developed a uniform methodology for recognizing, mapping, and classifying landslides. That methodology continues to evolve as new technologies are tested and incorporated into the workflow. CGS landslide inventory maps are prepared by geomorphic analysis of multiple sets of vintage and recent stereo-paired aerial photographs and topographic maps, field reconnaissance, and review of previous mapping. Landslides are digitally compiled on scanned or digital images of U.S. Geological Survey 7.5-minute topographic maps or other similar digital base maps. For each landslide, selected characteristics are recorded in an associated database using a standardized classification scheme. To streamline the interpretation and data entry process, CGS is testing the use of digital stereo models for on-screen viewing, interpretation, digitizing, and attribution. LiDAR and other remote-sensing imagery, where available, are also incorporated into the workflow. LiDAR significantly improves the identification and detail of earth flows and large, deep-seated rockslides in forested areas compared to aerial photographs used alone.

Landslide inventory maps are used by CGS to prepare landslide potential and hazard zone maps. CGS has found that landslide inventories and multi-level susceptibility or potential maps are most useful for the technical user, and that hazard zone maps are the most effective tool in conveying landslide hazard information and policy requirements to local agency planners and the general public. Selected CGS landslide maps and data are currently available to users as PDFs on the CGS website, on CDs, or as paper copies of digital and non-digital mapping. GIS files will be available in the future.