BASE MAP - The base map for the digital compilation is a Digital Raster Graphic (DRG) of the U.S. Geological Survey, 1:24,000-scale topographic map of the Olalla 7.5' quadrangle (1994), which has a 20-foot contour interval. The image inside the map neatline is georeferenced to the Universal Transverse Mercator projection. The horizontal position accuracy and datum of the DRG matches the accuracy and datum of the source map. The base map layer is a digital image, but no information other than location is attached to the lines. The base map is provided for reference only.
The hillshade for the digital compilation was created from a U.S. Geological Survey 10-meter Digital Elevation Model (DEM) of the Olalla 7.5' quadrangle (1994). The DEM and resulting hillshade are georeferenced to the NAD 27 Universal Transverse Mercator projection. The vertical accuracy of DEM data depends on the source and resolution of the data samples, and is probably 7 meters, but may be as much as 15 meters. The hillshade was created with a sun aziumuth of 45 degrees, a sun height angle of 50 degrees, and vertical exaggeration of 2.