Quaternary geology of Alameda County, and parts of Contra Costa, Santa Clara, San Mateo, San Francisco, Stanislaus, and San Joaquin Counties, California: A digital database

MAPPING METHODS
Geological units were mapped on 1:24,000 scale U.S. Geological Survey topographic maps using 1939 black and white aerial photographs showing the county before much development had taken place. The mapping was supplemented with 1:12,000- and 1:24,000-scale color aerial photography flown in 1965 and 1974 respectively. The mapping was also aided by observations made on turn of the century topographic maps at 1:62,500-scale with 25 foot (7.6 m) contour intervals. These maps were most useful in recognizing natural stream channels which have since been modified. The position of water boundaries, the stream channel to water transition, the length and position of mapped stream channels (see below), the distribution of artificial levees, and the apparent distribution of artificial fill were plotted to conform with the most recent 1:24,000 scale topographic maps.
Mapping units were delineated by: 1) landform morphology, 2) relative topographic position, 3) relative preservation of surface morphology, 4) tonal contrasts on aerial photographs, 5) relative soil profile development (compiled from the U.S. Soil Conservation Service, 1917), and 6) other features such as differences in vegetation density and type. Landform morphology refers to the shape of a particular landscape element. Examples are the distinctive conical shape of alluvial fans and the shape of levees that usually border active stream channels, sloping away from them to blend into flat basin deposits. Other criteria listed in the description of geological units are also used to distinguish one element from another, but they are particularly useful for delineating units within a specific landscape element. A surface on an alluvial fan, for example, might be differentiated from another because of its higher topographic position, greater drainage density, and stronger soil profile development. Geologic units defined this way are called allostratigraphic units (American Association of Petroleum Geologists Bulletin, 1983).
Digital Compilation
The geologic map information was digitized from stable originals of the geologic maps at 1:24,000 scale. The author manuscripts (pencil on mylar) were scanned using a Altek monochrome scanner with a resolution of 800 dots per inch. The scanned images were vectorized and transformed from scanner coordinates to projection coordinates with digital tics placed by hand at quadrangle corners. The scanned lines were edited interactively by hand using ALACARTE, color boundaries were tagged as appropriate, and scanning artifacts visible at 1:24,000 were removed.
Base Maps
Base Map layers were prepared from scale-stable printing negatives of the U.S. Geological Survey San Francisco (1978 edition), Stockton (1989 edition), Palo Alto (1983 edition), and San Jose (1978 edition) 1:100,000 topographic maps, which have a 50 meter contour interval. Scanned and vectorized images were transformed from scanner coordinates to projection coordinates with digital tics placed by hand at map corners. The images were then trimmed interactively by hand using ALACARTE to conform to the area of the geologic coverages, and the four portions were combined. Small mismatches at the boundaries caused by slight differences in the original scans remain in the three base map coverages. These base map layers are digital images but no information other than location is attached to the lines.