1 1 1 U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY Shaded-Relief Map of the San Francisco Bay Region, California By SCOTT E. GRAHAM and RICHARD J. PIKE1 The most detailed and accurate portrayal of the region's topography to date was created from 35,000,000 digital elevations spaced 30 meters apart OPEN-FILE REPORT 97-745 B 1997 This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. This database, identified as "Shaded Relief Map of the San Francisco Bay Region, California," has been approved for release and publication by the Director of the USGS. Although this database has been subjected to rigorous review and is substantially complete, the USGS reserves the right to revise the data pursuant to further analysis and review. Furthermore, it is released on condition that neither the USGS nor the United States Government may be held liable for any damages resulting from its authorized or unauthorized use. 1MENLO PARK, CA 94025 Shaded-Relief Map of the San Francisco Bay Region, California by Scott E. Graham and Richard J. Pike Summary Topography, the configuration of the land surface, plays a major role in the initiation of one type of landslide common to the San Francisco Bay region, the debris flow (Ellen and others, 1997), and in the recognition of such other modes of slope failure as slumping, translational sliding, and earthflow (Wentworth and others, 1997). Accordingly, a topographic portrayal at both local and regional scales is helpful in assessing the likelihood of slope failure and mapping the extent of its past activity. This report, one of six separate but related documents (San Francisco Bay Landslide Mapping Team, 1997), provides digital data and plottable map files that depict Bay region topography in detail by analytical hill-shading, a old technique at last made practicable by the digital computer. Hill shading, or chiaroscuro, portrays the three-dimensional shape of the land in two dimensions by intensity of the Sun's shadows. At least as old as Leonardo da Vinci's 1502-3 sketches of the Tuscan landscape (Horn, 1981), manual (artistic) executions of the technique can economically show only small areas accurately and in detail. However, Lehmann (1799) and later Wiechel (1878) developed analytical treatments that would eventually lead to automation. Thus quantified, the technique was finally adapted to the computer by Yoeli (1967). Analytical hill- shading in its current, digital, form has unique advantages for the portrayal and analysis of surface features: ´ it combines fine detail with large-area coverage; ´ it shows terrain accurately and in its true complexity, two properties often lost in sketches and diagrams of large areas; ´ the view is continuous„limited only by size of the digital data set„ unlike relief models, aerial photos, and radar images; ´ the map is free of distortion and the vegetation and cultural features that mask topographic form on satellite images; ´ positions of Sun and viewer can be varied to obtain a different perspective on a landform or to enhance certain terrain features. Most of these advantages are evident in the 11 digital shaded-relief images presented here, at 1:125,000 (county maps) and 275,000 (regional map) scale, the most detailed and accurate physiographic views to date of San Francisco Bay region and its ten constituent counties. Containing a vast amount of information, these computer portraits faithfully reflects the cumulative work of tectonism, fluvial activity, marine erosion and deposition, and other geologic events and processes shaping the region's landscape (for example, Brown, 1990). The 11 maps are numbered here, and in the other reports bearing the Open-file number 97-745, as follows: 1. San Francisco Bay Region 2. Alameda County 3. Contra Costa County 4. Marin County 5. Napa County 6. Santa Clara County 7. Santa Cruz County 8. San Francisco County 9. San Mateo County 10. Solano County 11. Sonoma County The new maps supersede earlier mechanized portrayals of the region. Included are those parts of the California (Edwards and Batson, 1990) and U.S. (Thelin and Pike, 1991) shaded-relief maps covering the Bay area. A 1:125,000-scale slope map of the ten-county region (U.S. Geological Survey, 1972), derived photomechanically from a contour map (U.S. Geological Survey, 1970), had previously been the only medium- scale relief portrayal of Bay region topography. Prior to this report, a satisfactory shaded-relief map had been prepared only for San Mateo county (Mark and Aitken, 1990). The data from which the new maps were computed consist of a digital elevation model (DEM), an (X,Y) array of terrain heights spaced 30 m apart. The data set was assembled from 204 separate DEM's derived from topographic maps in the USGS 7.5' series (see the quadrangle index map in Pike, 1997). Because all constituent DEM's were created by the recent and higher-accuracy "Level II" procedures that interpret elevations from digital line graphs of the map contours (U.S. Geological Survey, 1993), these maps are free from the striped artifacts that mar shaded-relief portrayals computed from older 7.5' DEM's. At 1:275,000 scale, the full Bay Region map comprises some 35,000,000 rectangular picture elements (pixels), each about 0.11 mm across. The array measures 7743 cells in the east-west direction and 7423 cells in the north-south direction. Pixels of the ten 1:125,000-scale county maps, at about 0.24 mm across, are below the size at which the human eye begins to resolve individual elements. On all 11 maps the gray tone of each pixel is a brightness value computed from a mathematical relation between ground-surface slope and azimuth and elevation of the observer and a simulated Sun (see, for example, Yoeli, 1967; Thelin and Pike, 1991). All images are illuminated from the north-northwest, halfway between the zenith and the horizon. Light and dark tones show steep areas, facing the Sun and in shadow, respectively; intermediate tones are gentle terrain. To enhance fine detail in low-relief terrain, we applied a modest vertical exaggeration. As a result, ridges appear about twice as high as they actually are, and valleys twice as deep. Details on the processing follow below. For references to the vast literature on relief shading see Horn (1981) and Thelin and Pike (1991). Drainage lines and nearshore bathymetry from an existing digital base (Aitken, 1997) enhance the relief-shaded portrayal on all 11 maps. To facilitate rapid location of specific areas of interest, particularly for emergency-response applications, the ten 1:125,000-scale county maps carry a detailed road network and major place names (Aitken, 1997). Similarly, we added a two-km buffer zone, mapped by a white- bounded black line, around each county to provide context for any slope failures and response activities near the county limits. Data Processing The digital compilation was performed in ARC/INFO, a commercial Geographic Information System (Environmental Research Institute [ESRI], Redlands, California). The database for shaded-relief maps consists of ARC/INFO GRID coverages for the region as a whole and for each of the ten counties (two versions of each set, the light-toned edition to be used as a base map for plotting other data), postscript map-image files for each of those 11 areas, and a postscript version of this text. See San Francisco Bay Landslide Mapping Team (1997) for information about filenames and how to obtain the data and plotfiles. The 7.5-minute DEM's were compiled into a single data set by the LATTICEMERGE command in ARC/INFO. Inconsistencies and edge-match problems between adjacent quadrangles were resolved using the FOCALMEAN function in ARC/INFO GRID. The hillshade grid (sfbr-sr) was prepared by the HILLSHADE command in ARC/INFO, using a Sun azimuth of 315 degrees, a Sun-elevation angle of 45 degrees, and a z-factor (vertical exaggeration) of 2.0. The light-toned hillshade grid was prepared by the HILLSHADE command in ARC/INFO, again using a Sun azimuth of 315 degrees and a Sun angle of 45 degrees, but a z-factor (vertical exaggeration) of 1.0. This grid was then recalculated from 255 brightness values to 100 values using the SLICE function in ARC/INFO GRID. The resulting grid was run through two additional grid algorithms (see below), which moved the hillshading towards the lighter end of the gray-scale spectrum yet still preserved the details of topographic features. Step 1: temp_grid = (ltgrid - [value GRD$MEAN]) * 15 / [value GRD$STDV] + 90 where ltgrid is the grid produced by the SLICE function Step 2: sfbr-srlt = con(isnull(temp_grid), 99, temp_grid <= 60, 60,temp_grid <= 99, int(temp_grid),99) where temp_grid is the grid produced by Step 1 Hillshade grids for the ten counties were prepared with the LATTICECLIP command in ARC/INFO, using a buffered county boundary-line (2 Km buffer) as the clip cover. The Digital Database No paper maps accompany this document or the other five reports in the San Francisco Bay Region Landslide Folio. Rather, the Folio is released as a digital database that can be obtained by either (1) connecting with a USGS Web page, (2) through anonymous ftp (file transfer protocol) over the Internet, or (3) sending a request, accompanied by a blank magnetic tape, to USGS. Available in two data formats (ARC/INFO Export-compatible or PostScript), the map files described here can be plotted on the user's equipment or that of a commercial vendor. The data files are as follows. (See San Francisco Bay Landslide Mapping Team (1997) for information on how to obtain the data.) sfbr-sr-dbsesc.ps -Postscript version of this text sfbr-sr.dbdesc.txt -ASCII version of this text ARC/INFO GRIDS sfbr-sr.tar -Tar file containing Bay Region hillshade grid al-sr.tar -Tar file containing Alameda County hillshade grid cc-sr.tar -Tar file containing Contra Costa County hillshade grid ma-sr.tar -Tar file containing Marin County hillshade grid na-sr.tar -Tar file containing Napa County hillshade grid scl-sr.tar -Tar file containing Santa Clara County hillshade grid scr-sr.tar -Tar file containing Santa Cruz County hillshade grid sf-sr.tar -Tar file containing San Francisco County hillshade grid sm-sr.tar -Tar file containing San Mateo County hillshade grid sol-sr.tar -Tar file containing Solano County hillshade grid son-sr.tar -Tar file containing Sonoma County hillshade grid sfbr-srlt.tar -Tar file containing Bay Region light hillshade grid al-srlt.tar -Tar file containing Alameda County light hillshade grid cc-srlt.tar -Tar file containing Contra Costa County light hillshade grid ma-srlt.tar -Tar file containing Marin County light hillshade grid na-srlt.tar -Tar file containing Napa County light hillshade grid scl-srlt.tar -Tar file containing Santa Clara County light hillshade grid scr-srlt.tar -Tar file containing Santa Cruz County light hillshade grid sf-srlt.tar -Tar file containing San Francisco County light hillshade grid sm-srlt.tar -Tar file containing San Mateo County light hillshade grid sol-srlt.tar -Tar file containing Solano County light hillshade grid son-srlt.tar -Tar file containing Sonoma County light hillshade grid POSTSCRIPT MAP-IMAGE FILES sfbr-sr.ps -Shaded relief map of the San Francisco Bay Region al-sr.ps -Shaded relief map of Alameda County cc-sr.ps -Contra Costa County ma-sr.ps -Marin County na-sr.ps -Napa County scl-sr.ps -Santa Clara County scr-sr.ps -Santa Cruz County sf-sr.ps -San Francisco County sm-sr.ps -San Mateo County sol-sr.ps -Solano County son-sr.ps -Sonoma County The map database itself is rather simple, consisting of attributed GRID cells stored in Universal Transverse Mercator (UTM) projection (Table 1). Table 1 - Map Projection PROJECTION UTM UNITS meters ZONE 10 DATUM NAD27 PARAMETERS none Content of the map database can be described in terms of the grid cells comprising the map. Descriptions of the database fields use the terms explained in Table 2. Table 2 - Field Definition Terms ITEM NAME name of the database field (item) WIDTH maximum number of digits or characters stored OUTPUT output width TYPE B-binary integer, F-binary floating-point number, I-ASCII integer, C-ASCII character string N.DEC number of decimal places maintained for floating- point numbers The gray-shaded picture elements (pixels) comprising the maps are rectangular cells shown on each map by a GRID coverage, consisting of 30-m cells derived from values of terrain steepness and azimuth as described briefly in the Introduction. The database format of the grid is described in Tables 3 and 4, which show the format of the GRID attribute and statistics tables. Table 3 - Contents of GRID Value Attribute Table (.VAT) ITEM NAME WIDTH OUTPUT TYPE N.DEC AREA 4 10 B -- COUNT 4 10 B -- Table 4 - Contents of GRID Statistics Table (.STA) ITEM NAME WIDTH OUTPUT TYPE N.DEC MIN 8 15 F 3 MAX 8 15 F 3 MEAN 8 15 F 3 STDV 8 15 F 3 References cited Aitken, D.S., 1997, Digital version of the 1970 Topographic Map of the San Francisco Bay Region: U.S. Geological Survey Open-file Report 97-500, three sheets, map scale 1:125,000. Brown, R.D., Jr., 1990, Quaternary deformation, in Wallace, R.E., ed., The San Andreas Fault System: U.S. Geological Survey Professional Paper 1515, p. 83-113. Edwards, Kathleen, and Batson, R.M., 1990, Experimental digital shaded-relief map of California: U.S. Geological Survey Miscellaneous Investigations map I-1848, scale 1:1,000,000. Ellen, S.D., Mark, R.K., Wieczorek, G.F., Wentworth, C.M., Ramsey, D.W., and May, T.E., 1997, Map Showing Principal debris-flow source areas in the San Francisco Bay Region, California: U.S. Geological Survey, Open-file Report 97-745 E, map scales 1:275,000 and 1:125,000. Horn, B.K.P., 1981, Hill shading and the reflectance map: Proceedings of the Institute of Electrical and Electronics Engineers, v. 69, no. 1, p. 14-47. [originally released 1979, in Workshop on Image Understanding, Palo Alto, CA, April, Proceedings: p. 79-120; reprinted 1982, in Geo-processing, v. 2, no. 1, p. 65-146.] Lehmann, J.G., 1799, Darstellung einer neuen Theorie der Bezeichnung der schiefen FlÉchen im Grundriss oder der Situationzeichnung der Berge (in German): Leipzig, paging unknown. Mark, R.K., and Aitken D.S., 1990, Shaded-relief topographic map of San Mateo County, California: U.S. Geological Survey Miscellaneous Investigations Map I- 1257-K, one sheet, scale 1:62,500. Pike, R.J., 1997, Index to detailed maps of landslides in the San Francisco Bay Region, California: U.S. Geological Survey, Open-file Report 97-745 D. San Francisco Bay Landslide Mapping Team, 1997, Introduction to the San Francisco Bay Region, California, Landslide Folio: U.S. Geological Survey Open-file Report 97-745 A. Thelin, G.P., and Pike, R.J., 1991, Landforms of the conterminous United States„A digital shaded-relief portrayal: U.S. Geological Survey Miscellaneous Investigations Map, I-2206, scale l:3,500,000 U.S. Geological Survey, 1970, Topographic map of the San Francisco Bay region: 3 sheets, scale 1:125,000. U.S. Geological Survey, 1972, Slope map of the San Francisco Bay region: 3 sheets, scale 1:125,000. U.S. Geological Survey, 1993, Digital elevation models„data users guide 5: Reston, Virginia, U.S. Geological Survey, National Mapping Division, 48 p. Wentworth, C.M., Graham, S.E., Pike, R.J., Beukelman, G.S., Ramsey, D.W., and Barron, A.D., 1997, Summary distribution of slides and earth flows in the San Francisco Bay Region, California: U.S. Geological Survey, Open-file Report 97-745 C, map scales 1:275,000 and 1:125,000. Wiechel, H., 1878, Theorie und Darstellung der Beleuchtung von nicht gesetzmÉssig gebildeten FlÉchen mit RÄcksicht auf die Bergzeichnung (in German): Civilingenieur, v. 24, p. 335-364. Yoeli, Pinhas, 1967, The mechanisation of analytical hill shading: Cartographic Journal, v. 4, no. 2, p. 82-88.