…from the ESRI Map Book, Volume 24…
“This map is a modified digital reproduction of the “Geologic Map of the Late Cenozoic Deposits of the Sacramento Valley and Northern Sierran Foothills, California,” by Edward J. Helley and David S. Harwood (USGS Publication MF-1790, 1985).
“This map was created by scanning the five-sheet set of the original Helley and Harwood map, georeferencing the scanned images, and digitizing the lithologic contacts and other geologic information in AutoCAD 2006. The digitized map was then colored and symbolized in ArcGIS Desktop 9.0 software. The accuracy of the digitized lines is within the accuracy of the originally drafted lines on the paper copy. In general, the width of the contact lines on the paper copy extends to about 20 meters (66 feet).
“Minor topological mistakes (such as identical rock units on both sides of a lithologic contact or unclosed polygons) and omissions (such as unidentified lithologic units) have been corrected to the best of the author’s geologic expertise. Comparisons were made between the five-sheet set and the original Mylar and colored field sheets (as available) in addition to various geologic maps.
“This map was prepared by Jonathan Mulder, engineering geologist, Department of Water Resources, Northern District, Geological Investigations Unit. Assistance with the geological interpretation was provided by Bruce Ross, engineering geologist. Assistance with the digitizing and map layout was provided by student assistants Casey Murray, Clint Andreasen, and Jeremiah Moody.
“Courtesy of the California Department of Water Resources.”