Digital Raster Graphics are provided online by the California Spatial Information Library (CaSIL) .
A DRG is useful as a source or background layer in a GIS, as a means to perform quality assurance on other digital products, and as a source for the collection and revision of DLG data. DRGs can also be merged with other digital data, e.g. DEMs or DOQs, to produce a hybrid digital file.
The maps are organized into 1x1 degree blocks covering the state. Within these blocks are the individual maps and ancillary data. Maps are named by their USGS identifier as described in Standardized Data Set Names .
There are multiple series of DRGs for different scales. The 1x2 degree series is at a scale of 1:250K. The 30x60 minute series is at a scale of 1:100K, and the 7.5 minute series is at 1:24K. Additionally, there are some 1:25K scale maps that are included in this distribution as belonging to the 7.5 minute series.
For the 1x2 degree series, there are 41 images covering the state of California and immediately adjacent areas. These DRGs were originally obtained from the California Department of Fish and Game (DFG).
For the 30x60 minute series, there are 127 images covering the state of California and immediately adjacent areas. These DRGs were originally obtained from the California Department of Fish and Game (DFG).
For the 7.5 minute series, there are 2853 images covering the state of California and immediately adjacent areas. Statewide DRG production was performed by the State of California, Stephen P. Teale Data Center, GIS Solutions Group (Teale). Additional acknowlegdements include: Governor's Office of Emergency Services (OES): compilation of DRG quad dates and other attributes; California Department of Fish and Game, Information Technology Branch: non-standard quad boundaries, quad currentness year information and enhanced index compilation.
The Digital Raster Graphic (DRG) is a raster image of a scanned USGS topographic map including the collar information. The source maps for DRGs were georeferenced to the UTM grid, but images DRGs for California are available in Albers equal area projection as well. Most DRGs were originally in the NAD27 datum, but have been converted and are available in NAD83 as well.
Information about the projection used by the DRGS can be found at CA Projection .
All California Digital Raster Graphics are available at ftp://casil.ucdavis.edu/casil/gis.ca.gov/drg, or http://casil.ucdavis.edu/casil/gis.ca.gov/drg .
Various mirrors sites of the CaSIL data also provide DRGs, such as the Sacramento Mirror: ftp://casil-mirror1.ceres.ca.gov/casil/gis.ca.gov/drg and http://casil-mirror1.ceres.ca.gov/casil/gis.ca.gov/drg .
Map interfaces are available at http://casil.ucdavis.edu/mapsurfer/ and http://ims.gis.ca.gov/website/casil/drgquads/viewer.htm .
The DRG data at CaSIL are organized by directories. The directories are named based on the series (C|F|0), projection(Albers NAD27|Albers NAD83), and type (trimmed|untrimmed|mosaic).
In the case of the 7.5 Minute (O) Series, the DRGs are further subdivided based on the USGS quad identifier (ie. 37122).
Users of web browsers can click on the desired quad on the client side image map to directly retrieve the appropriate GeoTiff file. A tooltip will show up to help identify the actual quad on the client side image map.
Further information about the California DRGs can be found at About California DRGs .