USGS Scanned Paper Maps (DRG)
The USGS has scanned the entire series of 1:24,000 topographic maps, calling this
product the Digital Raster Graphics (DRG) series. They are geo-referenced, and were
scanned at 4 meters/pixel accuracy. TopoDepot has the full series available on both
the TopoDepot website, and the SkyTopo county DVD series.
USGS Digital Raster Graphics (DRG) are precision scans of the existing paper maps.
DRGs are always produced by the USGS in UTM coordinates, NAD83. However, TopoDepot
makes it possible to change to other coordinates/datums, for example State Plane
NAD27. TopoDepot uses a precise method of raster coordinate projection.
The resulting images are compressed into JPEG files. The size of a single quad DRG
is about 2MB. A JPEG World file is also produced for use in most CAD and GIS software.
We use JPEG since it is compatible with most software, including all versions of
AutoCAD (V14 and up), and offers reasonable compression for photographs like these.
(You can also view it in any web browser.)
Additionally, if you choose DWG files, the image is combined into the DWG file at
the correct location and scale. Any version of AutoCAD (V14 or greater) can be used
to view these images. AutoCAD Map is not required.
If you choose multiple quadrangles, a JPEG file is created for each quad. Adjacent
quads will overlap slightly to give a seamless effect. Multiple quads are handled
correctly in AutoCAD.
This layer can be useful either as a backdrop and check for our vector data, or
stand-alone as a valuable presentation tool. The images are geo-referenced, so you
can plot GPS or other data on top of these maps.
Data Source: USGS-Microsoft Terraserver project. http://www.terraservice.net
More information:
http://rockyweb.cr.usgs.gov/nmpstds/drgstds.html
