About the Images
The
U.S. Geological Survey (USGS) provides the Microsoft Research Maps
site with Digital Orthophoto Quadrangles (DOQs) and Digital Raster Graphics
(DRGs) of the United States.
From the USGS you can order, and in some cases download, forms of data that, in given applications, can be used in conjunction with the DOQs and DRGs. These data include digital elevation models, digital line graphs, digital raster graphics (scanned topographic maps), and satellite image data.
For more general information about USGS, please visit the USGS Web site.
DOQs
DOQs are produced by a process that begins with the scanning of photos obtained almost exclusively from the National Aerial Photography Program (NAPP), an interagency effort coordinated by the USGS. Each photo covers an area a little more than five miles on a side, representing about one-fourth of a standard, 7.5-minute USGS topographic quadrangle map.
Once scanned into digital form, the images undergo a computer "orthorectification" routine, making them geometrically like maps. The process of orthorectification removes distortions created by the cameras taking the photos. The perspective of a camera causes things like foreshortening where, for example, parallel lines seem to converge near the camera. It is important to remove these distortions so accurate information -- such as distances -- can be gleaned from the final product.
The DOQs are modified, which entails sectioning them, resampling them, and compressing the results in files that are displayed on the Web site. Microsoft performs this process with technical assistance from the USGS.
Click here for more information on USGS Digital Orthophoto Quadrangles (DOQ).
DRGs
DRGs are USGS colored topographical maps that have been scanned in and digitized. They display major political boundaries such as cities and state and county boundaries, lakes, streams, in addition to lines identifying the topography of the area. Colors are used to depict certain aspects of the map, and all USGS DRG files share a common color map. For example, red indicates roads and sections, green indicates timber, light blue indicates water, white indicates map background, and brown indicates land contours. DRGs are the digitized version of the popular USGS topographical paper maps sold throughout the country.
A DRG is made by scanning a printed map on a high-resolution scanner. Like the USGS DOQ images, DRGs are georectified and put into the Universal Transverse Mercator projection. The image is compressed to produce a TIFF file that ranges from 5 to 15 megabytes in size.
Click here for more information on USGS Digital Raster Graphics.
