A digital raster graphic (DRG) is a scanned image of a U.S. Geological Survey (USGS) standard series topographic map, including all map collar information. The image inside the map neatline is georeferenced to the surface of the earth and fit to the Universal Transverse Mercator projection [10].

Besides DRG images there are other file types including, but not limited to, arial photography and topographic images that combine the image with georeferencing information. Originally the image (TIFF) and georeferencing information (TWF - TIFF World File) were packaged separately. The GEOTIFF [11] format, which contains both the image and referencing data, is an available option though many GIS systems currently have difficulty reading this new format.

Portion of a Digital Raster Graphics File.

Uses

Background images serve to reinforce spatial reliability in a conceptual model and are effective visualization tools for orientation and presentations. Geographic orientation allows the modeler to visualize the relationship between model input/output and topographic features of a region.

• The Digital Raster Graphics Program [12] has a database of all the 7.5' topographics images for the entire United States provides this product for a base price and additional fees for each additional 7.5' quadrangle ordered. There are also a number of web sites that have compiled free DRG images [13] for individual states.
• The PASA data system developed by a joint effort between Penn State University and the State of Pennsylvania has a small library of *.drg files. In addition the PASDA ftp site has almost the entire state in DOQQ format (Digital orthoquarterquads) which have the same name of their representative topographic quadrangles but with a NW, SW, NE, SE assigned indicating which section of a 7.5' quadrangle they cover.

Backdrop images such as .drg, .tif, and geo-referenced .tif files (such as a tiff world file, *.tfw) are very useful for a hydrologic conceptual model.  The images must be "geo-referenced" in order to properly utilize images in connection with already existing line and shapfile data.  If lattitude and longitude data are available for the site, the image can be registered in WMS.  If the image is to be used in other GIS applications the Map to World-File extension in ArcView can be useful.

Topographic images can be located and purchased at the USGS National Mapping Information page.  The USGS page also has an interactive "Map Finder" which can be useful if you don't have the exact geographic coordinates available.  In addition, hard-copy 7.5 min topographic maps can be purchased online at this site.