DRG (topo maps)

• the USGS produces topographical maps ("topo maps" for short) covering (nearly) the entire USA, and distributes them in digital form known as Digital Raster Graphics (DRG)
• they usually scan the paper maps at 250 dpi, but i've also seen some DRGs scanned at 400 dpi
• the actual files are in the common image format TIFF, in 8-bit color, with an accompanying TFW (TIFF World File) which gives the UTM extents of the map
• DRGs can be ordered from the USGS for $1 per file, or a scaled-down version can be browsed online at TopoZone
• free DRGs are available for about half of the US states, see Locations - USA
• DRGs are used as the basis for the USGS DLG files (roads and rivers) and DEM files (elevation produced by extracting the topo map contours and converting them to a grid)
• however, there is a wealth of untapped data on these maps, data which has never been fully extracted digitally, including:
  • building locations and outlines
  • bridges and tunnels
  • power lines
  • designation of areas as schools, hospitals, cemeteries, golf courses
  • in some areas, bathymetry near coastlines including mud, reefs, and rocks
  • water tanks, radio towers
  • tons of other cultural features

Working with DRGs

• extracting areas of interest from DRGs, and merging data from multiple DRGs, can be tricky process because of the 'collars' around the edges of each map
  • Global Mapper (inexpensive, for Windows) can do this, provided you already have the DRGs
  • the BigTopo program in the iGage - All Topo Maps product can also do this nicely, and is sold at around $80 per U.S. State for the DRGs and software bundled
• the VTP tool BExtractor can automatically extract locations for buildings represented with a simple square dot, but can not automatically extract any other kind of data
• commercial tools such as Feature Analyst ($1k-$16k depending on license type, ArcGIS plug-in) could be used to extract all kinds of useful information from a DRG

International Topo Maps

• many other countries and international agencies have produced their own topo maps, which often have similarities to USGS maps in their mapping conventions
• to the right is a section of a Russian military topo map (link to full image)
• as you can see, it also uses brown lines for elevation contours, black rectangles for buildings, filled double lines for highways, etc.
• other countries, such as the United Kingdom Ordnance Survey maps, have significantly different standards for their topo map features
• see the International pages for sources of DRGs around the world