DRG Map

http://www.google.com/imgres?um=1&hl=en&sa=N&biw=1366&bih=583&tbm=isch&tbnid=hMCJU9OS73gweM:&imgrefurl=http://egsc.usgs.gov/isb/pubs/factsheets/fs08801.html&docid=ZTPaoJuKsveX7M&imgurl=http://egsc.usgs.gov/isb/pubs/factsheets/fs08801.graphics/drg.jpg&w=386&h=310&ei=Oj57T4bUHaX9sQLK7py2BA&zoom=1&iact=hc&vpx=352&vpy=196&dur=3397&hovh=201&hovw=251&tx=75&ty=96&sig=114711917576470782219&page=1&tbnh=163&tbnw=203&start=0&ndsp=10&ved=1t:429,r:1,s:0

DRG stands for digital raster graphic which results from scanning a paper USGS topographic map onto a computer for use.  This particular example features DRG analysis’ from Puerto Rico, Carribean Islands and Alaska through a single series.  It is scanned at a minimum resolution of 250 dots per inch.