DLG (Digital Line Graph)

http://pubs.usgs.gov/of/2003/of03-471/domier/index.html

This is an example of a digital line graph. Much like the DRG, it is a digital representation of what was originally a paper map. Though a little harder to produce than a DRG, the DLG is helpful in showing us cartographic information, such as roadways, things like elevation (contour lines) and other helpful information. They are derived from aerial photographs and other cartographic sources that allow it, like the DRG, be a more accessible and functioning map, more than something that's just on a paper.