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What are 'RF3' and 'DLG?'

The U.S. Environmental Protection Agency's (EPA) Reach Files are a series of national hydrologic databases that uniquely identify and interconnect the stream segments or 'reaches' that comprise the country's surface water drainage system. The three versions of the Reach File that currently exist, known as RF1, RF2, and RF3-Alpha respectively, were created from increasingly detailed sets of digital hydrography data produced by the U.S. Geological Survey. The EPA enhanced these hydrography datasets by assigning a unique reach code to each stream segment, determining the upstream/downstream relationships of each reach, and, when possible, identifying the stream name for each reach.

Digital Line Graphs (DLG) are vector files containing line data, such as roads and streams, digitized from USGS topographic maps. DLG's offer a full range of attribute codes, are highly accurate, and are topologically structured, which makes them ideal for use in geographic information systems (GIS).

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