National hydrography dataset--linear referencing

http://pubs.usgs.gov/fs/2012/3068/

Explains how the NHD specifies point locations within streams by using a code to indicate the reach (including the hydrologic unit) and a measure, which indicates how far the point is from the downstream end of the reach.

by Simley, Jeffrey D. and Doumbouya, Ariel T.

U.S. Geological Survey Fact Sheet 2012-3068

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