NWI digital data files are records of wetlands location and
classification as developed by the U.S. Fish & Wildlife Service.
The classification system was adopted as a national
classification standard in 1996 by the Federal Geographic Data
Committee. This dataset is one of a series available in 7.5
minute by 7.5 minute blocks containing ground planimetric
coordinates of wetlands point, line, and polygon features and
wetlands attributes. When completed, the series will provide
coverage for all of the contiguous United States, Hawaii,
Alaska, and U.S. protectorates in the Pacific and Caribbean.
Coverage includes both digital data and hardcopy maps. The NWI
maps do not show all wetlands since the maps are derived from
aerial photointerpretation with varying limitations due to
scale, photo quality, inventory techniques, and other factors.
Consequently, the maps tend to show wetlands that are readily
photointerpreted given consideration of photo and map scale. In
general, the older NWI maps prepared from 1970s-era black and
white photography (1:80,000 scale) tend to be very conservative,
with many forested and drier-end emergent wetlands (e.g., wet
meadows) not mapped. Maps derived from color infrared
photography tend to yield more accurate results except when this
photography was captured during a dry year, making wetland
identification equally difficult. Proper use of NWI maps
therefore requires knowledge of the inherent limitations of this
mapping. It is suggested that users also consult other
information to aid in wetland detection, such as U.S. Department
of Agriculture soil survey reports and other wetland maps that
may have been produced by state and local governments, and not
rely solely on NWI maps. See section on "Completeness_Report"
for more information. Also see an article in the National
Wetlands Newsletter (March-April 1997; Vol. 19/2, pp. 5-12)
entitled "NWI Maps: What They Tell Us" (a free copy of this
article can be ordered from U.S. Fish and Wildlife Service,
ES-NWI, 300 Westgate Center Drive, Hadley, MA 01035, telephone,
The data provide consultants, planners, and resource managers with information on wetland location and type. The data were collected to meet U.S. Fish & Wildlife Service's mandate to map the wetland and deepwater habitats of the United States. The purpose of this survey was not to map all wetlands and deepwater habitats of the United States, but rather to use aerial photointerpretation techniques to produce thematic maps that show, in most cases, the larger ones and types that can be identified by such techniques. The objective was to provide better geospatial information on wetlands than found on the U.S. Geological Survey topographic maps. It was not the intent of the NWI to produce maps that show exact wetland boundaries comparable to boundaries derived from ground surveys. Boundaries are therefore generalized in most cases. Consequently, the quality of the wetland data is variable mainly due to source photography, ease or difficulty of interpreting specific wetland types, and survey methods (e.g., level of field effort and state-of-the-art of wetland delineation). See section on "Completeness_Report" for more information.