The U.S. Fish and Wildlife Service's (Service) objective of mapping wetlands and deepwater habitats is to produce reconnaissance level information on the location, type and size of these resources. The maps are prepared from the analysis of high altitude imagery. Wetlands are identified based on vegetation, visible hydrology and geography. A margin of error is inherent in the use of imagery; thus, detailed on-the-ground inspection of any particular site may result in revision of the wetland boundaries or classification established through image analysis.
The accuracy of image interpretation depends on the quality of the imagery, the experience of the image analysts, the amount and quality of the collateral data and the amount of ground truth verification work conducted. Metadata should be consulted to determine the date of the source imagery used and any mapping problems.
Wetlands or other mapped features may have changed since the date of the imagery and/or field work. There may be occasional differences in polygon boundaries or classifications between the information depicted on the map and the actual conditions on site.
Exclusions - Certain wetland habitats are excluded from the National mapping program because of the limitations of aerial imagery as the primary data source used to detect wetlands. These habitats include seagrasses or submerged aquatic vegetation that are found in the intertidal and subtidal zones of estuaries and nearshore coastal waters. Some deepwater reef communities (coral or tuberficid worm reefs) have also been excluded from the inventory. These habitats, because of their depth, go undetected by aerial imagery.
By policy, the Service also excludes certain types of "farmed wetlands" as may be defined by the Food Security Act or that do not coincide with the Cowardin et al. definition. Contact the Service's Regional Wetland Coordinator for additional information on what types of farmed wetlands are included on wetland maps.
Precautions - NWI wetlands data are developed using a biological definition of wetlands and may not be consistent with wetland boundaries established according to the federal regulatory definition of wetlands under the Clean Water Act. The data do not account for changes in the landscape that may have occurred after mapping (see metadata). These data should not be interpreted as representing the presence, absence, or extent of wetlands that may be covered under one or more federal, state, Tribal, or local laws. If you are planning to conduct activities that could affect waters or wetlands, we recommend that you contact your local USACE regulatory office to ensure compliance with applicable federal wetland regulations: https://regulatory.ops.usace.army.mil/offices/
