Update Local Wetland Maps

It is much easier to protect wetland resources when you have good maps of their locations and types. In addition, wetland maps provide “red flag” or up-front information or prioritization to inform local plan review and Clean Water Act Section 404 evaluations.  Some states and local jurisdictions have a detailed and reasonably accurate wetlands mapping layer. However, many jurisdictions rely solely on the National Wetlands Inventory (NWI), maintained by the U.S. Fish and Wildlife Service (FWS).  The NWI is the most comprehensive digital coverage of United States wetlands available and serves as a primary source of wetland mapping data for the country. However, the NWI does have a number of data limitations that suggest an update may be beneficial before it is used in subsequent steps of the WARPT:

• As of 2010, digital NWI maps are available for approximately 61% of the country; much of the data are over 20 years old (Tiner, 2009).
• Federal funding has typically only allowed for an average of less than two percent of the national wetlands map to be completed each year (Awl et al., 2009; Tiner, 2009).
• In addition, the NWI typically does not include wetlands smaller than one to three acres, ephemeral wetlands, farmed wetlands, and certain wetland types that are difficult to interpret from aerial photos.

Several options that are available for updating wetland maps include digitizing directly from digital imagery, manual stereoscopic interpretation, and using wetland indicator layers to identify potential wetlands.

Determine if an Update to your Local Wetland Maps is Necessary

Whether your community relies on the NWI for its wetlands mapping or has a state, regional or local wetland map, the following questions should be considered to determine if an update to these maps is necessary to help strengthen local wetland protection:

• How old are the data? Has there been much recent development activity since the maps were created?
• What is the resolution of the data or minimum mapping unit? 
• Are there known issues or inaccuracies with the data?

To determine the status of wetland mapping in your area, check with the following sources:

• Your local planning or natural resources department may have developed a wetlands map, or have included wetlands as part of a local land use/land cover layer.
• Association of State Wetland Managers Wetland Mapping highlights the latest state-level wetland mapping efforts and State Wetland Programs provides information and links for individual states to determine the status of state wetland maps.
• To determine whether digital NWI maps are available, refer to  Regional Wetlands Coordinators.  At the main NWI page, you can download digital data, order hard copy maps, and obtain metadata that describes when the maps were created for your region and identifies the minimum mapping unit.
• Natural Resources Conservation Service (NRCS) Wetland Determinations (aka “Swampbuster” maps) were used to determine compliance with swampbuster provisions in the 1985 Farm Bill and are available as paper maps only for individual sites. However, many of these wetlands are isolated and small (<1 ac), which are easily missed on NWI and other wide-scale maps or imagery.  They may be useful to supplement your NWI by digitizing the wetland boundaries. Contact your local or regional NRCS office to access these maps.
• If the wetland maps available from the above sources are outdated or there has been a lot of recent development activity in your community, the U.S. Army Corps of Engineers (USACE) Section 404 Permit Wetland Determinations may be useful to identify wetlands for which some type of permitted impact or mitigation has been authorized   You can request this information from your USACE district office.

In general, if your wetland maps are more than 20 years old (10 if there has been a lot of recent development), has known inaccuracies, or is of a relatively small scale (e.g., smaller than 1:40,000), you should consider updating the maps.  On the other hand, if your wetland maps include very small wetlands (e.g., less than 1 acre), as well as wetlands associated with intermittent and ephemeral streams, they are probably of sufficient detail to protect wetlands locally and you can continue on to:

• Identify priority sites for wetland conservation and/or restoration
• Estimate values provided by wetlands
• Protect wetlands locally using regulatory or voluntary measures

Options for Updating Wetland Maps

Several options are available for updating local wetland maps (Table 1.1).  The most accurate yet resource intensive methods include digitizing or photo-interpreting wetlands directly from digital imagery or high resolution aerial photos.  To offset the associated costs, agencies and organizations may want to collaborate on a regional basis to acquire imagery (which has many uses besides wetland mapping) and/or mapping wetland resources. Potential partners may include: land trusts, non-profits, transportation and utility departments, universities, federal and state agencies, private consultants, and regional governing bodies. For additional information on identifying partners and building mapping coalitions, refer to Stetson (2009), Christie and Stetson (2009), and NACo (2007).

One option is to hire a government agency like the U.S. Fish and Wildlife Service (FWS), one of its mapping contractors, or a mapping company/organization with experience applying the Federal Geographic Data Committee's (FGDC) wetland mapping standard. The U.S. FWS may be interested in a wetland mapping project if it covers a relatively large geographic area and falls within one of their priority areas. Costs for these services vary with the type and density of wetlands in a geographic area, the recency of the NWI data, and the availability of digital data sources (e.g., land use/cover and soils).

Some of the wetland mapping contractors utilized by the U.S. FWS include:

Contact your U.S. FWS Regional Wetland Coordinator for other possible contractors.

Some additional partners for wetland mapping include:

• Statewide GIS Coordinators know about mapping efforts within the state where they are located.
• USGS National Spatial Data Infrastructure (NSDI) Specialists are knowledgeable about mapping efforts within federal agencies.
• The Farm Services Agency does aerial photography on a regular basis.

A less intensive method of improving local wetland mapping is to use wetland indicator layers to identify potential wetlands using Geographic Information Systems (GIS). Ralph Tiner of the U.S. FWS has developed a method to identify potential wetlands based on hydric soils, while Munoz et al. (2009) describe such a method for identifying potential isolated wetlands.   The State of Wisconsin has used the former method to develop a map of ‘potential wetlands’ (Figure 1.1).   The Munoz et al. (2009) method may be most useful in regions with a known abundance of isolated wetland types that are insufficiently mapped.  However, the method may need to be adapted for the isolated wetland types in your region.  This analysis can be done with in-house GIS staff in conjunction with a wetland consultant, or may be contracted out if GIS capabilities are not available within your jurisdiction.  If the results are to be incorporated into local wetland maps and assigned wetland functions, field verification of all ‘potential wetlands’ is needed to confirm that they exist.

Whichever mapping method is used, you should be aware of the recently implemented Federal Geographic Data Committee (FGDC) wetland mapping standard. Compliance with these standards is required for all federal agencies and other organizations that use federal funds to map wetlands. However, states, local governments, and non-profit organizations are also encouraged to utilize the wetland mapping standard in an effort to contribute to the national wetland mapping effort being conducted by the U.S. FWS. An average of less than two percent of the national wetlands map is completed per year due to funding limitations. The mapping standard will allow diverse groups to produce wetlands mapping data that is compatible and consistent in quality so that it can be included in the NWI (Awl et al., 2009).

The basic FGDC wetland mapping standard requirements are listed below. For a more comprehensive list, refer to FGDC (2009). In addition, the U.S. FWS has created a companion document to the FGDC wetland mapping standard (Dahl et al., 2009) that describes the technical procedures and requirements for wetlands map data. It explains the appropriate application of wetland classification, wetland mapping process, and how to achieve the data quality requirements now required by the wetland mapping standard. The U.S. FWS, in conjunction with USGS, have developed customized GIS tools for performing data checks on wetland map data. These Attribution and Verification Tools are extensions to the ESRI ArcMap desktop software and have been designed to address geo-positional errors, digital anomalies, and logic checks for data included in the NWI. A Wetland Mapping training course is also available from the U.S. FWS for learning how to successfully submit standards-compliant wetlands geospatial data to the NWI.

• Imagery should have a minimum resolution of 1 m
• Wetland classification following Cowardin et al. (1979)
• Topological verification (how point, line, and polygon features share coincident geometry)
• Valid attribute coding for wetland habitat type
• Metadata should conform to the most recent FGDC Content Standard for Digital Geospatial Metadata (CSDGM).

For additional information on the process of contributing your data to the Wetlands Master Geodatabase, please visit http://www.fws.gov/wetlands/WetlandsLayer/ContributedData.html.