National Wetland Inventory Frequently Asked Questions

Q. Who uses these maps?

A. Wetland inventory maps are used by private citizens, developers, consultants, landowners, educators, water resources managers, fish and wildlife managers, and local units of government. These maps are used to assess the need for wetland permits, evaluate the completeness of permit applications, identify wetlands that might be impacted by proposed development projects, screen for development constraints, estimate potential wetland mitigation costs, evaluate flood storage potential, identify locations for wetland restoration, and assess the quantity and quality of habitat for wetland dependent species.

Q. What is the smallest wetland mapped by the NWI update?

A. The Minnesota NWI update meets or exceeds the federal wetland mapping standard which requires a target mapping unit (TMU) of at least 1/2-acre. The TMU is the smallest unit that is consistently mapped. Wetlands smaller than 1/2-acre were mapped if they were readily visible in the aerial imagery, which was not uncommon. Wetlands smaller than 1/20th-acre were not mapped.

Q. What was the process for reviewing data and ensuring map accuracy?

A. Wetland mapping procedures included a multi-step quality assurance process with on-screen review of draft data, field checking selected sites, and validation using independently collected field data. Initial review of all draft data was performed by a senior photo-interpreter for the mapping contractors. Subsequent review was performed by DNR staff and other stakeholders using an online data review application.

Q. How reliable and accurate is the updated NWI?

A. Map accuracy includes identification accuracy, classification accuracy, and positional accuracy. Accuracy for identifying wetlands is about 90%, while the accuracy for classifying wetlands is about 75%. If we group some of the more difficult wetland classes (i.e. littoral versus limnetic and aquatic bed versus unconsolidated bottom), the wetland classification accuracy improves to about 80%.

Positional accuracy is difficult to evaluate for wetland maps. Positional accuracy measurement relies on comparing positions of well-defined points in the mapped data and the same points derived from a high-accuracy dataset such as survey grade GPS, however, wetlands rarely have well-defined boundaries. Apparent wetland boundaries can change seasonally or annually depending upon climate conditions and other factors. The estimated root mean square error is +/- 3.0 meters. This means that 95% of well-defined wetland boundaries can be expected to fall within about +/- 5.9 meters (+/- 19.4 feet) of the true location. The NWI map data are not intended to be used to identify wetland boundaries for wetland permitting purposes.

Q. What can I do if I'm aware of errors in the NWI (wetlands erroneously included or omitted, inaccurate type)?

A. If you find an error in the NWI, we welcome your feedback.

Q. What does it mean if the NWI shows wetlands on my property?

A. The NWI has no legal or regulatory status. It is not a jurisdictional wetland determination. Wetlands were mapped using remote sensing information and cannot be expected to be able to identify all wetlands or determine jurisdictional wetland boundaries precisely enough for the purposes of wetland permitting. Qualified wetland professionals should perform onsite wetland delineations for this purpose. The NWI is a planning and assessment tool that indicates a high probability of the presence of wetlands in a location. It may be helpful in determining whether wetland-related permits may be needed for activities in a specific location.

Q: What's the difference between the National Wetlands Inventory and the Public Waters Inventory?

A: The NWI is an attempt to map all wetlands in the state (above the minimum size threshold of 0.5 acres), in addition to streams and lakes. Public waters inventory maps only show the lakes, streams and wetlands regulated by the DNR under the Public Waters Permit Program. As a subset of the state's wetlands, public waters are mapped on the NWI, denoted with a hatched pattern overlay.