"NWI digital data files are records of wetlands location and classification as defined by the U.S. Fish & Wildlife Service. 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 area features and wetlands attributes. Wetland Attributes follow the Cowardin Classification System (1979). The digital data as well as the hardcopy maps that were used as the source for the digital data are produced and distributed by the U.S. Fish & Wildlife Service's National Wetlands Inventory project."
"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, andsurvey methods (e.g., level of field effort and state-of-the-art of wetland delineation) (see section on "Completenesss_Report" for more information."
"Federal, State, and local regulatory agencies with jurisdiction over wetlands may define and describe wetlands in a different manner than that used in this inventory. There is no attempt, in either the design or products of this inventory, to define the limits of proprietary jurisdiction of any Federal, State, or local government or to establish the geographical scope of the regulatory programs of government agencies. Persons intending to engage in activities involving modifications within or adjacent to wetland areas should seek the advice of appropriate Federal, State, or local agencies concerning specified agency regulatory programs and proprietary jurisdictions that may affect such activities. 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). CREDIT Acknowledgment of the USFWS, National Wetland Inventory would be appreciated in products derived from these data.
"Attribute accuracy is tested by manual comparison of the source with hard copy printouts and/or symbolized display of the digital wetlands data on an interactive computer graphic system. In addition, WAMS software (USFWS-NWI)."
"Line segments are a set of sequentially numbered coordinate pairs. No duplicate features exist nor duplicate points in a data string. Intersecting lines are separated into individual line segments at the point of intersection. All nodes are represented by a single coordinate pair which indicates the beginning or end of a line segment. The neatline is generated by connecting the four corners of the digital file, as established during initialization of the digital file. All data crossing the neatline are clipped to the neatline and data within a specified tolerance of the neatline are snapped to the neatline. Tests for logical consistency are performed by WAMS verification software (USFWS-NWI)."
"All photointerpretable wetlands are mapped given considerations of map and photo scale and state-of-the-art wetland delineation techniques. The target mapping unit is an estimate of the minimum-sized wetland that should be consistently mapped. It is not the smallest wetland that appears on the map, but instead it is the size class of the smallest group of wetlands that NWI attempts to map consistently. Users must realize however that some wetland types are conspicuous and readily identified (e.g., ponds) and smaller wetlands of these types may be mapped. Other types (drier-end wetlands and forested wetlands, especially evergreen types) are more difficult to photointerpret and larger ones may be missed. In forested regions, the target mapping unit varies with the scale of the aerial photographs given acceptable quality (e.g., captured during spring, leaf-off condition for deciduous trees), as follows for the Northeast, Southeast, and Northwest: 1:80,000 = 3-5 acres; 1:58,000 = 1-3 acres, and 1:40,000 = 1 acre. This means that where 1:58,000 photography was used, the NWI maps should show most wetlands larger than 1-3 acres. In the treeless prairies (e.g., Upper Midwest), 1/4-acre wetlands are typically mapped due to the openness of the terrain and occurrence of wetlands in distinct depressions. In forested regions, small open water and emergent wetlands may also be mapped where conspicuous. For Alaska, the target mapping unit is 2-5 acres, while for the Southwest, 1-3 acres is the target. Map users must pay close attention to the photo scale used to prepare the maps. Also, users should be aware that black and white imagery tends to yield more conservative interpretations than color infrared imagery, except when the latter was acquired during a dry year, complicating wetland detection. In most areas, farmed wetlands are not mapped, with exceptions including prairie pothole-type wetlands, cranberry bogs, and diked former tidelands in the Sacramento valley. Mucklands and other farmed wetlands are usually not shown on the maps. As mentioned in the "Use_Constraints" section, no attempt was made to separate regulated wetlands from other wetlands, as these decisions must be based on criteria established by Federal and state regulatory agencies. Maps produced by photointerpretation techniques will never be as accurate as a detailed on-the-ground delineation, so the boundaries on the NWI maps should be considered generalized, especially in areas of low topographic relief (e.g., coastal plains and glaciolacustrine plains). Partly drained wetlands may also be conservatively mapped, since they may be difficult to photointerpret and in many cases, require site-specific assessment for validation. For more information on the limitations of NWI maps, consult "NWI Maps: What They Tell Us" (National Wetlands Newsletter Vol 19/2, March-April 1997, pp. 7- 12; a copy can be obtained from the U.S. Fish and Wildlife Service, ES-NWI, 300 Westgate Center Drive, Hadley, MA 01035)."
"Horizontal positional accuracy for the digital data is tested by visual comparison of the source with hard copy plots."