The National Wetlands Inventory - Version 2, Surface Waters and Wetlands Inventory was derived by retaining the wetland and deepwater polygons that compose the NWI digital wetlands spatial data layer and reintroducing any linear wetland or surface water features that were orphaned from the original NWI hard copy maps by converting them to narrow polygonal features. Additionally, the data are supplemented with hydrography data (high resolution NHD 931v210 or 931v220), buffered to become polygonal features, as a secondary source for any single-line stream features not mapped by the NWI and to complete segmented connections. Specific information on the NHD version used to derive Version 2 and where Version 2 was mapped can be found in the 'comments' field of the Wetlands_Project_Metadata feature class.
This dataset is continually updated in various areas of the Nation with wetlands data from the USFWS, other federal, state and local agencies, tribes, non-governmental organizations and universities. To identify these updated areas review the Data Source layers on the Wetlands Mapper, Projects Mapper on the Mapper homepage, or the Wetlands Project Metadata featureclass that is included in the data downloads. The Wetlands Project Metadata feature class may also include a URL to link to a document that identifies project speciic techniques, source imagery and conventions used for that specific project. All data additions comply with the the FGDC Wetlands Mapping Standard, FGDC-STD-015-2009 and the Wetlands Classification Standard, which can be found at: http://www.fws.gov/wetlands/Data/Data-Standards.html.
The National Wetlands Inventory - Version 2, Surface Waters and Wetlands Inventory was derived using an ESRI Data Model with 481 logical and topological processes. Linear NWI features retained their original NWI Code and were buffered to specific widths based on the following rules: Palustrine = 6m, Riverine Perennial = 8m, Riverine Intermittent = 6m, Marine, Estuarine and Lacustrine = 8m. The NHDFlowline features were buffered to the width based on the FType field using the following general rules: CanalDitch = 4m, Intermittent = 6m and Perennial = 6m. The NHDFlowline features were coded with NWI Codes based on the following general rules: CanalDitch/aqueduct = R5UBFr, CanalDitch/all others = R5UBFx, Intermittent Stream = R4SBC, Perennial Stream = R5UBH. Further refinement of the NWI Codes was implemented based on spatial intersect with known NWI features and specific FCode information from the NHD dataset. Topological modifications were conducted to remove overlapping features. In general buffered NWI linears were clipped into NWI polygons and both the NWI polygons and buffered NWI linears were used to erase buffered NHDFlowlines. Further topological refinements were conducted within the the buffered linear feature layers based on intersections with known NWI features and using a hierarchy based on NWI Code Sub-system and Water Regime. For more specific information on the logical and topological processes involved in deriving this dataset visit: http://www.fws.gov/wetlands/Data/Wetlands-V2-Product-Summary.html
Original stable base hard copy maps of wetland and deepwater habitats were created based on USGS state and quadrangle boundaries. These maps were converted to digital files using various software packages (WAMS, ARC and others). The digital files were stored as ESRI Import/Export files corresponding to a single 1:24,000 USGS quadrangle. These digital files were imported and converted to ESRI Coverage format and checked for topological and attribute errors. All coverages were converted from a UTM map projection to an Alber's Equal Area map projection and the horizontal datum was converted from NAD27 to NAD83 were necessary. Polygons attributed as "Uplands" were removed from the dataset and polygons were merged at quadrangle boundaries where the quadrangle line divided polygons with the same attribute. The data was loaded into a seamless SDE geodatabase for the conterminous United States. These steps were conducted using both Arc Macro Language (AML) and ArcMap editing tools. All point data from the original ESRI Coverages were buffered by 11.28 meters (1/10 of an acre) and incorporated into this polygon feature class. Further data improvements included the conversion of all old wetland codes that contained 'OW' to the new code containing 'UB', 'BB' to 'US', 'FL' to 'US', and 'SB' to 'US' except for 'R4SB' codes. Old Water Regimes were also changed, 'W' to 'A', 'Y' to 'C' and 'Z' to 'H'. All polygons labeled as 'OUT', 'No Data' and 'NP' were removed from the database.
NH GRANIT clipped original NWIPlus data to match the New Hampshire state boundary and used NHSP 1983 as the output spatial reference system.
The NWI wetland classification system is hierarchical, with wetlands and deepwater habitats divided among five major systems at the broadest level. The five systems include Marine (open ocean and associated coastline), Estuarine (salt marshes and brackish tidal water), Riverine (rivers, creeks, and streams), Lacustrine (lakes and deep ponds), and Palustrine (shallow ponds, marshes, swamps, sloughs). Systems are further subdivided into subsystems which reflect hydrologic conditions. Below the subsystem is the class which describes the appearance of the wetland in terms of vegetation or substrate. Each class is further subdivided into subclasses; vegetated subclasses are described in terms of life form and substrate subclasses in terms of composition. The classification system also includes modifiers to describe hydrology (water regime), soils, water chemistry (pH, salinity), and special modifiers relating to man's activities (e.g., impounded, partly drained). For a complete listing of the NWI wetland classification system, see Cowardin, L.M., V. Carter, F. Golet, and E. LaRoe. 1979. Classification of wetlands and deepwater habitats of the United States. U.S. Fish and Wildlife Service. 103 pp The NWIPlus classification includes additional attributes - referred to as LLWW descriptors - that describe Landscape position, Landform, Water flow path, and Waterbody type. See NWIPlus_AttributeCodes.pdf (https://granitweb.sr.unh.edu/metadataforviewers/commonviewers/relateddocuments/NWIPlus_AttributeCodes.pdf) for a full listing of these extended attributes. For additional reference information, see Tiner, Ralph W. 2014. Dichotomous Keys and Mapping Codes for Wetland Landscape Position, Landform, Water Flow Path, and Waterbody Type Descriptors: Version 3.0. U.S. Fish Wildlife Service. 96 pp