The NHD Value Added Attributes are a set of 20 new attributes that are computed from the NHD flow table and assigned to each drain feature in the stream network. "The VAAs are designed to enhance the capabilities of NHD in three areas: 1) Navigation/Traversal; 2) Analysis; and 3) Display. The VAA's are a component of NHDPlus, an integrated suite of application-ready geospatial data sets that incorporate many of the best features of the National Hydrography Dataset (NHD), and National Elevation Dataset (NED). The NHDPlus includes a stream network, improved networking, naming, and “value-added attributes” (VAA’s). For more information on NHDPlus see the NHD newsletters; Introducing the NHDPlus-April 05 and NHDPlus: Powerful Applications Made Affordable-Aug. 05

VAA Data will be available on the Vermont Geographic Information System (VGIS) in fall 2006.


Navigation/Traversal:  The native NHD contains geometry and a flow table that support coordinate tracing and tabular routing, respectively. With the values added attributes, three additional traversal methods are supported: 1. SQL queries - There are a set of attributes that enable navigation both up and downstream with relatively simple SQL queries. The use of SQL queries to navigate the NHD makes large scale navigations feasible in interactive and web mapping applications. 2. From/To Nodes - A number of existing models, such as USGS/WRD's SPARROW model, use node navigation. The nationally unique from and to nodes in the VAAs support this; and 3. "Substance" routing - Using the nationally unique hydrologic sequence number and several other related attributes, a very compact routine can be written to route up or down the stream network using the VAAs. This is useful in applications such as dilution modeling and impact assessment. These attributes can also be used to quickly determine whether two points on the network are on a flow path and which point is upstream.

Analysis:  The analysis attributes are varied in their purpose. There is a unique identifier assigned to each stream from headwater to mouth which can assist in analyzing a complete stream or in relating older river mile index data to the NHD. Strahler stream order is assigned to each feature in the network providing this popular attribute for subsetting streams by size. There is a unique identifier assigned to each independent network. There is an attribute that provides the distance from the current feature to the network terminus. And finally, there is an attribute that provides the number of stream miles upstream of the current feature.

Display:  There is one attribute specifically designed to improve performance of large mapping applications particularly where Internet connectivity band-width is concerned. This attribute is called Thinner and is used to display increasingly more dense stream networks as the application zooms in on the map."