NHD Reach Indexing Tool Overview
State water quality agencies require a method for linking surface water attribute information to a detailed streams coverage. Many agencies have accomplished this by directly editing the attribute tables of a hydrography coverage such as USGS Digital Line Graphs (DLG’s) or EPA’s Reach File version 3.0 (RF3). When different state agencies use different mechanisms for attributing their hydrography coverage, it can cause incompatibility between the surface water data layers within a state, or between states. It would be desirable for agencies to use a common hydrography coverage and a common methodology for georeferencing surface water entities in order to promote compatibility between data sets and provide the ability to aggregate surface water data to regional or national scales.
The National Hydrography Dataset Reach Indexing Tool (NHD-RIT) is an interactive GIS application that allows users to georeference surface water data using the National Hydrography Dataset (NHD). The process of identifying and assigning Entity ID’s or attributes to portions of the NHD is referred to as Reach Indexing.
The NHD-RIT assigns Entity ID’s or attributes to user selected portions of NHD through the creation of event tables that can be displayed as themes in ArcView using ESRI’s dynamic segmentation model. Not only does this protect the integrity of the nationally consistent NHD dataset, but it also provides the user more flexibility for delineating entities than is available through other methods. Event tables allow for the creation of event “reaches”, which can represent an entire reach in NHD, or only a portion of a reach. This allows the user to delineate any desired surface water entity, even if it is not coincident with the start and end point of reach segments in NHD.
The NHD event tables created by the NHD-RIT provide three fields to store attribute data entered by the user: Entity ID, Attribute Program and Attribute Value. Entity ID is frequently used to store a unique identifier for a surface water entity (like a Fish Consumption Advisory) that can be linked to an outside database that contains detailed information about the entity. Attribute Value can be used store a non unique descriptor about the surface waters, such as habitat type or substrate. Attribute Program can be used to identify the Agency or Program that is the source of the information.
Who can use the NHD-RIT?
The NHD-RIT can be used by any organization that has a mandate to monitor and track surface water information. It is designed for users who are familiar with ArcView. On site training for the NHD-RIT is available to states who are interested in using event tables for the EPA 303(d), 305(b) or WQS mapping initiatives. Please contact OW Support at firstname.lastname@example.org or call 1-800-844-0638, if you are interested in training. While the NHD-RIT will function with NHD coverages download from the NHD web site, it is currently recommended that you use the 1:100,000 releveled data to ensure compatibility with EPA's reach indexing initiatives. The releveled NHD data can be obtained by contacting OWSUPPORT at email@example.com.