NHD Reach Indexing Tool Overview
The Problem.
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 Solution.
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 owsupport@rti.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 owsupport@rti.org.
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