NHHD organizes these hydrographic features into a fully connected network within the GIS and furthermore defines relationships between features to orient flow in a continuously downstream direction. NHHD is a subset of the National Hydrography Dataset (NHD), a nationwide dataset that is housed and distributed by the United States Geological Survey (USGS). The NHHD has many functions and applications and is probably best known for its use as the cartographic representation of surface water features found on USGS 1:24,000 scale topographic quadrangle maps. It is the responsibility of the New Hampshire Geological Survey (NHGS) to oversee the development, maintenance and enhancement of the NHHD.
NHGS has provided quality control and project oversight for the entire NHHD development effort and in May 2008 formalized its role as the official data steward through a Memorandum of Understanding with USGS. At a minimum, this represents a long-term commitment to maintain the accuracy of NHHD. However, the overall objective is to enhance the utility of NHHD for a broad range of regulatory and non-regulatory users. NHGS has been successful in obtaining grant funds from the US Environmental Protection Agency to support these efforts.
In addition to its significant use as an inventory of New Hampshire surface water resources at a scale of 1:24,000, the NHHD network architecture supports other extremely useful functions, as highlighted below.
A special feature of NHHD is that it enables the simulation of “network connectivity” between water bodies and watercourses so that real world flows and relationships can be modeled through “virtual navigation” or flow tracing along stream centerlines.
Another important component of the dataset is that it provides a standard framework for referencing other water resource-related features of interest. These so-called “events” can be specific point features such as dams, USGS stream flow gauging stations, water withdrawals, water quality sampling sites or features that exhibit a greater spatial extent such as areas of rip-rap along stream banks or portions of channel classified as riffles or pools, etc. Locations of events are uniquely defined by a distance measure from the upstream end of the closest stream reach in the network. Events of any type can be readily identified by selectively tracing the network in either the upstream or downstream direction. Because NHHD is part of a nationally consistent framework for linear addressing and analysis, water-related information linked to stream reach addresses by one organization (national, state, local) can be shared with other organizations and easily integrated into many different types of applications to the benefit of all.
Users of the NHHD include state agencies, non-profit organizations, academia and the public at large. The partnerships that over time contributed in great measure to the successful development of NHHD have an obvious stake in maintaining the long-term value and benefit of their investment. As data steward, the NHGS welcomes input on potential dataset improvements and new applications, as well as reports of inaccuracies or other data quality issues. NHGS is particularly interested in expanding the different types of event data that are linked to NHHD and insuring the long-term compatibility of these data. Maintenance of the New Hampshire Watershed Boundary Dataset (NHWBD) represents a related activity as part of the stewardship agreement, acknowledging the obvious dependency between drainage networks and drainage divides. Ongoing stewardship of the NHHD and NHWBD is critical to improve their accuracy so that conditions on the ground are represented in a meaningful and consistent matter.
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