The National Hydrography Dataset (NHD) is a spatial database that
describes surface water features and their inter-relationships. As a nationally
standardized database, the NHD provides a framework for linking a wide
variety of water related data from multiple sources to the surface drainage
network. By using NHD, such information as flow rates, downstream risk
potentials, environmental impacts and water quality may be integrated to
enable advanced modeling and scientific analyses.

Production of High Resolution NHD: GDM is one of the nations most
experienced, accurate and affordable providers of high resolution 1:24,000
NHD. GDM builds upon cooperative effort between the EPA and the USGS,
which created a nationwide coverage of 1:100,000 NHD. While this scale is
useful for regional analyses, more detailed studies require higher resolution
data. To meet this need, GDM has developed a streamlined process and
capability to create high-resolution 1:24,000 scale NHD data. In addition to the increased spatial accuracy, high-resolution NHD data provides a fivefold
increase in the density of the stream network and a twenty-fold increase in the
number of water bodies. Centerlines are added to represent flow paths through
two-dimensional features and attributes are supplemented using the
Geographic Names Information System (GNIS). The resulting NHD database
retains linkages with the original 1:100,000-scale data, while providing users
with greatly increased content and accuracy.

Production of Local Resolution NHD: Taking NHD to the next step, and offering a bit more detail, GDM also offers Local Resolution (LR) NHD. Standards for LR NHD differ from local, state and federal entities. LR source data comes in a variation of small scales, data formats, collection processes, attribute schemes, spatial accuracies and geographic extents depending on the end user’s needs. This knowledge of the data can be acquired through metadata, or through first-hand knowledge of the data itself. Any hydrographic data can be used to create LR NHD, with proper assembly. Properties of the original data will be retained throughout LR NHD assembly without any additional preparation. GDM will conform to spatial accuracy and attribution requirements based on the needs of the end user.