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Digital Shorelines for Boston Harbor

Shoreline Definitions and Background
(condensed summary of a recent briefing by Lockwood, 1997, by permission)


Background: The shoreline is one of 27 global "geo-indicators", identified by the International Union of Geophysical Societies. Shorelines in the United States are a federal responsibility. Proposal for creation of the U.S. Coast & Geodetic Survey, whose responsibilities included shoreline mapping, was initiated by President Thomas Jefferson. The organic act was signed in 1807. Responsibility for domestic purposes is within NOAA/NOS, and externally in DOD/NIMA.

Shoreline and water level definition: The terms "coastline" and "shoreline" are held as synonymous by a leading treatise on coastal mapping (Shalowitz, 1964). Both are defined as the line of contact between land and selected water elevations. "Coastline" occurs in several recent Congressional acts. In common usage it is often used as a permanent high-water boundary. For more generalized quantitative measures "coast" tends to be employed, whereas "shoreline" refers to more detailed delineation. For tidal waters the shoreline is normally Mean High Water (MHW). For non-tidal waters it is normally mean water level. MHW and Mean Low Water Level (MLW) refer to the average high and low tidal levels for the previous 19 years. Mean Lower Low Water (MLLW) level refers to the lower of the two daily tidal levels in those areas where diurnal tides dominate. A corresponding definition applies to Mean Higher High Water level (MHHW). By an 1980 international convention "Lower low water level" is a standard for all nautical charts, as providing the lowest water levels likely to be encountered in navigation.

Baseline definition: Baseline is a term used in international law to indicate reference line from which the marginal sea and other offshore zones are measured. The official U.S. baseline is the low-water (MLW) line along the coast.

Shoreline data uses: legal, administrative, regulatory/cadastral, sovereignty, federal-state boundaries, geography, maritime navigation, nautical charts and bathymetric maps, coastal flooding, erosion, and subsidence assessment, military concerns, and engineering & construction.

Data sources & processing issues: topological "closing lines", coincident geometry, geodetic control reference standards, georectification of orthophoto and satellite imagery; metadata (documentation of quality, source & other qualification), certification, validation, and attribution. Attributes refer to the nature of shoreline features, such as rocky, sandy, muddy, and the like.

Stakeholders and issues: State agencies, landowners, flood insurance, utilities (pipelines), engineering works, tourism, recreational questions, environmental managers & shore protection.

Related research fields and topics: bathymetry, geology, environmental studies & biology, hydrology (rivers), cultural studies, transportation.

Standards & accuracy: Horizontal accuracy is referenced to the National Map Accuracy Standard. The vertical standard is referenced to tidal data measured by NOAA/NOS. Other more descriptive reference standards are exemplified in Table 1. For discussion of the details of elevation contours and datum references see Crowell et al, 1991.

Responsible agencies: NOAA National Geodetic Survey (formerly Coast and Geodetic Survey), DOD/NIMA (formerly Defense Mapping Agency) (federal); Massachusetts Geographical Information Systems office, (MassGIS), part of Executive Office of Environmental Affairs, Boston, MA. A memorandum of understanding exists between NOAA/NOS and the Minerals Management Service of the Department of Interior, regarding definitions and mutual responsibilities relating to the coast.

References: Shalowitz, 1964; Ellis, M.Y, 1978; Harrington, C.E., 1993; Fritz, 1994; Teller, 1996.


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