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Chief, Hydrographic Surveys Division CDR Gerd Glang Silver Spring, MD 20910 Hydrography is the science which deals with the measurement and description of the physical features of bodies of water and their littoral land areas. Special emphasis is usually placed on the elements that affect safe navigation and the publication of such information in a suitable form for use in navigation. A hydrographic survey may be conducted to support a variety of activities: nautical charting, port and harbor maintenance (dredging), coastal engineering (beach erosion and replenishment studies), coastal zone management, and offshore resource development. The one data type common to all hydrographic surveys is water depth. Of additional concern to most surveys is the nature of the sea floor material (i.e. sand, mud, rock) due to its implications for anchoring, dredging, structure construction, pipeline and cable routing and fisheries habitat. The primary use of hydrographic surveys is for nautical charting. This requires the precise location of least depths on dangers to navigation and depths significant to surface navigation. The precise location of aids to navigation is also required. Side scan sonars are often deployed to facilitate the detection of submerged dangers to navigation. Tide or water level measurements are also recorded to provide a vertical reference (Mean Lower Low Water) for water depths. All NOS hydrographic data are now collected and processed with specialized computer systems which store the data in digital form and generate graphic displays of the hydrographic data on mylar "smooth sheets" at a variety of survey scales, usually at 1:10,000 and 1:20,000-scale. The criteria for the smooth sheet scale are based on the scale of the nautical chart of the area and the amount of hydrographic detail necessary to adequately depict the bottom topography and portray the least depths over critical features. Hydrographic Questions?
Revised Tuesday April 17 2007 by OCS Webmaster |