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Frequently Asked Questions about
Hydrographic Survey Data

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I purchased data for ------------ area and there are spots which lack digital data coverage (holes), how can I get these data?

Answer: There are many surveys which exist in analog (paper) form. These surveys are maintained by the NOS/Office of Coast Survey located in Silver Spring, Maryland. To request these data, or contact the Office of Coast Survey, please see their website

How accurate are the data and what is the density of soundings in ------- area?

Answer: The NOS hydrographic survey database is a historical set of surveys. In fact, some of the digital data sets are digitized from surveys that were conducted in the late 1800's! Accuracy is difficult to determine for these surveys. Recently (after 1965) rigid standards were maintained for survey control. These accuracy standards and requirements are stated in the NOS Hydrographic Surveys Specifications and Deliverables publication, which focuses on the minimal criteria needed for both in-house and contract NOS surveys. The Hydrographic Manual Fourth Edition is an older (4th edition published in 1976) and more comprehensive document that covers all stages of the hydrographic survey process at NOS, and can be purchased from the Hydrographic Surveys Branch, Data Control Section (301) 713-2710. Data density requirements are covered in the Hydrographic Manual.

Are there any recent data for the area, the data I received were surveyed in the late 1800's?

Answer: Hydrographic surveys are completed continuously by the NOS. There are various reasons to conduct new surveys such as: safety concerning vessel traffic, topography changes due to environmental impact, and gaps in data coverage.

To find out if there are any new surveys in your area of interest, please check the NOS/Office of Coast Survey website

In addition, the NGDC maintains worldwide trackline bathymetry using a database system called GEODAS which may be used to augment the NOS data and fill the data gaps.

Are these data in a format that can be easily transferred and used in a GIS?

Answer: Yes, the data can easily be ported to a GIS. The data can be saved in various ASCII formats which can be ingested by a GIS.

I noticed that there are "holiday areas" off the coast of ------, is this area going to be surveyed in the near future?

Answer: The point of contact for new surveys is the NOS/Office of Coast Survey website,

What tidal datum is used on NOS surveys?

Answer: The tidal datum to which soundings from recent (post-1980) NOS surveys are referenced is Mean Lower Low Water (MLLW) while the currently used vertical datum on the Great Lakes is the International Great Lakes Datum of 1985. Pre-1980 surveys conducted along the Pacific coast and Alaska also utilized MLLW. The Mean Low Water (MLW) datum was used on the Atlantic coast of the US prior to 1980. Due to changes in sea level, the absolute elevation of these datums have been modified over the years to correspond to various tidal epochs (19-year periods). For more information on tidal datums contact the NOAA/NOS National Water Level Observation Network.