Hydrographic survey is the science of measurement and description of features which affect maritime navigation, marine construction, dredging, offshore oil exploration/drilling and related disciplines. Strong emphasis is placed on soundings, shorelines, tides, currents, sea floor and submerged obstructions that relate to the previously mentioned activities. The term Hydrography is sometimes used synonomously to describe Maritime Cartography, which in the final stages of the hydrographic process uses the raw data collected through hydrographic survey into information usable by the end user.
Hydrography is collected under rules which vary depending on the acceptance authority. Traditionally conducted by vessels and with Echo sounding, surveys are increasingly conducted with the aid of aircraft and sophisticated electronic sensor systems in shallow waters. United States Navy SEALS and Seabee Underwater Construction Technicians also have the ability to conduct hydrographic surveys. The SEAL/UCT operators are normally called upon before amphibious landings in order to survey the landing beaches.
When surveys are used for the purposes of chart making/distribution or dredging of state controlled waters they are commonly conducted by or under the supervision of national organizations. Coordination of those organizations voluntarily joined with the goal of improving hydrography and safe navigation is conducted by the International Hydrographic Organization. In the United States, hydrographic acceptance is conducted by NOAA for maritime navigation, the Army Corp of Engineers for dredging and marine construction and occasionally the Environmental Protection Agency (on projects such as the GE/Hudson River Super Fund site).
Companies, Universities and investment groups will often fund Hydrographic surveys of public waterways prior to developing areas adjacent those waterways. One example of this would be surveys completed for Riverboat Casinos that ply inland rivers. These large ships require relatively shallow water, but it has to meet minimum depth requirements. Private surveys are also conducted before dredging operations and after these operations are completed. Steel mills and companies that have large private slips and docks have their facilities and open water near their facilities surveyed regularly.
Modern surveying relies as much on software as hardware. Equipment can be installed on inflatable craft, such as Zodiacs, small craft, AUVs (Autonomous Underwater Vehicles), UUVs (Unmanned Underwater Vehicles) or large ships, and can include sidescan, single beam and multibeam equipment.
After data is collected, it has to undergo post-processing. A massive amount of data is collected during the typical Hydrographic survey, often several soundings per square foot. Depending on the final use (navigation charts, Digital Terrain Model, volume calculation for dredging, topography, Bathymetry) this data must be thinned out. It must also be error corrected (bad soundings,) and corrected for the effects of tides, waves/heave, water level and water temperature differences (thermoclines.) Usually the surveyor has additional data collection equipment on site to record the data required for correcting the soundings. Final output of charts can be created in a combination of specialty charting software or a CAD package, usually Autocad.
NOAA maintains a massive database of survey results, charts, and data on the NOAA site.
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