Charting the bathymetry and nature of the sea floor requires locating and measuring the geographic position (horizontal) and depth of the sea floor (vertical) below a survey vessel, and then referencing those measurements to a standard reference frame or datum (control).
Vertical control required for measuring ocean depth
A network of geodetic marks supports three-dimensional hydrographic positioning. This vertical control allows accurate determination of the depth of the sea floor.
Depth information from hydrographic surveys must be reduced to a sounding or vertical datum (Mean Lower Low Water for NOAA nautical charts) for charting purposes. Reducing measured depths to a uniform vertical datum requires the application of tidal and water level height information.
NOAA’S Center for Operational Oceanographic Products and Services (CO-OPS) plans tide and water level requirements for Coast Survey hydrographic surveys. CO-OPS analyzes historical data and tidal characteristics for project areas; specifies operational control stations and general locations for installation of subordinate water level stations; and provides the tidal zoning used during survey operations.
Horizontal control required for positioning depth information
A terrestrial network of geodetic marks supports two-dimensional hydrographic positioning. This horizontal control is necessary to position depth information accurately on the earth.
Positioning depth information similarly requires positions to be referenced to a horizontal datum (North American Datum of 1983 [NAD 83] for NOAA nautical charts). The horizontal datum is a set of constants specifying the coordinate system used for control, that is, for calculating coordinates of points on the earth.
The Differential Global Positioning System (DGPS) allows positioning of survey vessels, and determines accurate sounding (depth) location relative to the vessel position. This requires taking into account several offsets, including: the characteristics of a survey system, depth of water, speed of sound profile for the water column, and transducer offsets.
Typically, NOAA field units use U.S. Coast Guard differential beacons to correct GPS for hydrographic positioning. USCG differential correctors are subjected to an internal quality control process, ensuring a high level of accuracy. Additionally, USCG differential beacons have already been precisely positioned; thus, it is generally unnecessary for NOAA field units to establish horizontal control points when using this positioning method. In remote survey locations or confined areas, such as a fjord, USCG differential correctors may be unavailable, or severely limited, and field units must establish at least one horizontal control point for installation of a portable DGPS control station.