Charting and mapping the bathymetry and nature of the sea floor requires being able to locate and measure the geographic position (horizontal) and depth of the sea floor (vertical) below a survey vessel and then reference those measurements to a standard reference frame or datum (control). VERTICAL CONTROL Vertical control refers to a network of geodetic marks that supports three-dimensional hydrographic positioning.   Vertical control allows us to accurately determine 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) is responsible for all planning of tide and water level requirements for OCS hydrographic surveys. CO-OPS analyzes historical data and tidal characteristics for project areas, specifies operational NOS control stations and general locations for subordinate water level stations to be installed, and provides the tidal zoning (both preliminary and final) to be used during survey operations. NOAA hydrographic field units are sometimes required to install, monitor, repair, and/or uninstall a temporary control water level station under the instructions from CO-OPS. HORIZONTAL CONTROL Horizontal control refers to the terrestrial network of geodetic marks that support two-dimensional hydrographic positioning. 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. Differential Global Positioning Systems (DGPS) allows positioning of survey vessels and accurate sounding (depth) location that must be determined 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 where a portable DGPS control (“fly-away”) station can be installed. RESOURCES NOAA's Center for Operational Oceanographic Products and Services (CO-OPS)  U.S. Coast Guard differential beacons National Geodetic Survey