Essential bathymetric vocabulary

Bathymetric attributed grid

Gridded data formatted as a bathymetric attributed grid (BAG) is a multidimensional hydrographic data file that meets standards developed by the Open Navigation Surface Working Group. Currently, BAG files contain elevation gridded data and Total Propagated Uncertainty (TPU) gridded data, as well as metadata.

Bathymetry Data Index

The Bathymetry Data Index (BDI) is a mosaic dataset within the BIS geodatabase that maintains a list of all gridded surfaces registered to the BIS. The BDI mosaic dataset is a series of tables within the geodatabase that manages the location of gridded bathymetric surfaces on disk. Since the BDI manages the location of gridded surfaces, rather than loading the data directly into the geodatabase, the performance can be optimized with speed and size. Subsequent surfaces can be generated from the BDI with the use of the Explore Bathymetry and Compose Surface windows. The surfaces created will also be created as a mosaic dataset referencing the same gridded bathymetric surfaces on disk.

Bathymetry

Bathymetry is the measurement of underwater depths and the description of landforms beneath a body of water. This information is gathered from 3D and profile analysis of depth data.

Water density

Density is a measure of mass per unit volume. Seawater is denser than fresh water and pure water since dissolved salts add mass to water without significantly changing its volume. Seawater density generally increases along with pressure and depth, while temperature generally decreases with depth. Together, these properties affect sound velocity in seawater, and depth calculations from echo sounders need to be adjusted accordingly.

Water depth

A depth is a measurement of the vertical distance from a given water level to the seafloor.

Echo soundings

Echo soundings are ocean depth measurements gathered from underwater acoustic sonar systems. These measurements are derived from sound pulses that are generated from a transducer, transmitted through the ocean, reflected from solid objects, and perceived by receiving sensors on the transducer. Echo sounding information is typically used to obtain ocean depths for seafloor mapping.

Golden soundings

Golden soundings, or persisting soundings, are depths that a hydrographer has deemed significant and should appear in point or gridded data, regardless of the product scale. For example, a hydrographer may find depths at the top of a mast on a sunken wreck that are significant for navigational safety and choose to always display them on a chart.

Hydrographic survey

A hydrographic survey is the science of measurement and description of water masses, water depths, the nature of the seabed, currents, tides, and other physical features of the ocean. Results from hydrographic surveys are used for navigation safety, ocean and coastal engineering, environmental protection, coastal zone management, marine archaeology, naval operations, and ocean sciences. Typically, hydrographic vessels conduct these surveys using echo sounding equipment; increasingly, surveys include light detection and ranging (lidar) bathymetry from aircraft and sophisticated electronic sensor systems in clear and shallow waters.

Lead line

A lead line is a mechanical instrument to measure water depths. Before echo sounding equipment was developed, mariners often used lead lines to measure the ocean floor. One technique required a mariner to lower a weighted line into the ocean to measure distances from the boat to the water bottom. The weighted line is referred to as a lead line, regardless of the material that actually comprises the weight. Lead lines are still used today, though echo sounding and other modern techniques provide faster data collection and much more detailed and accurate information.

Marine lidar

Light detection and ranging (lidar) is a remote-sensing technique that uses lasers to determine distance to and other properties of a target. To determine shallow water depths (up to about 70 meters) for hydrographic surveys, airplanes equipped with lidar technology will fly an arc across an area of interest, aiming red and green lasers that reflect off the water surface and the seafloor, respectively. The time difference between these two is used to compute water depths.

Multibeam echo sounder system

A multibeam echo sounder system (MBES), also known as Swathe or Swath echosounders, transmits sound energy to the seafloor and analyzes return signals, or echoes, to determine the depth and character of the seafloor. The process is similar to that of a single-beam echo sounder system (SBES), but unlike an SBES—which transmits a single beam directly downward in a water column—an MBES transmits sound energy across a fan-shaped area, known as a swath, to the water bottom. The system measures the time it takes for the sound energy to travel from the transmitter to the seafloor and back. Those times are corrected for sound velocity in the water column, the movement of the boat, vessel orientation, and tidal variation and then interpreted as depths.

Multitransducer echo sounder system

An array of single-beam echo sounders, arranged on each side of a vessel and dragged along the ship's track, is known as a multitransducer echo sounder system. This is also known as a sweep sonar system.

Salinity

Salinity within water refers to the amount of salt, such as sodium chloride and bicarbonate, dissolved in a body of water. Salinity generally increases with pressure and depth, while temperature generally decreases with depth. Together, these properties affect sound velocity in seawater, and depth calculations from echo sounders need to be adjusted accordingly.

Side-scan sonar

Side-scan sonar is a system for detecting objects on the seafloor. Like other sonar systems, side-scan sonar sends sound energy to the ocean floor and measures the strength of the return signal. Unlike single and multibeam echosounders, however, side-scan sonar provides imagery rather than depth information.

The images are derived from the strength of the signal that returns to the receiver. Seafloor surface objects return strong signals, which display as light areas, while shadows from features on the seafloor return dark areas. The system is typically used together with a single-beam or multibeam echosounder in a hydrographic survey.

Single-beam echo sounder system

Single-beam echo sounder systems (SBES) measure water depth vertically underneath a vessel. In general, an SBES includes a transmitter, which sends the pulses directly downward in the water column, and a receiver, which detects sound waves returned to the ship after they reflect from the seafloor. The SBES can be mounted to the hull or to the side of the ship. The SBES records two-way travel times for the sound waves; to interpret these measurements as depths, they must be corrected for the speed of sound in water from sound velocity profile measurements, tidal variation, and the movement of the boat.

Sound velocity profiler

Calculating accurate ocean depths requires knowledge of the speed of sound in waters within a study area. This is not a straightforward metric; sound will travel at different speeds in water, varying with temperature, salinity, and pressure-depths of water masses. To measure sound velocities with precision, mariners can deploy a sound velocity profiler instrument from a stationary ship to create a detailed vertical profile of sound velocity at a location. That information can be used to correct depth measurements during echo-sounding data processing.

Soundings

Soundings are depth measurements taken from the surface through a water column, generated when a mariner or a machine sends some kind of probe into the ocean. Historically, soundings were taken from simple instruments such as lead lines; today, mariners can collect echo soundings.

Temperature of seawater

In general, seawater temperature decreases with depth, though it varies locally and over space and time. Time and space variations are highest in shallow water, where factors such as solar energy, surface currents, and atmospheric variations can have a large impact.

Tidal datum

The sea surface typically rises and falls twice daily as it responds to varying gravitational pull from the moon and sun. The high and low waters occur in adjacent phases and are given names such as higher high water, lower high water, higher low water, and lower low water. The average measurements taken at a given location during each of these phases is referred to as a tidal datum. For example, the average of higher high water measurements taken at one tide gauge is the tidal datum of higher high water there.

Uncertainty grid

The BAG file specification requires an uncertainty grid coincident with depth information. The uncertainty grid is a two-dimensional array of positive values, given in units of meters, that describes the vertical uncertainty at every pixel for which an elevation value is also present.

Since the uncertainty grid can be created in a variety of ways, this information is typically captured by the BAG's metadata XML file. Per the BAG format specification, the unknown state for uncertainty, or the raster NoData value, is defined to be 1000000.

Vertical datum

To create accurate charts and maps of the seafloor, the geographic position of points on the seafloor must be referenced to known locations on Earth in both vertical and horizontal space. A vertical datum is the vertical geographic reference against which vertical measurements are referenced and corrected.