Hydrographic Surveys
Level 2

bathymetry noun Measurement of the depth of large bodies of water. bathymetric or bathymetrical adjective bathymetrically adverb hydrography noun 1. The scientific description and analysis of the physical conditions, boundaries, flow, and related characteristics of the earth's surface waters. 2. The mapping of bodies of water. hydrographer noun hydrographic adjective hydrographically adverb The Concise Columbia Encyclopedia

Mapping the ocean floor or channel bottom is a specialized field of surveying, called hydrographic surveying. The maps that hydrographic surveyors produce are called bathymetric maps. Mariners and those responsible for the maintenance of the nation's waterways need current bathymetric maps to do their jobs.

Ship captains and pilots need timely and accurate channel information to navigate safely. Channel conditions change, sometimes quickly, during fluctuations in water level or currents. When obstructing shoals form, those planning to navigate the channel must be warned of the situation to avoid grounding and damaging their vessel or its contents. Maintained navigation channels are surveyed on a regular basis and the results mapped to provide updated information to mariners.

Channel condition survey for a project, 43 feet deep.
The survey lines are at 50-foot intervals. Shaded areas are shoals

Making a Bathymetric Map

Colorized Bathymetric map of Crater Lake, Oregon. The blue regions are the deepest portions of the lake

On land, surveyors can see important surface features and know they are shown accurately on the finished topographic map. Underwater surfaces are not visible to the hydrographic surveyor, who must rely entirely on remote measurements. This is done with great accuracy by making many measurements, plotting them and drawing isolines, or lines connecting points of equal depth, to represent the bottom surface. The greater the number of data points, the less risk of missing important underwater features (like a shoal).

It is important to note that unlike land surveyors, who measure the height of the land, hydrographic surveyors measure the depth of the water over the bottom. The data are then interpreted to show what the surface features must be.

Hydrographic Survey Principles
At one time, lead-weighted lines or poles with calibration marks were dropped over the side of the survey boat to measure the water depth. This tedious method has been replaced by the use of sonar instruments. Sonar equipment emits a sound wave and records the time it takes for the sound wave to travel the depth of the water, bounce off the bottom, and travel back to the boat. The time is translated into distance using a variation of the following basic equation:

The variables accounted for in the actual calibration of the equipment include such things as the salinity and temperature of the water. State-of-the art sonar equipment used for bathymetric surveys is extremely accurate, usually better than ± 0.1 foot per 10 feet of depth.

As important to the survey as determining the depth is determining the exact position of the measurement. Like land surveys, the position of the elevation data point is found by a triangulation of known points. Before the advent of global positioning, the reference points were land-based.

A complicating factor in making bathymetric maps is that all depth measurements are relative to the surface of the water. But, the water surface of rivers, estuaries, and oceans is not at a constant elevation. Measurements must be adjusted to compensate for tidal and seasonal variations in water depth measurements. In tidally influenced areas, the elevation of the water surface changes while the survey data are being collected. On the ocean, hydrographic surveyors must compensate for both tidal variations and the lifting and lowering of the survey boat by passing waves. In all cases, the surface of the water is adjusted to another constant datum -- mean lower, low water (MLLW) in coastal areas. In water bodies not affected by tidal variations, the depth is referenced to another datum. If the channel lies above a dam, the datum may be the minimum pool elevation specified for that reach of the river. Otherwise, there will be a locally adopted low-water gradient that will be indicated on nautical charts and channel maps.

Hydrographic surveying is a technical area that has greatly benefited by the development of powerful microcomputers, computer software, and global positioning. Because it relies on remote measurements, the accuracy of bathymetric maps is closely tied to the number of data points. But, if the data reduction, adjustments, and interpretation are done by hand, the more points measured, the greater the task and, in some cases, the greater the opportunity to introduce an error. Today, hydrographic survey technology has evolved to the point where the subsurface can be digitally recreated quickly and efficiently by a knowledgeable crew with state-of-the art equipment.

How Triangulation Works
BYTE Online Magazine, April 1995
GPS (Global Positioning System) location technology uses a trigonometric formula known as triangulation. There are two methods of triangulation; one uses an unknown location, and the other uses a known location. Vehicle location systems such as the one used by Day & Night and OnStar use the unknown-location method.

Find Yourself ... Study Trigonometry! Trigulation to find the location of an unknown point is an application of trigonometry. In a flat plane, the location is found by forming a triangle with the unknown point and two known points as the vertices. This is a conventional land surveying technique and is an application of the Angle-Side-Angle equation. In global positioning, the location of the point is established by the intersection of transmission signals from three satellites - an application of three-dimensional trigonometry concepts. The GPS system is akin to celestial navigation, which uses triangulation with the position of three stars. Whether it's done in two dimensions or three, on the ground, from a satellite, or with stars, trigonometry can help you find yourself!

A transceiver in a truck receives signals from three satellites. From these signals, the software in the transceiver can determine the truck's position. The satellites orbit the earth at about 12,000 miles from its surface, but a satellite is rarely directly overhead. The varying positions of a satellite in its orbit create a variety of distances from the transceiver.

If a truck receives a signal from a satellite located 13,000 miles away, the location of the truck is narrowed down to a 13,000-mile sphere centered on that satellite. If another signal comes from another satellite located 18,000 miles away, the location is narrowed further to the points where the 13,000-mile and 18,000-mile spheres intersect. Add a third satellite, located 16,000 miles away, and the area of intersection is narrowed to two points--one on the ground and one impossibly high above the earth.

The distance between a truck's transceiver and a satellite is calculated with the old "velocity times travel time" formula (d = V x t). The satellite signal contains the time it was sent from the satellite.

To work correctly in a GPS, each clock in every transceiver has to be synchronized to the nanosecond. Each satellite has four atomic clocks: One is the working clock, one serves as a backup, and the other two are responsible for keeping the first two in sync.

When the transceiver receives a signal, the time included in the signal is immediately recorded. The GPS's software then calculates the difference between the send time and the current time to determine the lapse. For example, the transceiver might find that the lapse is 0.1 second. Using the aforementioned formula, velocity (186,000 mi/sec) times time traveled (0.1 sec) equals the distance from the satellite (18,600 miles).

After determining the transceiver's distance from all three satellites, the software in a GPS can accurately determine the transceiver's latitude, longitude, and altitude. Then, based on the transceiver's unique identity number, it can transmit its position using a different geosynchronous satellite communications network.

How long does all this take? If a truck uses the same three satellites during the operation, it can take as little as 2 seconds. Otherwise, the calculation can take up to 30 seconds.