Hydrographic Surveys
Level 1

bathymetry noun Measurement of the depth of large bodies of water. 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. hydrographic adjective Hydrographic surveying accurate measurement of points and lines of direction on the earth's surface for the purpose of preparing maps or locating boundary lines. Hydrographic surveying records such features as bottom contours, buoys, channels, and shoals in bodies of water and along coastlines. The Concise Columbia Encyclopedia

Making Sure the Channel is Safe
Ship captains and river pilots can't see the bottom of the channel. They have to trust the charts and maps that the Coast Guard and the Corps of Engineers make. But the condition of the channel bottom can change, sometimes quickly during a flood or drought. When a shoal forms on the channel bottom, those planning to navigate the channel must be warned to protect the safety of their ships and cargo.

To keep the information on the charts up to date, navigation channels are surveyed often. 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.

Below is a piece of a survey of a navigation channel. The outline of the channel is shown and the numbers are the depth of the river in the channel. The surveyors draw isolines, or lines connecting points of equal depth, on the map so that captains and ships' pilots can get an idea of the "hills and valleys" underwater. The valleys do not pose much of a problem to ships, but if the depth of water over the hill is not as deep as the channel is supposed to be, boats and ships could run aground and be damaged. When there is a significant area of water that is too shallow, dredges are used to dig it deeper again.