|Navigation | Lessons | 5 | Hydrographic Surveys Level 1|
Making Sure the Channel is Safe
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.
Channel condition survey for a project, 43 feet deep.
The survey lines are at 50-foot intervals. Shaded areas are shoals.
For dredging, survey information and maps are used to locate the work area. Engineers use the survey information before dredging to figure out how much material is in the channel. After dredging, another survey is done to see if the channel has been successfully cleared of shoals. When engineers compare the survey before the dredging to the survey after the dredging, they can estimate how much sediment was taken out.
Pre-dredge channel cross section - Hatched area is to be removed
Post-dredge channel cross section
Making a Bathymetric Map
To do this, hydrographic surveyors measure the depth of the water over the bottom. They make many, many measurements, plot them, and draw isolines. The more measurements that the surveyors have, the less risk of missing important underwater features, like rocks or shoals.
Colorized Bathymetric map of Crater Lake, Oregon.
The blue regions are the deepest portions of the lake
Webdate: April 23, 2002