Isobaths

An isobath is a line on a bathymetric map that connects points at equal depth below a specified reference surface, typically mean sea level or a chart datum such as LAT or MLLW. It is the depth-contour analogue of a land elevation contour and is used to depict the shape of the seafloor or lake bottom.

On hydrographic charts and bathymetric maps, isobaths help readers understand seabed morphology, identify features such as shelves, slopes, canyons, and basins, and estimate water depth for navigation and engineering. The spacing between isobaths indicates slope: widely spaced lines signify gradual depth changes, while closely spaced lines indicate steep gradients. Depth values along the lines are usually labeled and expressed relative to the chosen datum.

Isobaths are generated from bathymetric data collected by sonar systems (such as multibeam or single-beam echo sounders), airborne lidar in shallow water, or satellite-derived measurements. After data processing, including datum corrections and noise reduction, a digital terrain model is created, and contour lines are drawn at selected depth intervals (for example, every 10 meters or every fathom).

Applications include navigation planning, port and harbor development, dredging and coastal engineering, and marine habitat mapping. Limitations arise because depths reflect conditions at the time of measurement and depend on the chosen datum; tides, sediment transport, dredging, and other dynamic processes can alter seabed depth, so charts and isobath patterns are periodically updated.