A bathymetric map is a map that measures water depth across an underwater area. Although many people think of bathymetric maps as measuring ocean depths, this type of mapping can also apply to seas and lakes. A bathymetric map is a lot like a topographical map, except that the features contained in it are underwater. It may use various representations, including color and contour lines, to represent ocean or sea depth in a particular area. Some bathymetric maps use what's called a digital terrain model (DTM) to show how underwater depth levels differ in a region.
When cartographers began to make bathymetric maps, depth was often found by lowering some type of physical probe down into a body of water. This method could be time consuming and inaccurate. In modern times, this method has been replaced by sonar to give mapmakers a much better picture of what's on an ocean floor.
A bathymetric map can serve many different functions. Many of these types of maps give navigators a better understanding of underwater features that could threaten the safety of the specific sea path for a boat or vessel. A bathymetric map can also be helpful in diving missions, where search parties are looking to identify something located on an ocean or sea bottom, from a lost ship to jettisoned cargo. Many interesting undersea diving missions that have unearthed long-sunk treasures or famous lost ships have taken advantage of bathymetric mapping to more easily reach an undersea location. Bathymetric mapping is also used for "paleobathymetry," the study of ancient changes to underwater topography.