An imaginary line on a map joining points of equal depth below a water surface is called an:

An isobath is a line on a map or chart joining points of equal depth below the surface of a sea, lake or other water body. It is the underwater equivalent of a contour line on land. The word comes from the Greek "iso" (equal) and "bathos" (depth).

Uses of isobaths:

• They show the shape of the sea floor or lake bed (its relief), including slopes, ridges, trenches and basins.
• They are drawn on nautical charts to help ships navigate safely and avoid shallow areas.
• They are used in planning harbours, laying undersea cables and pipelines, and fishing.
• They help scientists study ocean currents and marine habitats.

Bathymetry:

• The measurement and mapping of the depth of water bodies is called bathymetry.
• A map showing isobaths is called a bathymetric map or chart.

How depth is measured:

• Earlier, depth was measured by lowering a weighted rope or line, a method called sounding.
• Today, ships use echo sounders (sonar), which send sound pulses to the sea floor and measure the time taken for the echo to return.
• Multibeam sonar can map wide strips of the sea floor at once.
• Satellites can estimate sea-floor shape from tiny variations in the height of the sea surface caused by gravity.

Units:

Depth on charts is shown in metres, or in older charts in fathoms (1 fathom = 6 feet, about 1.83 metres).

The 200-metre isobath:

The 200-metre isobath is often used as a rough boundary marking the outer edge of the shallow coastal sea areas, beyond which the sea floor slopes steeply into deep water.

Mapping the ocean floor:

• Despite modern technology, a large part of the ocean floor has not yet been mapped in high detail.
• The international project Seabed 2030 aims to map the entire ocean floor by 2030.
• India's Deep Ocean Mission includes mapping and exploration of the deep sea.

Why not the other options?

• ✗An isohyet joins places of equal rainfall.
• ✗An isohel joins places with equal sunshine.
• ✗An isobar joins places of equal atmospheric pressure.