# How is bathymetry done without sonar?

Or better yet, how WAS it done? I understand that depth sounding was done with a lead weight attached to a long rope or wire, but how were the leadsmen able to tell when the lead weight reached the bottom? Especially when the water is far too deep to see the bottom. Part of what inspired the question was the story of the first depth soundings at Challenger deep done in 1875. So I am inquiring about the exact details of how depth sounding was done.

When a weight is suspended from a rope and is hanging either in mid air or mid water the rope is in tension and is tight. In this situation, when the weight is lifted and lowered the rope remains tight at all times. The tension in the rope can be felt at all times. If the rope is coiled onto a winder, as the weight falls the rope unwinds from the coil.

When the weight hits the bottom, the rope will eventually stop unwinding from the coil and become slack. The rope has no tension. To determine the depth to be bottom from the length of rope slowly wind the rope back onto the coil until it first become tense and there is no slack in the rope. The length of rope at that point is the depth of the sea floor.

• i wonder when a rock and a rope was first used to measure waterdepth,was it before or after the development of language in human history. Commented Jun 25, 2023 at 4:38
• @trondhansen "the development of language" was almost certainly not a discrete event, but a long process that extended over hundreds of thousands of years. The development of rope (as something to tie around something else) probably fell somewhere in that long time range. Commented Jun 25, 2023 at 11:15
• Importantly the rope must be neutral or slightly positive buoyancy. If the rope is heavy and long (hundreds of meters), the difference when the weight hits the bottom is harder to detect.
– jpa
Commented Jun 25, 2023 at 11:51

Fred has it wrong for the right reason.

The rope has weight (or if you follow @jpa, negative weight), so you want to detect the difference between (weight of rope) + (weight of sinker) and (weight of rope).

In which case, you want as light a "rope" as possible, and as heavy a sinker. You probably also want some moderately accurate load indicator on your winch - running it over a pulley suspended on a suitably stiff spring would do, or you can get more sophisticated.

@JPA's buoyant rope will eventually suspend the weight of the sinker and leave a pool of rope on the sea's surface.

When the Challenger did the first oceanographic survey, I think they used "piano wire" (single strand, high-tensile steel) because they were routinely sounding in 5km of water.

@RBarryYoung (Beanpole Barry, of "& Dr Toxic" fame ? ) - there is archaeological evidence of twisted fibre ... well back into the Neolithic. Pierced bead decorations imply either a cord for stringing them or thread for sewing the beads onto clothes.

Actually ... there are pierced ostrich shell fragments in the South African deposits ... not Bloombos Cave (of the grooved ochre), but nearby. It was in Nature a few years ago. Sorry, details not in mind, but a hundred thousand years ago if not more.