Today’s blog is the first of a short series from Andy Watkin. When not writing code Andy is often to be found poring over charts. Lately he has been thinking about soundings…
In the beginning, someone stuck an oar over the side of their boat and found that the water was deep enough at that single point for their rowing boat. Now we can measure the depth of a whole area simply by flying a small (suitably equipped) airplane over it and display all these depths on a screen.
As part of the development of the chart display for Nuno™ Navigator, I looked at the way soundings have evolved.
A single line of soundings indicates measurements made during a voyage – rather than a dedicated survey where the ship passed back and forth over the area. This was common in Captain Cook’s time and is still obvious on some current charts.
As each sounding was measured manually with a lead and line there are only a few and they fit easily onto the chart. A lead was a substantial weight: www.nmm.ac.uk. Great skill was required to throw it forward so that by the time it hit the bottom the line was vertical (thus allowing for the forward movement of the ship). At the depths above, they probably had to heave to each time.
The depths were marked on the line using bits of leather, bunting, etc, as shown in www.navyandmarine.org. Of course, on old sailing ships, the line had to be readable in the dark as well as in daylight – so each of the mark had a unique feel. Taking soundings from the deck of a heaving ship in the dark in a gale (with little idea where you really were) must have been a terrible job.
There is a whole language around the use of the lead and line, complicated by not marking all the depths.
· “By the mark ten” is 10 fathoms (60 feet).
· “By the deep eleven” is one fathom deeper than the 10 fathom mark. A fathom is of course approximately the distance between outstretched finger tips, which was presumably used to estimate the intervening lengths.
The lead line was not just used for measuring the depth. When the only positioning devices were a sextant and a sharp pair of eyes, finding out what the sea bed was made of could be a vital clue to your position. A hollow in the end of the lead was filled with tallow (which is sticky) and would bring up a bottom sample if you were lucky. This is why charts are marked with “mud”, “shingle”, “sand” etc.
Rather than measuring one depth at a time with a piece of string, a ship can use a side scan sonar to build an image of the sea bed for a significant width either side of the ship, all the while steaming along. The sensor can be built into the hull of the ship or towed in a ‘fish’ – a floating torpedo shaped housing. A fish can be controlled (it has underwater wings) to stay at a constant depth providing the ship’s speed is maintained constant. An advantage of using a fish is that, being entirely under water, it is not subject the pitch, roll and yaw experienced by a ship.
The sonar images can be remarkably detailed. www.abc.se contains some good examples.
The most amazing technique though is LiDAR - the word is a composite of light and radar. Two lasers are shone downwards and swept from side to side from an aircraft. One is a frequency (color) designed to be reflected from the sea surface. The other is a frequency designed to penetrate the sea and be reflected by the sea bed. Comparing the two, with a lot of computer processing allows the depth over the entire width of the scan to be determined – and the aircraft is moving forward at a rate so huge areas can be covered by repeating the process quickly enough. The depth that can be measured is limited by the sea opacity and does best in shallow areas; which is ok as that is where we generally care about most.