The last part of the article from Andy about soundings

There are a lot of numbers to display in a small space – even on a paper chart 40” wide.

Conventionally, soundings are displayed...

	without thousands separators or units
	with a single decimal digit displaced ½ a line down instead of with a decimal point (if less than about 30 feet)
	with a bar under the number for drying heights, instead of using a minus sign

Units are implied by the chart – in the UK, non-metric charts were black and white and metric charts are colored. Everything is done to try and prevent someone misreading a dangerous depth for a safe one.

Sounding Units in S-57 (ENC)

ENC is one of several chart products based on the S-57 chart standard. Part of the ENC specification is that all depths and heights be specified in metric units (meters).

But the surveys in the US have been done in English units (feet). When a sounding in feet is converted to meters to be stored in the ENC format and is then converted back to feet for display, rounding errors creep in...

We could argue that 0.1’ here and there does not make the chart useless, but it does make the chart significantly more crowded with all those decimal digits. In Nuno Navigator, depths from US sourced chart data are rounded to whole feet, on the assumption that the survey was in feet in the first place.

Soundings can also be very crowded – clutter on the chart, particularly when the chart is displayed at a less detailed scale than it was intended for. The ECDIS display standard (ISO 61174) also specifies rather large text and symbols – larger than are needed on a modern LCD display. The standard S-57(ENC) presentation draws sounding values using turtle graphic symbols instead of text – symbols cannot benefit from clear-type etc.

Taking all this together the display of soundings changes from

At a smaller scale the difference is even more marked

The Nuno Navigators soundings might be a bit small to read, but the original is too crowded to read and the existence of small text is the clue you need to know that there is something there – all you have to do is zoom in a bit to read it. Everything else about the chart remains intelligible – you can still see the contours clearly for instance.

Highlighting Soundings

The “Properties at Point” display is an important element of an ENC chart. It shows all the detail of the last thing you clicked on and is one of the big advantages of a vector chart.

It is important to know what you actually clicked on.

More from Andy about soundings…

The traditional lead line sounding made point measurements. The cartographer positioned them on the chart and interpolated between them by eye to draw depth contours. Cartographer and navigator alike had to assume that the gaps between the soundings were not a significantly different depth from the soundings themselves.

A worried cartographer could possibly order the surveyor to take more soundings but there would always be gaps and assumptions.

Modern sounding techniques are fundamentally different – side scan sonar imaging is effectively continuous – there are no gaps. No rocks will be missed. This can give the navigator a much higher degree of confidence than from point soundings. The soundings drawn on the chart can be positioned at the shallow bits such as isolated rocks.

So on a chart produced from a modern survey, the soundings are worst case, whereas from a traditional survey they are probably average and the spread of depth may be quite uncertain.

(caveat. Where it was particularly important to know there was a safe depth, traditional soundings were bolstered by using a drag line – if nothing caught on the line, which was set at a fixed depth with floats, then the area was entirely deeper than that.)

The source Data Diagram (SDD) on a chart tells us which type of survey was used:

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…

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.

Modern Soundings

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.