Many paper charts include a Source Data Diagram (SDD). This is small inset displaying the charted area which indicates something about the origins of the information used to compile the chart. There can be some important stuff here.

This SDD shows that some of the soundings come from a lead line survey in 1832. In other words... and think about this carefully ... a hundred and eighty years ago somebody stood on deck with length of hemp rope with knots or marks on it and a heavy weight at the end. From the numbers he shouted out you are going to decide if there is enough water to avoid grounding your boat. To be fair most commonly used waterways are much more recently (and accurately) surveyed than this but even so it can be worth checking. The chart may be completely up to date but the original survey could have been a long time ago.

Of course with your shiny electronic charting system you may think that this sort of consideration is not an issue any more. Sadly this is not true. Most electronic charts are created from paper charts and this will probably be the case for a while. Now clearly the underlying accuracy of the survey data is a concern. The designers of S57 had a think about this and came up with the notion of a ‘Category of zone of confidence in data’. This is chart meta-data - data about the data. Areas are defined and for each area the quality of the underlying survey data, the Zone of Confidence (ZOC) is classified as one of:

This looks quite promising. Instead of telling me something about where the data came from they are going to tell me directly just how accurate it is.

These meta data objects are designed to be used at the compilation scale of the chart however this does not seem quite right to us. The information is really part of an overview of the chart, a summary, so in Nuno we are introducing an overview window. This displays the Zone of Confidence areas and has some other nice uses too.

A second potentially useful bit of information for the overview comes from 'Nautical publication information' objects. This is more meta data which is a reference to a specific paragraph from a nautical publication. Quite usefully this is often a note about the paper chart which was used to create the electronic cell and in particular the source date of the paper chart.

So in Nuno we have put this information together into a nice little inset window which can be easily displayed or dismissed. It gives you a handy overview of the main chart view and its surrounding area. It also supports panning and zooming which can be a neat way to move the view around larger areas. 

You can click on the little button I have circled in red to make the overview disappear. Technically this is called an affordance (just in case you wanted to know).

Sad to say there is small hiccup in this scheme and this is because of another value for Zone of Confidence which does not appear in the table above. The value is U and this means ‘data not assessed’. Which is to say that the creators of the electronic chart cells have chosen not to specify anything about the quality of the chart data. To my mind it is a bit unfortunate that this value even exists however it gets worse because for the most part all the NOAA data is classified as U. A few newer cells use B but most of them are just U.

I was recently at an IHO meeting to discuss S-100 which is the chart standard currently being designed to replace S-57. One topic was a consideration of ways to display the S-100 equivalent of this sort of data quality value. There was, as usual, much discussion on this, but to my amusement nobody pointed out that unless the chart producers actually encode this information then it does not really matter how it should be displayed. S-100 is a long way off but for now, please NOAA, could you start adding more zone of confidence information? It is really quite important information. The Nuno overview is useful in its own right and it displays the date of the source chart for each area. It also displays the data confidence level so if more of this were actually in the cells then we would really have an electronic equivalent of an SDD. Come on NOAA – we are all ready for you.

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: