Here are a couple of thoughts:

1. Most National Hydrographic Offices, and hence the government behind them, make some sort of assurance or even guarantee about the quality of their charts.

2. Electronic (ENC) charts are generally considered to be an improvement over paper charts. It is easier to be more accurate in using them and updating them for example.

Putting these two assertions together you might be tempted to think that the governments can now make and even stronger statement as to the reliability of their chart data. However scratch under the surface a little bit and this notion can come unstuck.

There are some fundamental differences between paper and vector charts; here I am just going to focus on display options. Paper charts don’t really have any, vector charts have lots. With a traditional paper chart the choice as to exactly what goes onto the chart and how it is displayed is determined by a cartographer (chart compiler). So when the chart is published it can come with as assurance about the accuracy of the data and (this is the important point so pay attention) exactly what the chart looks like. Two skippers in completely different vessels with totally different equipment will be looking at exactly the same image.

With ENC charts, the data will be just as accurate but control over the image has been diluted. If we were to compare several different types of chart viewer using the same data, showing the same area at the same scale then chances are that they would show a different image. Sure they would all be similar but sometimes the devil really is in the detail. In fact (I can already hear the pedants) if you were to try and match the display settings on each of these units them you would still find that there were some differences between the images.

The starting point of this problem is that there is too much data to display. There are some 180 different classes of symbol which are arranged into groups. 20 of these groups must always be displayed and 90 are optional. Of the optional groups 51 are normally visible, switched on and the remaining 39 are usually switched off. Ok – so if you are worried about switching something off that you should really be displaying then you might consider simply switching everything on. Here is what it looks like:

On the left is the paper chart and on the right that nasty mess is all the ENC data. These two charts are intended to tell you the same thing so clearly this is sub-optimal. There is too much clutter on the vector display – something will have to go. In fact clutter is possibly the biggest problem with ENC data. So, you need to switch some stuff off – but what? Actually doesn’t it strike you as odd that right out of the box this chart is virtually unusable and so things have to be switched off? When are these things ever going to be switched on and why? How are you going to decide what should be on or off? This is the crunch; there is no easy way. I guess you could read a manual, learn about S52 viewing groups, brush up on cartography and give it a stab but that is actually quite a tall order. It is quite a lot to expect of your average mariner before they can use and electronic chart. It is also a moving target. You may well get the display looking just so but as soon as you start changing scales it can all go a bit pear shaped and if you use charts from a different producer then all bets are off. With paper charts you need to learn what they mean, with ENC you need to first decide what they should look like.

In all probability what you are really going to do is to take some reasonable default values. Fiddle with anything that you can understand and stop once you have a half decent display. This is a quite rational approach and this is one reason why all the ENC displays mentioned above are going to look different. There are many other reasons and these can rapidly get very technical but I hope you are getting the gist of this now. So the chart data may all be reliably accurate in line with the assurances of the Hydrographic Office but that is not going to help if the thing you just hit was not being displayed on the chart.

With Nuno we have taken the fairly pragmatic approach that the skipper is more likely to be interested in the chart as a navigation aid rather than a computer game. To this end we have attempted, to the best of our ten years’ experience in messing with ENC data, to make the chart display just work. There is a lot of clever stuff involving dynamic positioning of symbols, subtle re-scaling, jiggling labels around, changing fonts, adjusting for display scale, merging multiple cells and so on. The end result is a half decent chart display with very little messing around. It is all on the chart, well apart from one switch which we have termed anchoring mode. This enables additional data to be displayed in a way that would be appropriate if you were looking for somewhere to tie up. So this switches sounding, bottom type and some other stuff on. This is a feature in the new version of Nuno and it is coming very soon now.

Globally the whole business of updating charts is in a bit of a mess at the moment and probably will be for a while. Most national hydrographic offices are fairly well set up for maintaining their paper portfolios. This is no great surprise as some of them have been in this business for a century or more. However with the advent of electronic charts a lot of things have to change. The whole production process needs to be reorganized (not cheap) and some of the basic ways of thinking about charts have to change.

Raster charts are commonly produced as facsimiles of the paper charts. In a simplistic system they are literally scanned from the paper originals. This can give rise to inaccuracies for a range of reasons not least of which are distortions in the paper and non-linearity in the scanner. Scanning at a resolution appropriate to the electronic chart can cause a rather jagged appearance which is most noticeable in diagonal lines and known as aliasing. The raster chart has far fewer dots (pixels) than the corresponding paper chart and this can compromise appearance. A better production method is to create the raster image from the same electronic chart images used to drive the paper printing process. There are still some issues here with re-projection which need to be handled correctly but in general this is a cleaner and more accurate process. It also allows image processing such as anti-aliasing which gives a better visual impression when the electronic chart is displayed.

In either production technique the updating of the raster charts follows on more or less naturally from the time served paper chart update process. It is then common for the vector charts to be produced from the raster data, more or less indirectly from the paper chart production mechanism. At the end of the update chain are the third party chart producers who copy the official data sets and repackage them in a variety of ways. Quite naturally these are the last types of charts to get updated.

But this is all changing because this is not a good way to handle vector data and vector data is the future. It is a separate issue as to why vector charts are the future or whether it is the right future but for now it definitely is the future. The IHO and other august bodies are committed to this and substantial funds are being invested. This matters because creating vector data from paper charts is just not a good way of doing things.

In the vector world everything is an ‘object’ like a light or a depth contour or a traffic lane. Each object has ‘attributes’ like the color red or a certain depth. Each object also has a position (the ‘vector’) so it is represented as a point, a line or an area. These objects can be quite readily managed in a simple database but the natural way of organizing them is to tie them more directly to the raw survey data. So when a report is received that a new wreck has been found it is just entered into the database – no intervening chart required.

Vector charts are now a cinch to produce. A cell is just a collection of all the objects in a given geographical area. The problem of actually displaying the chart data is palmed off to the ECDIS or ECS. Paper charts and raster charts are a bit more problematical because the traditional role of the cartographer has been taken out of the loop. Have you ever wondered why a modern chart display simply just does not look as good as a paper chart? Well this is why, there is no longer a cartographer involved to lay out the chart and make it look ‘just so’. Instead we have a dumb computer, and they are all dumb, attempting to reproduce the sort of work that takes a human many years of training and experience. It just doesn’t work so well.

Now to be fair the automated vector to raster production processes are getting better but none the less there is still a complete role reversal. The raster chart becomes a second class citizen to the vector chart and the paper chart ends up at the bottom of the pile. Instead of being the primary focus for updates it becomes the last. There are undoubtedly many advantages with vector charts and with paper charts generated from vector databases. However, for the foreseeable future, they are never going to match the visual quality of the charts that have become standard fare for the mariner for many years.

Last in the chain will always be the third party chart producers. Fortunately as their update feeds become predominantly more electronic (just another output from the database) then these updates should become more timely. Ultimately they should be able to match the official charts for accuracy.

Just now we are in a great transition phase. Some hydrographic offices are pushing ahead with vector only systems while others just deal with traditional paper charts. Most are somewhere in between and possibly a bit unsure of which way to go or how it is all going to shake out. Meanwhile the mariners can look forward to improved electronic charts coverage and more rapid updating. They can also anticipate charts which do not look so good and which, in some cases, are going to cost more. Maybe this is just a classic engineering compromise.