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Scale and Purpose

Here are some thoughts from Andy about what happens when we change the display scale.


What is the difference between a chart used at 1:1,000,000 and one used at 1:10,000?

The most obvious, for charts of the same size, they cover very different amount of area.

The actual numbers are staggering. Consider a chart drawn at a 10th of the scale – a length on the ground is displayed ten times shorter. This is ten times less detailed on both axes – one hundredth of the area. An area that was represented on a screen of 1 million pixels (a typical laptop) now has to be represented in only 10,000 pixels! That is a serious reduction. It is just 100 pixels by 100 pixels.

Image at 1:10,000clip_image002

Image of the same geographic area at 1:100,000


Incidentally, the range of useful scales for marine charts is about 1:2,000 to 1:20,000,000. That is an area ratio a million times greater than the above example.

The less detailed one has to leave a lot out. Otherwise it would be a sea of overlapping symbols.

How do we choose what to leave out?

The cartographer does this.

Traditionally, paper charts at different scales were quite separate. The cartographer drew each one by hand, more or less independently. Before even getting to the point of creating detailed charts, lots of decisions have already been made about which areas to even bother with. The mid oceans are only charted at 1:3,500,000 or worse because it doesn't help the mariner to have more detail. There is nothing to hit. It is 7, 14 or even 28 days to land. The detailed charts were only created for navigationally interesting places, ports, narrow channels, the coast because we don’t want to hit it.

With electronic vector charts, the attraction from the chart producer’s point of view is to have a single database of chart data and publish it at different scales – thus reducing the number of layers of chart that he / she has to maintain. In principle, you might feel that this approach allows you to produce a chart at any scale. This is re-enforced by the ability in many navigation programs to display the chart at any scale. But I don’t think this really works.

For a start, the detailed information required to make detailed charts still only exists for the interesting areas. Without that information, a detailed scale chart doesn’t give you any more information, it is just bigger. This is exactly the same as with town street maps. Once the scale is detailed enough to include all the roads and their names, you don’t gain anything – that bumper sized street guide just uses more paper to display the same information in a more spaced out way.

Continuing with the road atlas theme, we have different types of map for different purposes – from national freeways, through state and county maps to individual town plans.

Navigation charts are just the same. In ENC vector charts we have these navigational purposes







Their purposes are, hopefully, fairly obvious from their names. They have a very commercial ship feel to them though – were does that beautiful cove with a sandbar across the entrance fit into the scheme? – it needs a detailed chart, but isn’t the sort of place that commercial shipping cares about. Well, in US waters, Approach is actually more or less continuous along the coastal strip. Harbor also covers a lot more than just harbors, covering a substantial fraction of the coastal strip. So really the navigational purposes are more like scale bands.

Paper charts are also purposed, though, as they are selected manually, it wasn’t so explicit – you just pulled the relevant ones out of the chart drawer - a passage chart for the middle and a harbor chart for each end.

Now look at an electronic chart display. How do we choose the right chart, the right navigational purpose now?

Well, it is a trick question, because typically, you don’t; the computer decides. It switches between purposes as you zoom in and out. This explains why the display changes by more than you expect for some zoom steps. It has just changed navigational purpose.

It is obvious that this will not always do what you want. The computer does not fundamentally know what your current navigational aim is – so as you sail along the coast, it will tend to display harbor charts for the ports you are merely passing by.

The alternative would be to choose the navigational purpose manually – so you always get what you ask for (which might not be quite the same as want you want, or even what you need).

Now we can explain why we don’t do that.

Assume you have a button for each navigational purpose. Say you press “Approach”. The computer displays Approach ENC only. If you are zoomed in to a scale of 1:2,000, you will see a small part of a very coarse chart. If you are zoomed out to a scale of 1:1,000,000 you will just see a few small areas of mush – chart that is so scrunched up you can’t read the detail it contains – and most of the screen will be blank because approach data only exists for the coastal strip. At some scale in-between, you will see a useful Approach chart.

Assuming that we don’t think that computers suffer when we make them work harder, the computer can do much better than this – it can automatically display the less good navigational purposes underneath the approach chart. And it could indicate where there are areas of a more detailed navigational purpose available.

In practice, indicating the areas where there is more detail does not help much – it is usually obvious from the chart where the coast is – that is where the more detailed purposes will cover. There is little point drawing a more detailed navigational purpose when the scale is such that it will be too scrunched up to be legible. It is better to display a less detailed navigational purpose that is nearer to the display scale. So the computer ignores ENC data that has a much more detailed compilation scale than the current display.

In contrast, it is always worth displaying the less detailed purposes underneath because they will still give some context however un-detailed they are – after all, blank areas of screen give no information at all.

Given these rules, if you press the button for the most detailed navigational purpose, Berthing, the computer can automatically display the best ENC data that is available for the current display scale.

In Nuno we decided that this was want you would want 90% of the time, so the complication of having all those buttons was not worth it. Pressto – an automatic system that almost always does want you want, and on the occasions when ideally, you would prefer something slightly different, it errs on the side of safety, by displaying a slightly more detailed chart.

Navigational purposes are the main reason why the content of the chart changes at certain scales. As you zoom in to a scale where it is worth displaying a more detailed navigational purpose, the chart is completely replaced by the new navigational purpose; it might be from a different survey; certainly the person who compiled the chart will have included more detail, both in objects such as buoys and in the wigglyness of the coast and depth contours.

For what it is worth, here are the scales of each navigational purpose…

Nav Purpose

Normal scale range



1:1,100,000 to 1:4,860,000



1:500,000 to 1:1,200,000



1:180,000 to 1:600,000



1:80,000 to 1:120,000



1:10,000 to 1:50,000


A few at 1:5,000

Berth (only 2 cells)



Because there is such a wide range of scale within each navigational purpose, it is possible to find an area and scale combination where some of the chart is displayed at one navigational purpose and the rest at the next one. This results in a discontinuity in the chart at the boundary because of the different surveys and detail level in the two purposes.

To summarize this discussion. The software tries to display the best detail available for the current scale. Knowing about the mechanisms involved in the choosing the data to display allows you to understand the discontinuities that sometimes appear and make the best use of the electronic chart.

Nearly a Source Data Diagram

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.

Traditional or simple?

ENC supports two types of symbology for buoys and beacons. These two symbol sets are referred to as ‘traditional’ and ‘simplified’.

Traditional symbols look a lot like those you would find on a paper chart. These are internationally agreed and some of them have been in existence for over a hundred years. So they are pretty familiar. They are used on all paper charts and as a consequence on all electronic raster charts. Excellent description of these symbols here.

Simplified symbols appear on ENC vector charts and were invented along with the ECDIS standard. Technically they are described in the S52 standard which dates from sometime prior to 1996 and is currently at edition 6.0 (March 2010). The DNC vector format also has a set of simplified symbols but these are different again.

In comparison the traditional symbols are more pictorial, more detailed and more descriptive than the simplified set. You might have guessed this from the name. An obvious question is why do these exist at all? The paper chart symbols are time server, proven, reliable and readily recognizable by anyone familiar with a chart. The simplified symbols are rendered (drawn on the screen) using just straight lines. This may have been easier to see on the coarse resolution monitors that were typical of 15 years ago.

Here are the symbols for a Lateral starboard hand conical  buoy with a Quick Green light.


Is one of these clearer than the other?

They both convey the same information so why invent a new standard? Actually this is a bit of a trick question because on a paper chart the buoy would be drawn solid black indicating it was green or black. ENC does not use filled symbols so the term ‘traditional’ needs to treated somewhat liberally. Why no fills? I am not sure but I would guess that the reasoning is that it might obscure something important underneath. So why doesn’t this happen on paper charts? Probably because the cartographer (chart compiler) makes sure that everything is drawn just so. It is hard for a computer to be suitably discriminating.


There are 58 traditional symbols but only 38 simplified ones.

So something has to go.

Here is a Pillar buoy with a ball on top. Long flashing white light. The Ball indicates safe water.

The traditional symbol shows you what the buoy looks like. It is a pillar with a ball on top. The simplified symbol just uses a red circle to indicate safe water. Maybe with an electronic chart you also have good positioning and so you are less concerned with visually identifying the actual buoy and more interested in that you are in safe water? Of course you can always inspect the properties of the buoy to find out what shape it is. Whatever the reasoning is I find it is not clear cut and certainly not an obvious justification for learning an additional set of symbols. You are going to need to know the paper chart symbols even if you prefer simplified on your ECS.

So which symbol set is best and why?

It seems to me that if you choose to use the simplified symbols then there should be a clear cut reason as to why they are better. The traditional symbols are familiar. Paper charts are not about to go away. Using a different symbol set means more learning and more chances to get it wrong. Why are there two symbol sets? Most ECDIS/ECS allow a choice for the display. Surely one of these sets is better than the other and that should be the end of it.

So what do you think? I’d really like to hear some opinion as to which you prefer to use and why. Does one set stand out better than the other? The simplified symbols use blocks of color which do not normally appear on a paper charts. So they certainly look different but is that necessarily an improvement? Were the originators of vector charts just showing off? The data is carefully set up so that information about an object and the way an object is drawn are quite separate. If you were so inclined you could create a whole different set of symbols and render the same chart data quite differently. Maybe this feature was so ‘clever’ that they just could not resist using it for something. What do you reckon?



The Nuno Software License

In a previous blog I was encouraging people to pay attention to what the software license actually meant. I shall now attempt to take my own medicine by describing the Nuno license sufficiently succinctly that you won’t doze off or lose the will to live before you get to the end.

On the way I may even convince you that this is a really good deal with no subtle clauses that are going to disappoint you.

Here is the small print – in headlines.


For an initial outlay of $100 you can have a license for a state of the art navigation system and a one year subscription to full support and update services. After a year you can choose to renew your subscription for a further $50.

If you want to know more; keep reading.

How to buy Nuno

On the Nuno website you create an account and pay for Nuno with your credit card. This gets you a license to use Nuno and a subscription for a year.

How to get Nuno and Install it

You may have already downloaded Nuno to try it out before you bought it. If not you can download it now. This will be the very latest version. As soon as Nuno starts up on your computer it will ask you for your account logon credentials. This is the same email address and password you used to create your account. Nuno will use these to activate over the Internet. Once activated Nuno is fully functional.

You can have Nuno installed and activated on two computers at the same time. This is so that you can have one PC for route planning and another, maybe a laptop, for use at sea.

You are now up and running with the latest version of Nuno and a complete set of up to date charts from NOAA.

During the next year

The subscription is valid for a year and entitles you to the following:

· You will be able to use our chart updating service. This is basically just one click to update all your NOAA charts.

· You will be notified of any updates to Nuno.

· Occasionally we find bugs or problems in the code. More often we want to roll out a bunch of usability and implementation improvements. You will be able to download, install and use the new version of Nuno with these fixes and improvements.

· In the next year we are planning to add several new features including Active Captain Integration, AIS support, S63 (commercial, encrypted ENC), auto-helm and a rolling road (whatever that is). You will be able to download, install and use the new versions of Nuno with these new features.

· If you encounter any problems or have any issues you will be able to contact us directly and probably have your question answered by one of the programming team.

After a year

The subscription expires in a year.

At the end of the year we will invite you to renew your subscription. This will cost just $50. If you renew your subscription then you can carry on with all the good things I have just described for another year.

Expired subscription

You can still use Nuno. It is yours to keep and use whenever you want.

If you ever lose your copy of Nuno you will be able to log into your account and download a fresh copy.

You can still update your charts but not via the update service. You will need to download updates directly from NOAA and then install them manually.

You will not be able to upgrade Nuno. Activation will be frozen at the latest version on the date that your subscription expired.

It is all on the chart but it is too much

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:

NY approach

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.

NY approach 2

Selling software by the pound

You might think, quite reasonably, that since you’ve been to the store, handed over some hard earned money and come home with a nice new plastic wrapped box, that you had just bought some software. It says software on the outside. It contains a disc with the software on it. It is on my desk. It is mine. I bought it.

Nope you are wrong.

What you actually bought is a license. Even though you physically have the disc in your hand what you really have is a license and a copy of the software which you are only allowed to use according to the terms of the license. You did read the small print before parting with those bills I hope.

I guess the nearest thing to actually owning software might be owning the IPR in the source code but even that is often not clear cut. Several parties may have contributed. In fact this is quite commonly the case especially for any larger system. It can get very messy, very expensive and if all you want to do is use the software then it is probably very little use. You really don’t actually want to own the software – you just want to be able to use it whenever you want – kind of like owning it.

Let’s get back to the store. The crux of all of this is not legal ownership so much as allowing you to do what you want to do with the thing you have paid for. So you buy a license which lets you take it home and use it. Feels a bit like I bought it and there doesn’t seem to be much practical difference so is that the end of it?

Well no – I wouldn’t be blogging about it if it were that simple (ok - I might, but in this case I’m not).

Software is very different to most things you buy. By comparison to, say, a puppy dog, it makes less poo (usually), it does not age and it is easy to copy. In some ways it is a bit like music and shares some of the music problems:


If I buy this CD can I use it in all my CD players?

Yes, of course (^_^)


Can I rip it and copy it to all my MP3 devices?

Errr… not so sure (-_-)


Can I copy it to all my friends and use it for my vids?

Try that we’ll be round to shoot the puppy (>_<)


Actually software has even more problems. If your favorite singer hits a bum note then you might not expect to get an upgrade to the album with it fixed (although this can actually happen). However you would, quite righteously, expect buggy software to be fixed (although this often doesn’t happen). Then there is all the business about upgrades and new features. What about support when you can’t work out how to use it? Finally there is the service aspect. Most modern software comes with some sort of internet based functionality whereby information is provided as a service. So how do you make a license for this lot? Well there are many ways and this is where it starts to get difficult – not just for you but for the company making the software as well. I did really mean it earlier about the small print. You haven’t bought the software so what on earth have you bought?

What does a license cover?

Here are some commonly occurring elements in software licenses:

Use of the software.

This is a fairly simple consideration and also the most important so it is what is usually focused on. These conditions will usually apply to a specific version of the software.

· The software can be used for a limited time. Often called a subscription.

· The software can be used indefinitely.


What is going to happen when (not ‘if’) bugs are found in the code? There are several common approaches including:

· Unlucky – learn to live with it.

· A fix will be released for this version. This may only be made available to people who bought it recently or who have paid extra.

· This will be fixed in the next version which also includes new cool features so we are going to charge for it.


It is not guaranteed but most software will continue to develop after the initial release. So what does the license say about upgrades?

· No such thing. Each new release is treated as a brand new software product.

· Ok, there is such a thing as an upgrade so we will charge a reduced price to existing customers.

· It is version specific. So the girlfriend 2.0 license might allow upgrades to girlfriend 2.1 but not to girlfriend 3.0 (with the impressive new features).

· Unlimited. The license allows all upgrades to the software as they become available.


You cannot make the tachyon emitter operate at full power and you have no idea whether to reverse polarize the matter stream. How can you get some help? Just what is actually meant by support covers a wide range of notions. Here are a few:

· None. Just Google it if you have an issue.

· Some information on a website. Bit static but you may find something useful if you are lucky.

· A forum, chat room or notice board. Bit more interactive but can have a poor signal to noise ratio. Useful to chat in a more general sense but less good if you have a specific problem which is stopping you working.

· Support by phone or email for a limited period of time. This is another type of subscription but it is actual real support though – you can ask a direct question and get a response. Might not be a very good one but that’s not the point since we are looking at license models.

· Pay per use support. There are various ways to extract your money such as a premium rate line or a requiring a credit card. This type of support can lead to lovely scenarios where you end up paying a company to tell them that there is a bug in their software.


This is often associated with some of the provisions above so for example software upgrades and support issues may be dealt with through a website that you need to log on to and this is part of the subscription.

Sometimes the service maybe for the provision of data, such as a global database of puppy names and what they mean.

· Always available. If you can run the software you can access the service.

· Part of the subscription. You can access the service while ever the subscription is up to date.

· Pay as you go. You pay for what you use when you use it.


So here is the quick and easy checklist for a software license. Go on… have a look at the small print. This is what really defines what you are buying - not the picture on the box.


How long can you use the software for? Image0009

How many computers can you install it on? 

Are there any other restrictions on use?


What is the provision for patches?

How long are patches going to be available?

How do you report a bug?


What upgrades are likely to be available for this version?

Support and Service

How much can I get before I have to start paying?

Transmittals, exchange sets and loose cells #2

The last post dealt with loose cells. Now we are going to consider Exchange Sets and finally Transmittals.

The Navigation software needs to import cells and updates. The result of this is stored in some kind of SENC. That is a System Electronic Navigation Chart (Google for SENC and you’ll find many definitions other than this one but just bear with me). The thing is that while a single .000 file can be displayed directly the cell update files need to be applied to the corresponding base file first. They also need to be applied in order. The SENC then is the result of installing base files, update files and possibly other files. The SENC is what your navigation system reads to display charts. An Exchange Set is the standards definition of the set of files that can be applied to a SENC.

The ENC product specification states that an exchange set contains a catalog file, one or more data files maybe a readme file and possibly some text and picture files. These last two are referred to as auxiliary files and contain information which is supplementary to the cells. That is, an object in a cell may refer to one or more auxiliary files to provide addition information.

The catalog file contains an entry for every other file in the exchange set. It describes where the file is in the exchange set (different organizations may use different folder structures) and also some checksums to ensure that none of the files are damaged. Bearing in mind the previous discussion about updating cells you might also hope that it contained the edition and update number of the cell files in the exchange set but sadly this is not the case. Some organizations put this information in a comment field but since this is not mandated it is not reliably present. So you can have update logic that uses the edition and update numbers from the catalog but it also needs to be able to fall back to reading this information from the cells themselves – which is perfectly possible but takes longer.

The standard describes how the volume label of the exchange set should be made up. If you are not savvy about volume labels then no matter because nobody really takes much notice of this. In principle an exchange set could be split across several volumes but I don’t think this ever happens. In the volume is a folder called ENC_ROOT which contains the catalog. The cell files may be in this folder or are commonly placed in sub-folders. A directory listing might look like this:


Does that bring back memories of those heady DOS days c:\ > ?

So the basic update algorithm is to locate ENC_ROOT\CATALOG.031, read it and then install all the files it mentions. Overall the mechanism is a bit clunky and can lead to some seriously cryptic error messages. It can be made to work but there are corners which have not been thought out very well. Management of the auxiliary files is one example of this. In fact some of the omissions are possibly evidence that the standard preceded any real implementation.

A Transmittal is a super set of an Exchange Set. Each producer of data tends to have their own take on the standard and often include additional files. This is allowed by the standard. The trouble is that if any software relies on the additional data then it can only be used with data from that supplier. This is a pity because some of the additional information can be quite useful. There is often a complete catalog of what is available (albeit just from that producer) and other information. Check for a folder called \INFO or the such.

The standard actually prohibits the use of compression (S57 Appendix B1 5.2 if you care) which seems a bit unfair. NOAA go ahead and compress it anyhow so when you download charts from http://www.nauticalcharts.noaa.gov/ they will arrive in a zip file. Decompress this and you will find the transmittal contains an exchange set pretty much as above. Each cell has its own folder which contains the base cell and sometimes quite a lot of updates. The readme file contains a catalog which is not in the most readable form but can still be quite useful.


The catalog information in a transmittal can make for a useful geographical display.

Transmittals, exchange sets and loose cells

S-57 data is transmitted from place to place is a variety of formats. This can be a tad confusing. It confuses me at times and it confuses my dog. This article is an attempt to cast some illumination on the subject.

To begin with we need to be fairly precise about terms. Specifically we are not simply dealing with S-57 but the ENC (Electronic Navigational Chart) product specification. S-57 describes a fairly generic mechanism for exchanging geographic data. ENC is a specialization of S-57; a product. This describes a sub-set of all that might be possible with the broader S-57 and ties it down to some specifics. Eventually we get to a list of allowed entities, how they relate to each other and how they get updated.

The world is divided into cells. Cells are regular and rectangular. They are not the same as charts. A chart typically is designed for a particular purpose and is the right shape and coverage for that purpose. Cells on the other hand are designed to give a complete tiling so often several cells are required to give the same coverage as a particular chart. Cells come in six different flavors called Navigational Purpose which roughly correspond to scales.



covers the most area















most detailed sort of cell

I use the word ‘roughly’ because there is no real consistency between producing agencies as to how the differences between Navigational Purpose are managed. In general the more detailed Navigational Purposes use a smaller compilation scale and so correspond to larger scale charts.

For each sort of cell there are two types of file that we are initially interested in: a base cell file and an update file. The base file is the first type of file that is ever issued for a cell and contains the all the basic chart data. Update files are produced later and contain incremental updates. That is they contain only the information needed to update a cell from its current version to the next version. So it is essential that updates are applied in order. This is what the Update Number is for.image

The name of a cell file is made up like this:


| | | |

| | | |----- EEE = update number

| | |-------------- XXXXX = individual cell code

| |------------------- P = navigational purpose

|----------------------- CC = producer code

So, for cell EC09M which is Chesapeake Bay from a compilation scale of 1:200,000 (navigational purpose Coastal) produced by NOAA :-

· US3EC09M. 000 is the base cell.

· US3EC09M. 001 is the first update to the base cell.

· US3EC09M. 002 is the second update

· And so on…

Unfortunately this reasonably simple scheme is not the end of the story. Occasionally the producer of the cell will roll all the updates to date into a re-issued base cell. In this case the cell will be published as a base cell (.000) but the update number will not be zero. So the publication sequence might look like this:

File Name

Update Number

US3EC09M. 000


base cell

US3EC09M. 001


1st update to the base cell

US3EC09M. 002


2nd update to the base cell

US3EC09M. 000


re-issued base cell which include updates #1 and #2

US3EC09M. 003


1st update to the re-issued base cell and also the 3rd update to the original base cell

US3EC09M. 004


2nd update to the re-issued base cell and also the 4th update to the original base cell

So how do you tell the real update number from a .000 cell? Well fortunately it is usually possible to read this information from the cell. So encoding the update number in the file extension is interesting but stops short of actually being useful because you may well have to read the update number from the file anyhow.

We are still not done though.

Occasionally the cell producer will create a whole new edition of the cell. This is comparable to a new edition of a paper chart. In this case a new base cell is created and the update number also gets reset because subsequent updates will apply to the new edition base cell and not the old one. In fact it would be and error to attempt to apply an update for a new edition to a cell of the previous edition. This means we also need to track edition number which is not part of the filename at all but can be read from the cell. Here is the complete publication sequence:

File Name

Edition Number

Update Number

US3EC09M. 000



1st edition base cell

US3EC09M. 001



1st update to the base cell

US3EC09M. 002



2nd update to the base cell

US3EC09M. 000



re-issued base cell which include updates #1 and #2

US3EC09M. 003



1st update to the re-issued base cell and also the 3rd update to the original base cell

US3EC09M. 004



2nd update to the re-issued base cell and also the 4th update to the original base cell

US3EC09M. 000



2nd edition base cell

US3EC09M. 001



1st update to the 2nd edition base cell

US3EC09M. 002



2nd update to the 2nd edition base cell

Confused yet? There is more to come but this is probably the worst of it. To recap:

· A file name ending .000 is a base cell containing a complete set of chart data which can be displayed. We refer to these files as loose cells and it is the way that several agencies distribute their chart data. You need code to actually read the edition number and update number from the cell.

· A file name ending .001 to .999 is an update to a base cell. This does not contain information that can be displayed directly; the update always needs to be applied to a base cell first. The correct base cell will have the same file name but with a .000 extension. Unfortunately several of these may have been issued and the update will only apply to one of them.

In Nuno we display the combined edition and update number as a single decimal number. So for example 2.003 is edition 2 update 3 and 11.589 is edition 11 update 589. This is quite a convenient representation because it allows rapid comparison of both edition and update numbers in an easy and familiar way:- 3.004 is more up to date than 2.845.

Ok that’s loose cells pretty much dealt with but this is not how S57 data is supposed to be distributed. Next time we’ll look at Transmittals and Exchange Sets.


Commercial shipping has a requirement to carry up to date official charts. Traditionally these have been paper charts. Every few weeks a chart agent will visit the vessel bringing new charts and also updating the charts on board. Updating consists of adding pencil annotations, overlaying bits of tracing paper and even pasting on a small, new pieces of chart. This is all tedious and time consuming so it is easy to see the appeal of digital charts and automated updating. However commercial vessels cannot simply switch over the digital charts. The official part of the requirement is important.

After quite a lot of wrangling and discussion between the International Hydrographic Office (IHO), other international ship controlling bodies, various national Hydrographic Offices, chart producers, equipment manufacturers and an assortment of other interested parties some standards emerged. S-57 was designed as a standard for transferring data between hydrographic offices but it got co-opted for sending charts to ships using a product specification called ENC. There are some other S-57 products such as AMLs and AIO but these came later. S-63 was designed as the license enforcement and encryption layer. Unfortunately this was published with errors in it which are still propagating issues today. S-52 was an attempt to decouple the presentation (symbol) for an object from the data representation. This started quite well but then got terribly complicated so that now S-52 is almost like a small programming language in itself. In consideration of the overall system requirements ISO61174 was produced. This describes equipment and software performance standards and is the main standard against which equipment is tested. A navigation system, hardware and software, which complies with all these standards is known as an ECDIS (Electronic Chart Display and Information System). An ECDIS can be used , with some qualifications, on a commercial vessel to satisfy the chart requirement.

Unfortunately the ECDIS standards are quite old. S-57 was frozen in 1996 and ISO61174 dates from 1997. Now old, as such, is not necessarily a bad thing, but these standards prescribe the behavior and performance of computer equipment which had not even been invented at the time. They are standards based on a prediction of what the future would be like. As such they are not a bad guess but are still well short of the mark when it comes to contemporary approaches towards interacting with computers. Nuno Navigator is not an ECDIS. At CherSoft we have developed software for ECDIS. In fact we have quite a long history of this. However in Nuno Navigator we have taken the best features of ECDIS, the bits that actually work, and brought them into a thoroughly modern and genuinely user friendly environment. Nuno Navigator is classified as an ECS (Electronic Charting System) rather than an ECDIS. It is not really intended for commercial vessels.


Free charts make for safer seas

Our coastlines are precious. All the bits of the planet are precious in one way or another but I am somewhat painfully reminded of how fragile the coast is by the Deepwater Horizon incident. Thankfully the worst of the escaping oil is over now but there is still the problem of cleaning up the mess. The cost of this is eye watering and even when the money is spent it will not have really undone all the damage. Evidence from some of the world’s other great oil spills such as the Amoco Cadiz, the Exxon Valdez and the Torrey Canyon can still be found.

It is unlikely that we are going to stop drilling for oil or shipping it around the world in the near future. In fact it seems probable that we are going to see even more of it. Of the major (>700t) oil spills caused by tankers 36% were caused by grounding. Of course oil spills aren’t the only bad effect of a vessel going aground. Loss of cargo, loss of life, loss of the vessel, damage to the environment – the list goes on. It is a serious problem and while there is no simple answer it is probably reasonable to suggest that better navigation would help.

Electronic position fixing and electronic charts are a major step forward in improving navigational confidence and hence safety. This is not really disputed although there is a very valid note of caution with regards to over reliance on computer technology. In general it is such a good idea the 85th session of IMO's Maritime Safety Committee (MSC) approved a proposal to make the carriage of Electronic Chart Display and Information Systems (ECDIS) under SOLAS chapter V Safety of Navigation mandatory from 2012.

So it is probably fair to say that electronic charts make for safer seas. In fact a country can help protect its coast line from some of the massive damage that we’ve been talking about by the simple expedient of publishing free digital charts. The US already does this and is to be applauded for the foresight. Other countries may follow. Typically public funds are used to conduct the surveys and compile the charts in the first place so there is an argument that it is already morally wrong to be selling this data. However in light of the potential costs of not making this vital information as widely available as possible it is an easy step to recognize that a government has a responsibility to publish nautical charts for free.

Here is a list of sources of free ENC vector chart data.

















Czech Republic








South China Sea


USA Coast


USA Inland



There may be others and I’d be very interested to hear about them.



April 2010 the MV Shen Neng ran aground on Douglas Reef – some 15 miles off her planned course. The (fragile) reef was badly damaged and several tons of heavy fuel oil was leaked.