Each vessel fitted with an AIS transponder transmits its identity, position, course, speed and other information over a VHF channel. This is compulsory for vessels over 300 tons and all passenger ships. Quite a lot of other vessels carry them as well.

So if your boat has an AIS receiver on board you can track these vessels if they are in range, typically 20 miles or so. If the receiver has a display at all it is usually a bit rude and crude – the real trick is to feed the AIS information into your chart display system. Now you can see the AIS targets overlaid onto your navigational chart and this starts to get useful.

AIS was intended and designed as a collision avoidance mechanism however you really need a graphical chart based display to properly realize this. Nuno™ now supports AIS (hooray!) and makes a great way to display the information.

Even on a smaller boat there is a lot to be said for getting an AIS receiver. They are pretty cheap these days and benefit in terms of situational awareness is brilliant.

Configuring AIS in Nuno™ is pretty easy. You need an AIS unit with a NMEA output and then you just plug it in. That’s it. Depending on your set up the AIS NMEA stream may be multiplexed onto your GPS feed or it may be a separate connection. Either way just plug it in and Nuno™ will find it.

AIS targets are displayed as little green symbols with a vector arrow representing speed and direction. Ticks on the arrow are at one minute intervals. Hover the cursor over the target to see additional information such as name, destination and cargo. The scope of the information being broadcast can vary quite a lot.

Here is a really handy technique: Create a pencil line and drop one end onto the marker for your boat and the other onto an AIS target. The line will display range and bearing to the AIS target. Even better the line will ‘snap’ to your vessel and the AIS target and update as they move. Very useful for keeping an eye on another ship.

Nuno™ with AIS capability is being released soon. Very soon. Maybe even today. This is available as a free upgrade to all users with a current license. There are lots of other improvements in this release and some great new features just around the corner.

How do you get it? Easy. Run Nuno™ up on a PC with an internet connection. If an upgrade is available it will tell you in the Support Centre pane and there will be a download link for you to follow.

Charts are square, countries aren’t, it’s a problem. There may be a vision of a utopian world where country boundaries are straight and align with a geodetic grid but the reality, as usual, is much messier. The tricky bit is that each national hydrographic office will create ENC cells with bits missing. The political ramifications of drawing a map of someone else's country can be severe. Politicians just don’t like this sort of thing and Google have managed to upset people to a remarkable degree by misplacing the odd line. So the only sensible thing to do is to create chart up to the national boundary and then stop. Of course by safe I mean politically safe. For the mariner it is a pain in the transom akin to sailing off the edge of the world.

This means we end up with multiple ENC cells of the same area at the same compilation scale but from different providers. The reason you might care about this is that you could easily end up with duplicate or similar features from both cells overlaid on each other. Depending on how your chart display software renders this it will probably look a mess, could easily be confusing and might even be dangerous.

Fortunately the good people at the International Hydrographic Office have already thought of a solution to this. Unfortunately it requires a degree of international cooperation and we (I am speaking for the whole human race now) are not very good at that. None the less several countries have been giving it a go following the principles of the Worldwide Electronic Navigational Database or WEND. This is a great idea and would be even better if it worked. I am not saying it won’t work but there is a way to go yet. One small step in the right direction has just been announced by NOAA and the Canadian Hydrographic Service. They have conceded that the boundary between the US and Canada is not straight so they have agreed on using a wrinkly one instead.

ENC cells are often cataloged and managed by the coordinates of the cell corners. This is fine most of the time and implies that the cell is square. However within the cell is a coverage object which really defines the shape of the area covered by the cell data. This can be a polygon of as much complexity as needed. So if two countries cooperate they can arrange the coverage boundaries along the international border, each country then charts its own bit and everyone is happy. The UK and France are a good example of where this works well. What is happening up on the Canadian border seems to be a little different though. They are still dividing up who does what but, see the above diagram, they are not following the border. Instead they are making sure the the cells will fit together properly but Canada is charting some of the US and visa versa. Why are they doing it this way? I have no idea. Please tell me if you can shed any light on this. Pragmatically the reason does not matter too much. The end effect will (should) be a set of ENC cells which join up nicely.

Nuno™ Navigator is named after the 15^th century Portuguese explorer Nuno Tristão. This Nuno was one of Prince Henry’s most valued and trusted captains. And this Henry was none other than Henry the Navigator, arguably one of the fathers of modern navigation.

Nuno is notable for his explorations of the West African coast which helped established Portugal as a leader in the emerging colonial world. Previously no one had attempted to venture out to this region which had been dubbed the Ocean of Darkness. Many terrifying myths surrounded it with claims that monsters swam in the seas and the overhead sun made the seawater boil. If a ship was lucky enough to survive these hazards it was only a matter of time before they would reach the edge of the flat world and plummet into the abyss. Nuno was only marginally perturbed by this but did realize he was going to need a good ship.

Typically the vessels used for long voyages were slow, heavy and not very maneuverable. This was no good. Nuno wanted an agile boat that could sail up rivers. Fortunately Henry stepped in with his newly designed Caravel sporting a dashing lanteen rig. Later this type of ship would be used to discover America and become the exploration vessel of choice but first of all Nuno took a prototype to Africa.

His first trip was to Capo Branco. The furthest any European had been beyond Rio de Ouro and a significant step across the Ocean of Darkness. Now I should mention at this point that although Nuno was heroic in many ways that one of the reasons he was doing this at all was to catch slaves. This sort of thing was quite acceptable back then (or at least it was mostly acceptable by most people except the slaves) so we will try to not let it distract us from the good bits which are about navigation and exploration.

Nuno did three subsequent expeditions each time pushing further south and pushing the limits of exploration and navigation. Beyond Capo Blanco was the Bay of Arguin. On his third expedition he reached the border of Senegal and announced that he had discovered sub-Saharan Africa. With his fourth and final expedition in 1446 he got south of Cap Vert and may have discovered Guinea-Bissau. There is some debate about this. However, wherever it was that he actually rolled up, there were also a lot of poison arrow wielding locals. They justifiably took exception to the slave thing and promptly killed Nuno and his crew.

So bold, innovative, adventurous, pushing the limits of technology and a great navigator. We like all these things. Many years ago we named our professional navigation system after Henry the Navigator so it seemed appropriate that our next generation system should be called Nuno.

The Wikipedia article on Nuno is a bit dry but informative.

These days Nuno has a couple of statues to his credit. A naval frigate was named after him and he has appeared on Portuguese coins and bank notes. My favorite is this article explaining how an abandoned statue of Nuno was found after the war in Guinea-Bissau and moved into the city. I will visit this one day and in the meantime can anyone send me a photo of it that I could post here?

So who does use a compass? Serious question. A compass is not the vital piece of kit it used to be. I am not suggesting that you should put to sea without one but it is quite possible to sail round all day without actually using it. I often use the compass just to hold a course. Without using any calculations or taking a bearing off the chart, I find it useful to keep me heading in the same direction. I mentioned this to my colleague Andy and he promptly pointed out that I was not holding a course at all but just a heading. Depending on stuff like wind and currents my actual course could be wandering all over the place. The compass just keeps the boat pointing in the right direction but not necessarily going in the right direction. None the less it still feels quite satisfying to hold the compass steady.

Another thing I find I use the compass for is picking up landmarks. No great accuracy required. Just measure the bearing to the tower or whatever and then mark it off on the chart to identify what you are actually looking at. Of course it is possible to be much more accurate with a bit of effort. We have just been updating the magnetic variation tables in Nuno™. The raw data comes from NOAA as a series of tabulated values which refer to the World Magnetic Model (WMM).

The World Magnetic Model is the standard model used by the U.S. Department of Defense, the U.K. Ministry of Defence, the North Atlantic Treaty Organization (NATO) and the International Hydrographic Organization (IHO), for navigation, attitude and heading referencing systems using the geomagnetic field.

Pretty good credentials eh? In Nuno™ we have code which can interpolate the model and so calculate the magnetic and secular variation for any point on the planet at a given time. Secular variation is the rate of change of magnetic variation. Variation is also sometimes called declination. The current model is valid until the end of 2014 by which time another set of data will be published. At school I was taught that the earth’s magnetic field is as if there were a bar magnet on the spin axis. The reality is a little more complicated, the Earth's main magnetic field is generated in the conducting, fluid, outer core, but it is still not a bad analogy. There is some more interesting detail here on the NOAA website. The field is not completely aligned with the earth’s spin axis. Currently the magnetic north pole is at 80.02° N, 72.21° W. The field is not completely even as you can see from this picture.

Do we need magnetic navigation when Global Position System (GPS) is available? GPS provides precise point location but only measures travel direction when in constant motion. A GPS receiver must collect several sets of latitude and longitude pairs to obtain direction. In addition, GPS signals may become blocked due to obstructions, adverse terrestrial and space weather, ionospheric conditions or being underwater. Hence, compasses complement GPS receivers to attain precise and immediate navigational headings for air, ground, and water-based systems. Electronic compasses and the WMM commonly co-exist in GPS receivers.

If you have fixed ships compass, say in a binnacle, then to be as accurate as possible you need to account for compass deviation. This is the effect of the boat itself on the compass. Anything close to the compass which can affect the earth’s magnetic field will cause deviation. Lord Kelvin patented a system around 1880, Kelvin’s Balls, which involved two spheres mounted on the binnacle. These were common place for a long while and can still be seen on older vessels. The effect of deviation is directional so if you point the vessel in different directions and measure the deviation it is possible to construct a correction table. You need to get organized a little bit to swing the ship like this but it is possible. If you find the right place you can obtain accurate bearings by sighting on known objects and so eventually you construct a deviation table. In our Henry product you can enter the deviation table – it looks like this:

So now it is possible for the system to quite accurately use true bearings (T), magnetic bearings (M) and compass bearings (C). A true bearing is the angle from true North, a magnetic bearing is a true bearing corrected for magnetic variation and a compass bearing is a magnetic bearing corrected for the compass on a particular vessel. The question is :- does anybody, would anybody, use this? We could quite easily add this feature into Nuno™ but, realistically, is trying to use a magnetic compass accurately a bit out-dated these days?

One little lurker that I should mention in closing is electrical equipment. When Lord Kelvin was adjusting his balls this was not much of a concern. Once set the deviation would stay fairly predictable. However anything electrical can generate a magnetic field and these can play havoc with a magnetic compass. A good experiment to try is to see what happens when you switch things on and off. Get your boat still and steady and then try messing with any bits of electronics near the compass. While you are at it have a look at what your mobile phone can do to the indicated direction. If you are lucky then very little will happen, but this is not always the case.

Who is for a deviation table in Nuno™?

On a paper chart there is not really enough space to describe colors and flashing sequences of a navigational light so a code was invented. ‘F’ is a fixed light, ‘Fl’ is a flashing light (which means it is off more than it is on) and ‘Oc’ is an occulting light. Interestingly an occulting light does not lean towards the dark side. It is on more than it is off. In my dictionary ‘occulting’ also means supernatural, not capable of being understood by ordinary human beings and hidden. Here is the complete list of light types:

• Oc Occulting
• Iso Isophase
• Fl Flashing
• LFl Long Flashing
• Q Quick flashing
• IQ Interrupted Quick flashing
• VQ Very Quick flashing
• IVQ Interrupted Very Quick flashing
• UQ Ultra Quick flashing
• IUQ Interrupted Ultra Quick flashing
• Mo Morse Code
• FFl Fixed And Flashing
• FLFl Fixed And Long Flashing
• FOc Fixed And Occulting
• Lit Unknown details

Which seems like quite a lot to me. Then there are codes for colors. ‘W’ is white. ‘G’ is green and so on. Finally there are indicators of period (time), height and range.

Here we have some raster and vector renderings of Point Hudson.

The last one is from Nuno (we think it looks best). As you can see there is some confusion over height units but they are all basically agreed that we have a flashing red light with a period of 2.5 seconds. In Nuno we have also gone a step further and illustrated the flashing sequence with a diagram.

This is a hover tip, a little floating window that appears when you hover the cursor in one position for a few seconds. We use a lot of these in Nuno.

For the most part reading the light characteristic is pretty straightforward but there are some rules about the characteristic string than can get a bit arcane. Such as: If a Characteristics field consists of Occulting and Flashing Light Characters that have a single period and are not Alternating and have no Grouping then these are merged into a single Light Character.

For example: Fl G Oc R 5s

The use of Green and Red in combination is quite common despite the fact that red/green color blindness occurs in around 8% of males (women don’t often have this problem). With something like a traffic light it is possible to distinguish the type of light from its position but this is not so easy for marine navigation lights.

There are three types of range that get discussed with regards to lights:

The geographic range is all to do with geometry. It assumes perfect visibility and so is really to do with the curvature of the earth. Here is a typical range table:

So from the bridge of a modest size vessel you should be able to see the Point Hudson light from over ten nautical miles away. Note however that the height is referenced to chart datum so in some circumstances the tide will affect visibility. In fact at low tide you will be able to see it from further away because it will effectively be higher above you.

Luminous range is to do with the power of the light and how well it penetrates the atmosphere. So assuming that the murk in the air is the the only thing blocking the light then this is a measure of how far away the light can be seen from. Obviously the brighter the light then the further away it can be seen. But what about color? Do some colors penetrate foggy conditions better than other? Anyone know?

The Nominal Range is what is written on the chart. Technically the nominal range is the luminous range when the meteorological visibility is 10 miles. Which of course begs the question as to what meteorological visibility is. Well one definition is that it is the greatest distance at which lights of 1,000 candelas can be seen and identified against an unlit background (does that help?).

 

So if it is a bit murky, meteorological visibility down to 5 miles, then the Point Hudson light will only be visible from less than four miles away.

Prolific marine blogger ‘Panbo’ reported that some of our competitors were doing interesting things with iPads and Windows based navigation systems. Andy Nibbs loves his iPad so he went off to investigate…

The iPad is ideal for an extra view on a Nav system. It’s a general purpose device when away from the boat which eases the sting on your wallet. It has a pretty good bright display and you can made it more rugged with off the shelf stuff. So you can have it in an exposed position next to the wheel while the laptop is in the dry below.

There’s a lot to be said for using an iPad in this way. The iPad is battery powered and connects wirelessly so it can work as an extra display on deck whilst your laptop is kept below. Note that you need a WiFi connection between the iPad and the Windows computer and a suitable app to configure your iPad as an additional monitor. Waterproof cases start at around $20.

The navigation software doesn’t need to do anything special – just work properly with multiple monitors. The iPad looks to your PC like an extra monitor.

Nuno’s route monitor view is just right for putting on an extra display.

Apps

I’ve tried iDisplay and Maxi Vista (both about ten dollars) and they both seem to basically work. Refresh rates aren’t the same as a real monitor.

iDisplay allows you to do some mouse work on the iPad display which is useful but I recommend using the iPad mainly as a display only view whilst a passage is underway. Your laptop’s display is probably better for passage planning.

Maxi Vista seemed to cope better with me checking my email on the iPad and then going back to it but it didn’t allow me to do any mouse work using the pad.

For both apps, you buy the app for the pad and it tells you what to do on the PC – which is to install some software to handle the PC’s side of the connection. Once that’s done, you run the app on the iPad and the app of the PC either detects the iPad and everything springs into life or you have to do something manual on the PC to start it up.

The iPad and PC need to be on the same network. That can be a peer to peer network with just a laptop and the iPad.

I found both the applications I tried suffered from a few problems. I’ve got a few reservations about whether the software is robust enough for use at sea just yet. If you already own an iPad then the cost of trying this is pretty low but don’t buy one especially and get disappointed. Hopefully the software will get better quickly – after all this has wide applicability away from boating.

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.

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.

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.

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.

1	Overview	covers the most area
2	General
3	Coastal
4	Approach
5	Harbour
6	Berthing	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.

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

CCPXXXXX.EEE

| | | |

| | | |----- 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	0	base cell
US3EC09M. 001	1	1st update to the base cell
US3EC09M. 002	2	2nd update to the base cell
US3EC09M. 000	2	re-issued base cell which include updates #1 and #2
US3EC09M. 003	3	1st update to the re-issued base cell and also the 3rd update to the original base cell
US3EC09M. 004	4	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	1	0	1st edition base cell
US3EC09M. 001	1	1	1st update to the base cell
US3EC09M. 002	1	2	2nd update to the base cell
US3EC09M. 000	1	2	re-issued base cell which include updates #1 and #2
US3EC09M. 003	1	3	1st update to the re-issued base cell and also the 3rd update to the original base cell
US3EC09M. 004	1	4	2nd update to the re-issued base cell and also the 4th update to the original base cell
US3EC09M. 000	2	0	2nd edition base cell
US3EC09M. 001	2	1	1st update to the 2nd edition base cell
US3EC09M. 002	2	2	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.