Navigation and charting

Navigation and charting.

For thousands of years navigation was the art of knowing where you are and going where you wanted to go without the help of sightings of land not to mention instruments.
We have difficulty imagining how the early Pacific ocean navigators about 3-4000 years ago on board their basic ocean going vessels, without using instruments, only by observation of the sun and the stars, the sea state and the wave direction and birds in the sky and on the water, could establish their position and travel from island to island to even the farthest away islands such as Pitcairn and Easter Island and the Hawaii islands in the north Pacific Ocean. Astro-navigation in its purest form.

DSC_0002A reconstruction of a Maori sailing canoe in Te Papa museum in Wellington N.Z.

Today satellites circling the earth provide the modern navigator with accurate continuous location, speed and direction information displayed on electronic charts on a screen, and only the nostalgic sailors among us carry a sextant a relic of the later developed tools of navigation. More often than not without the required tables for calculations. Great fun though for the enthusiasts.

A much earlier developed instrument, the magnetic steering compass is still in use on every yacht although for steering by autopilot the GPS reading of the true course is superior to the magnetic course provided by a compass.
It is generally assumed that Marco Polo brought the compass to Venice on his return in 1295 AD from the Middle East or China where it was used since 850 AD as a navigation tool. Some historians believe that the compass came to the Mediterranean with the Norseman in the 11th century. At that time it was no more than a magnetised needle tied on a wooden bloc floating on water. In the 15th century it developed into a pivot mounted needle on a compass rose and was the basic instrument of European maritime exploration in the middle ages.
The first European circumnavigators from Magellan in 1520 followed by French and Dutch explorers and Cook from 1768 till 1779 to sail the Pacific had to draw their own charts often to a surprising degree of accuracy given the lack of precise time measurement.
Now we take for granted charts based on WGS84 satellite positions and surveyed in detail.

The basic set up for electronic navigation on a modern yacht is a chart plotter connected to wind instruments, depth meter, speedometer, radar, AIS  etc or a PC, laptop, iPad or tablet computer loaded with a chart reader, electronic charts and a GPS connection.
It is not only direction, speed and position information directly projected on electronic charts which supports safe navigation but also overlays from instruments such as radar and AIS contribute to safety at sea.

AIS show of anchored and moving coal freighters in the Coral Sea
AIS show of anchored and moving coal freighters in the Coral Sea

AIS stands for Automatic Identification System. AIS systems transmit and receive over VHF ships data such as name, type of vessel, position, course and speed. Ocean going vessels over 300 tons are equipped with this system. The receiver shows the positions of nearby ships on a screen and calculates, taking into account its own position, course and speed, if a collision or close encounter is likely. In which case it shows on the screen which vessel is the dangerous target and it gives an audible alarm. For yachts a lower cost Class B AIS is available with similar features. This system greatly enhances safety and is well worth having on board. Make sure to choose the type that transmits as well as receives so that other vessels become aware of your presence. It is false economy to buy a system that only receives.
In the next few years all fishing vessels will be equipped with AIS but that is currently not yet the case.

Electronic charts.

Most yachts today are equipped with electronic charts with the GPS position and direction of travel of the yacht projected on the plotter screen as well as radar overlay and AIS signals displayed on the chart.
Information such as wind direction and speed, speed over the ground (SOG), depth, course over the ground (COG) and bearing and distance to waypoint can also be displayed on data screens in the cockpit.

But be very aware one needs not only an accurate GPS position but also a chart based on a recent survey with the right chart datum namely WGS84. Newly digitalised charts based on old surveys will not do. We have anchored on the slope of a mountain in the Marquesas, cut corners off the entrance to Tanna in Vanuatu and sailed on the verge of the road along the Baha in Mexico. Not all waters are accurately charted and one needs to be vigilant at all times. Never totally rely on the electronic chart in unknown territory, use all tools available, your eyes for starters.

More and more sailors rely on electronic charts only and saving on piles of charts which need careful storing and cataloguing. Cataloguing can be done on chart catalogues highlighting with a pencil which ones one has. Most of us have only the small scale charts that give a broad overview of the passage and use the electronic charts and guide books to find preferred anchorages in remote places.
A sensible collection of small scale charts is necessary just in case the electronics fail.

Electronic charts are ideal for passage planning when used appropriately. One puts a destination waypoint on the small scale chart which gives a track on the chart. By zooming in to a large scale and following this track by scrolling the chart with the cursor along the track one is able to find details such as reefs or small islands which are not marked on the small scale chart.
Departing from Nuku’alofa in Tonga to Opua in NZ one will find Minerva reef which is accurately charted on a large scale chart of its own but not on the small scale chart. One sailor who did not do his preparation this way sailed onto South Minerva reef busy talking on the HF radio with a friend, stranded on the reef lost his yacht and was very lucky to be rescued by someone anchored inside this beautiful reef which is essentially a convenient and safe anchorage in the middle of a deep ocean but you have to go through a narrow well marked entrance on the electronic chart to reach safety.

chart tableIt must be said that a laptop connected to the chart plotter makes life for the navigator much easier since manipulating the chart with the mouse is quicker and easier than with the controls on a chart plotter.


Weather information up to 5 or 7 days ahead now can be requested as email files over satellite telephone or HF radio and can be overlaid on the electronic chart giving a picture of weather conditions ahead. One can mark points on the track on the chart at distances 12 or 24 hrs of sailing (or motoring) apart. Give these points a time and date and overlay the latest weather grib file. This gives a picture of what to expect wind wise along the route. All based on mileage assumptions, wind strength and fuel availability in light wind conditions. This is not 100% reliable but a great help nevertheless.

Grib overlay 2 days ahead P1010959This weather overlay shows the wind direction and strength for 2 days ahead at waypoint 6



Grib files are computer generated weather forecast information files based on the global GFS model. They contain data on wind direction and strength and surface pressure on a 30 by 30 NM grid for every 3 hours up to 180 hours ahead depending on request. They are produced for free without human interference by the US federal agency NOAA (National Oceanic and Atmospheric Administration) and also in Wwave3 format. For details and differences between the two consult:
These files can be downloaded as email attachment after an email request containing details such as the area of interest and the times and hours ahead.
The request email needs to be in a specific precise format. It is helpful to have an example email in the drafts folder of ones email client and use an adjusted copy of this for a later repeat request. See also communications.
The received grib file can be viewed with a grib viewer such as Viewfax which can be downloaded for free from

useful sites:

Details are available about the practical side of set-up and request email by sending a blind email to:

Internet sites also provide useful graphic information and forecast weather charts.
A internet search on “weather forecast charts” will reveal many government and commercial weather and climate services.
Useful sites:    www.passage    amongst others

The New Zealand weather and climate service produces excellent free weather forecast charts available over the internet. Not to be confused with grib files. You need internet access to download them. They cover SW Pacific, Australia, N and S- Atlantic, Europe even the UK. Make your choice on the websites such as

NZ nov dec 2012 133Updated every 6 hours the forecast files for NZ north-island from Metvuw show isobars, wind direction and strength at 10 m height and rainfall. Fronts are not included but the isobars and rainfall can give an idea about them to the experienced observer.



On the subject of radar and weather one encounters regularly often thundery squalls particularly on he equator and in the trade wind routes.
Radar is a great help to gauge their size and direction of travel sometimes helping to avoid a squall particularly when it is a thundery squall.
Once inside a squall the radar is less useful to get out of it.

P1010973 mast and sail shadowsThe mast, particularly when the scanner is mounted on the mast and the wet sails give blind spots of which one is not always aware.




We prefer radars that have a minimum radius of 1/8th of a mile or 230 meters range rather than a maximum range of 48 miles.
The short range of 1/8th of a mile is much more useful in cramped anchorages.

Another good thing of electronic charting is that one can make and save a track to assist future trips, very useful under less than ideal conditions or on an unreliable digital chart.
Once blown off the anchor in a narrow rocky inlet in Dusky Sound in NZ in a howling squall with a heavy down pour in the middle of the night we managed to escape through the narrow entrance with the help of the radar and the track on the chart made when slowly motoring in during daylight hours.

Lightning strikes.

Be aware that computers are sensitive to the corrosive air, can’t swim and don’t like lightning strikes. One needs at least one spare computer with charts a chart viewer and GPS connection carefully packed away in a dry place. To keep the battery charged take it out of the laptop or charge the spare laptop once a month. Buy a separate GPS dongle for your laptop in case lightning destroys the ships GPS. On the plus side one can use nowadays iPads (at least the models with a sim!)or tablet computers with built in GPS receiver for navigation which can be taken out into the cockpit. In case of lightning one protects these easily in a cage of Faraday such as the (microwave) oven or an sturdy aluminium camera case. One might try a couple of layers of aluminium foil but I advise many rolls to make a thick layer to avoid destruction from a side flash or an electro-magnetic pulse when a lightning strikes even nearby.

Logging and watch keeping.

Although electronic charting programs can keep a log, it is advisable to keep a paper log on board to keep track of progress. Such a log can easily be made on an Excel sheet with the advantage that one can incorporate exactly what one needs.
For coastal passages one would like to log every 2 hours where as for long passages every 4 hours is good enough.
Log keeping on a long voyage every 4 hours is not meant to suggest a watch keeping schedule based on 4 hours. Traditionally sailors staid 4 hours on watch and 4 hours off watch and with a large crew 8 hours off. For a crew of two that would be a dream.  More realistic is a 2 hour watch to remain vigilant and a 2 hour sleep. If repeated from mid afternoon until late next morning one will achieve 8 hours sleep a day and have meals and a happy hour together.

Screen Shot 2015-10-05 at 09.24.56Sometimes suspicious Customs personnel asks to see the log as happened to a fellow cruiser on voyage from Australia to New Zealand.


CTS is course to steer, COG is course over the ground and SOG is speed over the ground. Dist is distance to waypoint. One can insert as many items as required depending on the size of the sheet.

Although the laptop with electronic charts is now top dog on the chart table one cannot do away with all more traditional skills of navigating and dead reckoning. When all electronics fail one has still a position on the paper log and should be able to work from there to the next port of call. One should know about variation and deviation to work out a compass course and one should be able to measure the speed of the yacht. A practical paper chart based navigation course is highly to be recommended.

Recommended websites:

Recommended reading:
Waka Moana voyages of the ancestors by K.R. Howe ISBN 1086953-625-8
Longitude by Dava Sobel ISBN 1-85702-502-4

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