Electronic navigation – DS & YM


Before reading this page you will need to have a copy of the training chart (and a real one if possible).

Why is this important?

GPS is currently the most popular, quickest, easiest and most accurate way of finding your position at sea away from land. However, you should never rely on your GPS totally as you never know when the battery will go flat or it will stop working.

Below as some things to consider when using a GPS and some quick handy hints.

Basic terms


Navstar GPS (Global Positioning System) or GPS for short, is the name given to the US Navy’s satellite positioning system which since the mid-1990s has been opened up for civilian use around the world.

In its simplest form, a GPS receiver will give you a position as a Long and Lat. displayed on a screen.

A GPS unit can be a handheld unit, generally with a small screen and powered by small batteries or a ‘fixed’ unit on a vessel generally with a larger screen and powered from the vessels main power supply.

Other GNSS systems

GPS is currently the most popular GNSS (Global Navigation Satellite System) which is the generic term for any satellite navigation systems that provide autonomous positioning with global coverage.

Other systems include:
– Galileo
– Beidou 


A chartplotter is a GPS unit with a built-in chart. It has the advantage of allowing you to see your position instantly as a symbol on the screen. As with a normal GPS, it can be a handheld or fixed unit.

A chart plotter should not be used without a paper chart as a backup as you never know when the batteries might fail.

The picture below shows the Navionics chart plotter that can be purchased for the iPhone or iPad.


One advantage of a chart plotter is that the chart can contain far more than just a digital version of a paper chart. Although it depends on the make and model of the chart by clicking on particular symbols on the chart you may be able to display data such as:
– Tidal Heights (as is in the right-hand picture above)
– Tidal Current
– Contact details (harbours, marinas, restaurants…)
– Photos and 3D views

You can view an online version of the Navionics charts (with reduced features) using Navionics Webapp.


There is often a limit to the amount of space on a GPS display so some common acronyms are often used:
– SOG: Speed Over Ground
– COG: Course Over Ground
– XTE: Cross Track Error (distance away from the straight line course).

How accurate is GPS?

The accuracy of GPS depends on a number of factors however, assuming the unit has a clear view of the sky, has a good signal from a number of satellites and has had time to acquire a position, studies have shown that GPS alone can give you a position with 15m, 95% of the time. For more detail see the GPS Basics website [GPS Basics…]

There are other technologies that can work alongside GPS to make it more accurate but in most cases 15m is very good.

Having said this you will never know when you have a position outside the 15m of the correct position and GPS alone is still beaten in by traditional methods in particular situations.

Out of the sight of land GPS is hard to be beaten. In a pilotage situation when navigating down a narrow channel a carefully choose transit can beat GPS every time.

Types of digital chart

Vector Charts

Vector Charts are made up of a finite number of data points which allow you to zoom in and out while, in theory, only displaying the information that is needed for that particular level of zoom.

For example, when full zoomed out only the basic outline of the coast and major lights are shown. As you zoom in more and more details is displayed which would have otherwise cluttered the chart when fully zoomed out.

Most modern chart plotter do the zooming with ease but you must make sure you have not turned off any important layer of information.


Raster Charts

Raster Charts are rare on leisure chartplotters. They are digital scans of a chart which cannot simply pixel late if you zoom too far. You will require a larger scale chart of the same area if you wish to see more detail.


Geodetic Datum

What is a geodetic datum?

When talking about Datum in terms of electronic navigation don’t confuse it with Chart Datum which is used to determine the depth of water on a chart.

This type of datum determines where the lines of latitudes and longitude are. Traditionally each country or region had its own way of deciding where the lines of latitude and longitude were. They mostly have the same basic concept ie. the Equator or there about is zero latitude and Greenwich, London or there about is zero longitude but when put alongside each other each datum is slightly different. In some cases these differences were significant however in most cases within a few hundred meters.

Why do we need to know about Geodetic Datums?

In the time before accurate electronic navigation these differences did not matter. For example, in the UK the traditional datum was OS36 (Ordinance Survey 1936) and all charts of the UK published in the UK used this datum.

It did not matter because as long as all the landmarks local to your position were in the correct place relative to each out you could use a compass and basic tools to plot your position on a chart. If you were to travel across a sea mass and moved from charts drawn in one datum to charts drawn in a different datum you would never notice the differences using traditional methods as they were relatively small.

In the age of GPS there had to be a standard Geodetic Datum that would work everywhere around the world. That datum is WGS84 (World Geodetic System 1984) and is the default datum for GPS.

Therefore, if your GPS is outputting a position in WGS84 the chart you use must also be in WGS84. In the last few year the charts in the UK have progressively been transferred to WGS84. Admiralty charts drawn in WGS84 are easy to spot as they will have “WGS84 POSITIONS can be plotted directly on to this chart” in large print in the margin (see picture).

If you have a chart without these words in the margin check for the datum in the small print below the title on the chart. If it is not WSG84 it is normally possible to change the settings of your GPS so that it outputs your position in you charts datum.

So what is ETRS 89?

Some charts within Europe use a datum called ETRS 89, for all practical navigation purpose this can be considered the same a WGS84. In fact the 1989 they were the same but as Europe move away from North America at an average rate of 2.5cm a year ETRS 89 is stays static relative to Europe. You might also see that chart updated in Europe (including the UK) at stated in ETRS 89.

Quick plotting methods

There are a number of methods for plotting your position on a chart at speed. This article from Sail Train talks you through the basics [Sail Train…]


Sail Train
Everything you need to know about GPS

Space Jam – alternatives to GPS

Chart plotter – friend or foe?

ESA – Navipedia
Everything you need to know about Satellite Navigation Systems.


Practise this methods in the real world and be ready to use this along side Course to Steer calculations.


The content of these pages is put together in good faith and is constantly evolving. It is possible that errors exist within this content. If you spot an error or would like to add anything to these pages please contact use via email.

Reading the content of these pages is not a substitute for completing a RYA Shorebased course or similar.