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This article first appeared in  Sportsboat & RIB Magazine

So you have the boat, have bought the safety kit, what next? Simple – buy a GPS! Paul Glatzel reports…

Do some basic research amongst friends with boats and you will find the most popular ‘accessory’ is a GPS, probe a little further to explore why they have bought a GPS  and things often start to get a little vague with references to  “ so I can tell how fast I am going”  or “it tells me my position”. Probe further still to see if they understand the data on the screen or how to work out where they are from the figures on the GPS  and with a fair few boaters you may be risking your friendship!

NB: Images for this article to be uploaded shortly

Whilst there are plenty of us with a GPS unit we rarely use even a limited percentage of its capability. Many of us are unsure how to link what we see on the screen to the charts we have or we are unsure what XTE stands for and whether what it says matters anyway.  Certainly, when I first used a GPS XTE, BTW and TTG meant nothing to me. Over the next page or two we’ll look at what a GPS can do and what the various abbreviations mean.  So what is a GPS?

GPS (or ‘Global Positioning System’) was created by the US Government for military use. A GPS receiver computes its position relative to a number of satellites orbiting the earth and represents its position on the earth’s surface as a “Lat & Long”. Gone though are the days when the sole use of GPS was to direct cruise missles through windows on behalf of the US Government, GPS is now endemic and is found in taxis, delivery vans,  boats and all manner of other day to day situations.

What is “Lat & Long”?

 Early map makers needed to create a way to define a position on the earth’s surface. By drawing lines between  the poles and horizontally parallel to the equator the earth became covered in a grid. The lines between the poles are known as Meridians of Longitude and it is well known that the 0° line (the ‘Prime Meridian’) runs through Greenwich in London. Every position on the earth’s surface is either on this line or East or West of it. The horizontal lines around the earth are referred to as Parallels of Latitude, these lines are measured as an angle relative to the equator positions are either on the Equator or North or South of it

So any position on the earth’s surface sits on a line of latitude and a line of longitude – giving an exact way to define a position.

 In the above example the boat’s position on the earth’s surface would be written as:

 30º 00’.0N, 030º 00’.0W

Rarely though does a craft sit perfectly on such a precise position. For example CSL’s offices are at 53º 42’.5N, 001º 02’.5W. which if you were relaying verbally would be “Five three degrees, four two decimal five minutes north,  Zero zero one degrees zero two decimal five minutes west”.

 Note: Each degree can be divided into fractions of a degree. Like hours, a degree consists of 60 minutes and each minute consists of 60 seconds.  Seconds are usually expressed as decimals now. (eg 42’.5 rather than 42’ 30’’)


A GPS is useful for: 

bulletDisplaying the position (lat & long) of the craft
bulletStoring waypoints that can be linked together to create a route
bulletShowing speed and the ground track.

There are two types of GPS receiver typically bought by boaters. At a simple level basic GPS units have small screens, cost £100 – 200, and only show the lat and long plus various other items of information. The next level up (and an increasingly common option that is coming down in price almost daily) are chartplotters.  A chartplotter is a ‘souped up’ GPS that contains electronic versions of the paper charts we use and overlays the position of the craft on those charts. One of the advantages of chartplotters is that whereas with  basic GPS units you need you to key in manually the lat & longs of waypoints a chartplotter allows you to position the  cursor on the chart and press a button to create the waypoint. This can be quicker, simpler and less prone to input error. One of the downsides of a chartplotter though is that it can be difficult to get the ‘overview’ that a paper chart gives. Zoom in and you focus on a small area and cannot see much beyond the immediate vicinity, zoom out and the detail of the chart is less visible – have a chart AND a chartplotter.

Key points to remember about GPS: 

bullet Ensure the GPS receiver is set to the correct datum – WGS 84 (this is then the same as newer charts meaning satellite derived positions can be plotted directly onto a chart.) – if you don’t there is an error between the chart and the GPS.
bullet The compass will only work when the set is moving – unless the set has an inbuilt fluxgate compass.
bullet Do not rely exclusively on GPS as its accuracy and existence relies on the US Government. They may need to switch it off or create a random accuracy error (called ‘Selective Availability’) from time to time.


Using your GPS to navigate between two points:   

bulletInitially choose and input your waypoints
bulletYou can then link the waypoints together to form a route. Care must be taken not to miss out any waypoints from your route.


GPS terminology

 Cross track error (‘XTE’): The distance the craft is off to one side of the line between two waypoints

Speed over ground (‘SOG’): The speed the craft is making relative to the ground  – rather than through the water. The difference is the effect of the tide. (eg speed of craft through water – 3 knots, tide in the same direction – 2 knots, SOG – 5 knots. If Tide is in opposite direction to direction of travel SOG would be 1 knot.)

Course over ground (‘COG’): The course that the craft is making relative to the ground, which may be different to the direction the craft is actually heading due to the effect of the tide.

Bearing to waypoint (‘BTW’): The bearing from the craft to the target waypoint.

Distance to waypoint (‘DTW’): The distance from the craft to the next waypoint.

Time to go (‘TTG’): The estimated time it will take the craft to reach the next waypoint.

GPS compass: There are two types of compasses that could be built into a GPS unit. Typically (and almost always found on older and cheaper models) is the standard compass that displays compass headings calculated by reference to the movement of the craft (they do not work if the craft is not moving), newer (and often more expensive) GPS units sometimes have an inbuilt electronic called a fluxgate compass (which is accurate irrespective of movement)

Selective availability: The random accuracy error induced by the US Government. This is presently switched off.


When you activate this route most GPS sets will show you a ‘rolling road’ with the next waypoint at the end of it. The trick is to keep your craft to the centre of the road The distance off to either side of that centre line is your ‘cross track error’ (XTE)

The problem with straying off the intended track is that danger may lurk if the XTE falls outside the acceptable limit. You will need to decide an acceptable XTE for your route.


Most GPS sets allow you to configure the screen to show any combination of the fields. Making the important data larger can make it far easier to use in rougher conditions or bright sunlight.  

So how do you plot the ‘lat and long’ that you see on the screen onto a chart to determine your position?

 Using dividers or a ruler you will need to mark off the ‘lat & long’ on the chart. The figure 34° 53’.866N is the latitude and is to be plotted on the vertical scale. Plotting .866 is impractical so round the number to something you can plot on your chart.


NB: The Lat & Long on the simulated GPS screen is not the same Lat & Long that is being plotted on the ‘chart’ above.

 Do the same for the longitude and at the intersection is where you are. 

GPS has many uses and given that sets are so cheap now they are an essential element of any boater’s toolkit. To get the most from your GPS read the manual, consider spending time with an Instructor to develop your skills and invest time fiddling with it and practicing plotting positions.


More information about using a GPSs and Chartplotters can be found in the new RYA Powerboat Handbook written by RYA Powerboat Trainer Paul Glatzel.

The book has been produced to support all of the courses in the National Powerboat Scheme and is the book recommended by the RYA for those keen on developing their powerboating skills.

The book retails at £12.99 and is available from the RYA website at www.rya.org.uk or via 0845 345 0400

Paul Glatzel runs a powerboat school in Poole Harbour – Powerboat Training UK.


Author: Paul Glatzel
Contact: www.powerboattraininguk.co.uk

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