When we first started short coastal cruising we used traditional chart work and position fixing methods, dead reckoning, compass bearings and cocked hats were practised.
At night school, on an Ocean Yachtmaster course, I learnt the art and science of Astro navigation and bought a plastic Ebco sextant to hone my skills. We used to holiday in the 80’s at the Ferry Boat Inn and early in the morning I would take the Mirror dinghy to the mouth of the Helford river where I could see the Sun and horizon. On my return I would inform family and friends that I had been able to determine my position line correct to 2-5 miles. This was normally greeted with hoots of derision and comments such as “we haven’t had to move from here, but know exactly where we are to a few yards, just ask us next time you want to know”. I still keep Annual Ephemeral tables and occasionally practice position fixing on paper, although pocket calculators and computer have simplified the calculations. A few years ago I replaced the plastic sextant with a very well made 2nd hand Russian model, bought for a few pounds in Singapore.
In the early 90’s I bought my first GPS. It was expensive, ( about $1000), heavy, ate batteries and would give lat/long accurate to about 50-100metres. It would store 20 waypoints and show bearing / distance to WP from current position. It was a marvellous tool.
Pre car satnav I bought some software to run on my laptop and linked this to the GPS. I used this to help me drive all round the UK and Europe in spite of the inaccuracies in the position and the maps. The software must have known I was using a nautical GPS because driving up the A38 it would normally show me in the Trent and Mersey Canal.
How things have changed. A few months ago I bought a fancy smart phone. As well as being able to browse the web for weather forecasts it has an inbuilt GPS, accurate to about 2 metres (it shows which side of the road your driving), which gives me street by street navigation anywhere in the world for free. We used it recently on a touring holiday in South Africa. OS maps can be downloaded ( again free) and the GPS will locate exactly your position on the map, able to follow footpaths. The phone does everything, except I find it difficult to make some phone calls.
The Raymarine chartplotter on “Sarah Jane” is equally as accurate, showing our position on the pontoon at Mylor. Linked to the autohelm it follows a track far better than we can. It gives position far more accurately than any traditionalist could have dreamed of.
Without a doubt GPS has in recent years transformed and simplified all types of navigation, at sea, in the car and on foot.
When GPS is working properly it is a fantastic tool for the sailor. That is the problem – WHEN IT IS WORKING PROPERLY.
All GPS users will know the symptoms, position indicator jumping around, unable to show a position, or wildly inaccurate position. We were once shown to be miles away from our known position and at 32000ft, not on the sea ( we later found out that the USA had invaded Panama).
Twice we have lost all satellite for some time. The first time was mid channel when both the Raymarine chartplotter and below decks Furuno lost the fix at the same time. No satellites were visible on either unit. By “coincidence a Nimrod was circling overhead during the time the signal was lost. The second time was a few miles from Plymouth, both onboard GPS showed no satellites. A third hand held GPS indicated no satellites. Again, by coincidence several naval warships were entering and leaving Plymouth at the time. In our case the most common cause of GPS not working is lack of battery voltage, after a few hours sailing using the autohelm the house battery voltage would drop too low and the GPS would switch off. This partly solved by fitting a wind generator and new battery.
To reduce the difficulties arising from the loss of GPS we routinely plot our position every ½ hour or hour on a traditional chart. As well as a the security this also gives a much better overview of progress and the effects of tide and wind.
Of greater concern inshore is the the possibility of inaccuracies in the electronic chart, omissions or out of position hazards or marks. Because of this when entering or leaving an unfamiliar harbour we rely chiefly on traditional chart pilotage, leading lights or lines or buoyed channel. The final GPS waypoint is one that brings us to the point where traditional pilotage takes over, but even then there are circumstances when its useful to be able to quickly plot an accurate GPS fix on the chart to verify you are where you think.
Make certain the chart GPS datum is the one programmed into the instrument. The majority of charts use WGS84, but a few use OSGB. Be careful when using a handheld GPS that the datum has not been set to OSGB36 for walking.
The GPS will give an accurate lat/ long position and it is possible to plot this directly on the chart, but this a little laborious and prone to human errors, especially if the boat is pitching around.
A much simpler method of position plotting is to use bearing and distance TO a fixed point on the chart.
Make the centre of a compass rose on the chart in use a waypoint. Program the lat/long of the chosen compass rose into the GPS as the destination waypoint. The GPS will then give your position’s bearing and distance to the compass rose WP. And using these two values your position can be quickly and simply plotted on the chart.
To help in transferring this fix to the chart I have made the very plotter shown below. The thin plastic sheets used were off cuts bought for a few pence from a sign maker.
The distance scale is taken from the latitude scale on the chart in use.
We use Imray C series charts for most coastal cruising. Many have the same latitude scale so one plotter has been marked permanently with distance for use with these charts.
Other plotters have been made without a distance scale. Stick a strip of masking or white electrical tape along the working edge, hold against the latitude scale of the chart used and mark of the distance.
When cruising we set the destination WP of the chart table GPS to the chart compass rose and and read off the GOTO information as required.
In the example below the plotter has been made for use with chart 5604.13 (one of the Admiralty Channel Island folio).
The compass rose from the chart as been entered as WP 5604-13 into the GPS.
At the time of the plot a GOTO showed :-
Bearing to WP 047o(T) and distance to WP 3.61NM.
The position is plotted as shown below.
At night school, on an Ocean Yachtmaster course, I learnt the art and science of Astro navigation and bought a plastic Ebco sextant to hone my skills. We used to holiday in the 80’s at the Ferry Boat Inn and early in the morning I would take the Mirror dinghy to the mouth of the Helford river where I could see the Sun and horizon. On my return I would inform family and friends that I had been able to determine my position line correct to 2-5 miles. This was normally greeted with hoots of derision and comments such as “we haven’t had to move from here, but know exactly where we are to a few yards, just ask us next time you want to know”. I still keep Annual Ephemeral tables and occasionally practice position fixing on paper, although pocket calculators and computer have simplified the calculations. A few years ago I replaced the plastic sextant with a very well made 2nd hand Russian model, bought for a few pounds in Singapore.
In the early 90’s I bought my first GPS. It was expensive, ( about $1000), heavy, ate batteries and would give lat/long accurate to about 50-100metres. It would store 20 waypoints and show bearing / distance to WP from current position. It was a marvellous tool.
Pre car satnav I bought some software to run on my laptop and linked this to the GPS. I used this to help me drive all round the UK and Europe in spite of the inaccuracies in the position and the maps. The software must have known I was using a nautical GPS because driving up the A38 it would normally show me in the Trent and Mersey Canal.
How things have changed. A few months ago I bought a fancy smart phone. As well as being able to browse the web for weather forecasts it has an inbuilt GPS, accurate to about 2 metres (it shows which side of the road your driving), which gives me street by street navigation anywhere in the world for free. We used it recently on a touring holiday in South Africa. OS maps can be downloaded ( again free) and the GPS will locate exactly your position on the map, able to follow footpaths. The phone does everything, except I find it difficult to make some phone calls.
The Raymarine chartplotter on “Sarah Jane” is equally as accurate, showing our position on the pontoon at Mylor. Linked to the autohelm it follows a track far better than we can. It gives position far more accurately than any traditionalist could have dreamed of.
Without a doubt GPS has in recent years transformed and simplified all types of navigation, at sea, in the car and on foot.
When GPS is working properly it is a fantastic tool for the sailor. That is the problem – WHEN IT IS WORKING PROPERLY.
All GPS users will know the symptoms, position indicator jumping around, unable to show a position, or wildly inaccurate position. We were once shown to be miles away from our known position and at 32000ft, not on the sea ( we later found out that the USA had invaded Panama).
Twice we have lost all satellite for some time. The first time was mid channel when both the Raymarine chartplotter and below decks Furuno lost the fix at the same time. No satellites were visible on either unit. By “coincidence a Nimrod was circling overhead during the time the signal was lost. The second time was a few miles from Plymouth, both onboard GPS showed no satellites. A third hand held GPS indicated no satellites. Again, by coincidence several naval warships were entering and leaving Plymouth at the time. In our case the most common cause of GPS not working is lack of battery voltage, after a few hours sailing using the autohelm the house battery voltage would drop too low and the GPS would switch off. This partly solved by fitting a wind generator and new battery.
To reduce the difficulties arising from the loss of GPS we routinely plot our position every ½ hour or hour on a traditional chart. As well as a the security this also gives a much better overview of progress and the effects of tide and wind.
Of greater concern inshore is the the possibility of inaccuracies in the electronic chart, omissions or out of position hazards or marks. Because of this when entering or leaving an unfamiliar harbour we rely chiefly on traditional chart pilotage, leading lights or lines or buoyed channel. The final GPS waypoint is one that brings us to the point where traditional pilotage takes over, but even then there are circumstances when its useful to be able to quickly plot an accurate GPS fix on the chart to verify you are where you think.
Make certain the chart GPS datum is the one programmed into the instrument. The majority of charts use WGS84, but a few use OSGB. Be careful when using a handheld GPS that the datum has not been set to OSGB36 for walking.
The GPS will give an accurate lat/ long position and it is possible to plot this directly on the chart, but this a little laborious and prone to human errors, especially if the boat is pitching around.
A much simpler method of position plotting is to use bearing and distance TO a fixed point on the chart.
Make the centre of a compass rose on the chart in use a waypoint. Program the lat/long of the chosen compass rose into the GPS as the destination waypoint. The GPS will then give your position’s bearing and distance to the compass rose WP. And using these two values your position can be quickly and simply plotted on the chart.
To help in transferring this fix to the chart I have made the very plotter shown below. The thin plastic sheets used were off cuts bought for a few pence from a sign maker.
The distance scale is taken from the latitude scale on the chart in use.
We use Imray C series charts for most coastal cruising. Many have the same latitude scale so one plotter has been marked permanently with distance for use with these charts.
Other plotters have been made without a distance scale. Stick a strip of masking or white electrical tape along the working edge, hold against the latitude scale of the chart used and mark of the distance.
When cruising we set the destination WP of the chart table GPS to the chart compass rose and and read off the GOTO information as required.
In the example below the plotter has been made for use with chart 5604.13 (one of the Admiralty Channel Island folio).
The compass rose from the chart as been entered as WP 5604-13 into the GPS.
At the time of the plot a GOTO showed :-
Bearing to WP 047o(T) and distance to WP 3.61NM.
The position is plotted as shown below.
Last year after leaving Dartmouth for St Peterport I decided to check the GPS by taking bearings of Start Point and Berry Head. The result was wildly out, showing us just off Start Point. I checked several times, with various results, but always miles from our known position. It was only recently I realised the cause of the error. My clip on sunglasses are held in place by small magnets fitted to the glasses frame.
