Finding Your Position

Book Extract

A position plotted by a navigator is marked with a circle and the time.

Keeping a sense of the boat’s position is vital in the three-dimensional world of the sea. Unfortunately, it is not as straightforward as on land. The sea has few features to recognise and no one to ask if you get lost, plus other problems too. Luckily there are instruments to help. As the boat travels forward the steering compass will show the direction that the boat is heading and the log will measure the distance travelled, but there are unseen forces acting on the boat:

The wind may push the boat sideways causing leeway. This affects different shaped boats to a greater or lesser extent, increases with the force of the wind and varies with the wind direction relative to the boat’s heading. Leeway is difficult to measure and has to be estimated from the conditions at the time and the type of boat.

The whole body of water may be moving too because of a tidal stream. This is the horizontal movement of the water caused by the ebb and flow of the tides. All vessels and other free-floating objects are affected to the same extent because it is the water that is moving. Tidal streams may increase or decrease the speed of the boat or push the vessel off the heading that the helmsman is steering. This does not show on the log and has to be calculated.

So checking the position regularly and keeping a record in the logbook is important. There are lots of ways of checking the position, some quicker than others, some more reliable than others and it is best to use a variety of methods to avoid putting all your nautical eggs in one basket. Never just assume that a position is correct. Look for corroborating evidence and apply the common sense test:

Does this make sense with what I can see and with the last known position?

Don’t assume you are wrong either, in fact don’t assume anything! Remember as well that by the time you have plotted the position on the chart you are not there anymore! Even if the boat is travelling at only 6 knots, in 10 minutes it will have travelled a mile.

One of the biggest changes in navigation in recent years has been the introduction of electronic position fixing devices with ever increasing accuracy. First Decca and Loran, and now GPS and other Global Navigation Satellite Systems have transformed the work of the navigator. GNSS can find the position and update it automatically every few seconds.

GNSS
A GNSS receiver, such as a GPS, uses information from several satellites to calculate its position. They are remarkably accurate, relatively inexpensive and worldwide.

Today there are several different systems, including GPS, Glonass, Galileo and others, and some sets can use signals from more than one system to find a position.

The set needs an antenna, many have an internal one, but in some cases having an external aerial will enhance the performance or may be required. A small portable receiver may not be able to update in a shielded location.

Some sets show the position as a latitude and longitude, sometimes to three places of decimals, which is perhaps a little misleading. The average calculated accuracy of a GNSS set is within 5 metres 95% of the time. The accuracy depends on how many satellites the set can see at any one time. HDoP (horizontal dilution of precision) is a measure of this and the set will give the value. The smaller the number the better. Many sets display the position on a chart image on a chart plotter or tablet or the position can be fed into other equipment on the boat such as a VHF / DSC radio or a radar set.

Once the set knows the position it can then compute other very useful information, but to get the best of all these other extra features you will have to read the instruction book! Remember that whatever other information the set displays, GNSS is essentially a position-fixing device and it knows nothing about depth of water or tidal streams. In fact, most boats that go aground on sandbanks or hit rocks probably have GNSS!

The GNSS can display the position in different ways, depending on the set:

• On a chart image
• Latitude & longitude
• Reference to a waypoint

Or possibly all 3 at the same time

These will not give a different position, and one cannot be used against another to check if the position is correct. It is the same information, just presented in a different way. GNSS cannot check itself; new data from another source needs to be introduced to make a worthwhile check of position. 

The display will depend on the capabilities of the set and the choice of the skipper. If the position is to be plotted on a paper chart it can be much quicker and easier to use the position shown with reference to a waypoint, rather than latitude and longitude.

If the plotter is not long enough to line up and keep the lines level and perpendicular, use the dividers for both latitude and longitude. 

Using dividers for latitude & longitude

On a small chart table, when the chart has to be folded, the latitude and longitude scales may be inaccessible so plotting the position with reference to a waypoint can be easier.

 

© Not to be reproduced without written permission from Fernhurst Books Limited.

Navigation: A Newcomer’s Guide is written by Sara Hopkinson. It gives you all the information you need to know to get to grips with navigation. It starts from scratch, uses no jargon and features diagrams and pictures wherever possible. It explains simply how to look at charts, find your position, look at tides, plan your passage and determine the course to steer. Sara Hopkinson is an experienced sailor, and a Yachtmaster Instructor and Examiner. She runs an RYA Training Centre in Suffolk which specialises in navigation, radio, radar and first aid courses.  She has also been a Coastguard Rescue Officer for many years and Deputy Station Officer of HM Coastguard, Holbrook. Sara has written books for the RYA and Fernhurst Books’ Skipper’s Pocketbook, VHF Afloat and VHF Companion.