Cautions to be observed when using GPS with Chartplotters and Paper charts.
Important - Read this list of cautions before using GPS chartplotters and/or GPS positions in conjunction with paper charts. Many people have sailed into danger and have come to grief by assuming that because the GPS satellite systems are extremely accurate that they are out of danger. There are hidden inconsistencies that you must be aware of. You may be sailing into danger without realizing.
Many people sail off blissfully ignorant of basic DR and the use and value of other navigation aids. They think that their up-to-date modern technology will keep them on course and safe with pinpoint accuracy. The smiling salesman that extolled the wonders of the latest technical marvels did not tell everything. If you read all the manuals and information that came with the GPS chartplotter you may have come across some warnings which gave you a hint that you had to use the system with caution.
If you have used a GPS chartplotter outside of Canada or the USA, you will have noticed that sometimes the chartplotter shows your boat sailing on land or when you are only a few boat lengths from shore it tries to convince you that you are a mile or more out to sea. This not because the GPS is inaccurate, but that the people who surveyed the seas to make the charts did not have GPS or any aids that were as precise.
The charts are quite accurate in themselves in a relative sense. If you navigate as you would before GPS came along you would find your way safely without any trouble. One point on the chart is quite accurate relative to all the other points.
The people who surveyed the waters to produce these charts picked a few points of land where they took astronomical sights with painstaking care to establish a datum to which they could anchor the position of the chart relative to its position in the world. In other words they established a Datum Latitude and Longitude from which they plotted all the land and sea features accurately using transits and cross bearings.
The problems they had were;
- Their timepieces were the best available but were not atomic clocks. Even an error of less than a second would move the position of the datum.
- Their sextants also were the best available but had small instrument errors.
- The people doing the surveying had varying degrees of skill, some more precise than others.
- Their methods of calculation assumed that the earth was a perfect sphere. it is not. Another small error.
Now GPS comes along. The GPS gives us an accurate Latitude and Longitude (relative to the earth). When you plot this latitude and longitude on a chart that has not been corrected for this difference in datum, it will not match the actual physical position where you are standing.
Most electronic charts in modern chartplotters are made from the data on paper charts, by scanning and digitizing the data so it will operate in these chartplotters. The most common charts used in North America and Europe are corrected so that they show your position accurately. However, most other charts in the world still do not.
What can you to do? How can you navigate safely all around the world?
The most important thing to get into your head is that you are the navigator, not the fancy GPS system which displays your boat position in the chartploter. The most expensive machine in the world is no more than another aid to the navigator. YOU!
The GPS is just a technical box of tricks that does what it is programed to do. It is nothing but one of your aids that you use to make your navigation decisions. Get this idea firmly in your head and you can sail safely where ever you wish to go. On the other hand if you let the 'Black Box' take over the responsibility of guiding your boat, you are going to land in trouble.
There are other aids to navigation. The best one is the old Mark IX eyeball. Use you eyes and your brain to cross check your position using charts, bearing compasses, sextants, radio direction finder, depth sounder,navigation lights, compass and Dr plots as aids.
❑ Carry paper charts for more information and to backup the electronic charts. GPS and all electronics are subject to failure. Lightning, electrical failure, terrorist EMP, military security, or electronics that just plain quit working are some of the many ways you could lose your fancy, expensive magic boxes. Then what are you going to do?
❑ Read the 'Coast Pilot' or 'Sailing Directions' for your area to find out about any datum errors and changes in lights, buoy and navigation regulations.
❑ Get a current 'List of Lights' so you can identify the correct light to guide you.
❑ Get a current 'Radio Aids To Navigation' book to use radio as an aid.
❑ Seek out local knowledge for areas in which you intend to sail.
❑ Learn to read the sea. The waves and swell can give you good clues.
❑ Learn to read the clues in the sky to predict the weather.
❑ Cross Check. Cross Check. Cross Check. Note the depth sounder reading. Does the reading agree with the depth indicated on the chartplotter. Take cross bearings on points of land, buoys, lights and other points to double check the electronic position. Learn to take vertical sextant angles on objects of known height to get a distance off.
❑ Keep a detailed log book.
❑ Do not approach land or dangers closely at night.
❑ Remember that depth soundings on charts and chartploters can change in area of fast currents like rivers and estuaries.
❑ Enjoy your sailing adventures. Stay safe. Treat the sea with respect.
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