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HSH Yacht Articles

The wrong side of the track


Jeremy R. Hood

Happy New Year! Do you have plans for an adventure this year or did Santa bring you a GPS for Christmas. If so, read on. Here is a short introduction to some of the features of your GPS that you may want to use on your next trip.

To be honest, in the past I have rarely paid attention to all of the extra features on my GPS. I pretty much know what they all do but somehow it has all been too much trouble to figure it out. For someone who learned his navigation using a #1 pencil, dividers, parallel rule and a chart, it seems easier just to continue that way because Iím familiar with it. But as a new yearís resolution I plan to develop my GPS skills! And so, here goes: time for me to review what these features all are and how to use them, and a chance for you to adopt my new years resolution as well.


A waypoint is any position on a chart that you choose. But ideally it will be a point that you want to get to out on the water either as a destination or as point to pass through on your way somewhere. Waypoints can record good fishing spots, or the location of a buoy at the beginning (or end) of a channel or even exotic destination that you plan to visit someday such as my favorite spot in the Eastern Caribbean, the Tobago Cays (Lat 12 38.0íN, Long 61 21.5íW). When you enter a waypoint into your GPS you can then obtain information such as the course and distance from your present position to the waypoint. But now comes the most important part about waypoints. JUST BECAUSE YOUR GPS TELLS YOU A COURSE AND DISTANCE DOES NOT MEAN THAT IT IS A SAFE COURSE TO TAKE! I can see from my hand-held Garmin 45 that my idyllic Caribbean destination is 2130 miles away on a course of 107 (M) but if I tried to take this course I would end up ship wrecked on Cuba! Waypoints have to be used with caution and only in conjunction with a chart! If the course from your present position to the waypoint is not a safe course then you will need to add some intermediate waypoints that can be followed to avoid the dangers. Often a series of waypoints are incorporated into a route.


Most GPS receivers allow you to enter hundreds of waypoints which can then be combined into a series of routes. To get to my destination in the Eastern Caribbean I could set a series of waypoints to take me from Galveston to the Florida Straits, then on weaving through the Bahama chain to the Virgin Islands and then south to the Tobago Cays. Such a route because of its complexity and the number of dangers involved (wrecks, reefs, islands, currents) could require a hundred waypoints but other routes can use as few as two or three. When I head down to Vera Cruz I usually have one waypoint immediately to the north of the town (about 600 miles from Clear Lake) then perhaps another two or three to take me in to the harbor entrance.

Most recent GPS receivers will allow you to set up ten or more different routes and will then give you a list of the waypoints with the course and distance from each point in the route to the next. One neat feature which I have only used once so far is the ability to use a route in reverse in order to return home along the same path.

Cross track error

Whenever you use a waypoint (either alone or as part of a route) the GPS plots a virtual rhumb line from one waypoint to the next just as you would probably plot the actual rhumb line on a chart from your starting point to a destination. Once you set off towards this waypoint the GPS will then calculate your distance off this line (Fig 1) and will display this information to you in any number of ways from a simple arrow pointing to the left or right to showing the actual track of the vessel alongside the rhumb line. This feature allows you to use a GPS to actually follow a line to a destination as if it were drawn in the sand of the sea bed. Each time you begin to head off the line the GPS will alert you and indicate the direction you have to steer to get back on the line.

When under power this can be a useful feature (though for me itís more tedious to watch the GPS than to use a compass) especially in tight navigational areas but under sail when you are limited by the wind direction, cross track error can only really be used when sailing well off the wind and even then, altering course to follow a GPS without attention to a changing wind or sail trim is not such a good idea and will certainly not win you any races!

Find out what the current is doing

One of the neatest features of a GPS is that it will allow you to calculate what the current is or, more strictly, what the effect of current and leeway are combined. By plotting on your chart a dead reckoning position (course steered on the compass with distance run using the distance/speed log) and then comparing it with a GPS position for the same time (Fig 2) you can see what the difference between the two is. Assuming a steady hand at the helm (and a calibrated distance log and swung compass), this difference will be the result of current and leeway combined. And when the leeway of the vessel is negligible, as when motoring perhaps, it will show the effect of the current. Knowing this can allow you to get out of an adverse current; take more advantage of it; or simply allow for the current in calculating any future course to steer.

With integrated electronics packages it is possible to get the electronics to do the calculation for you and to display figures for the current but remember, this is still a combined effect of current and leeway (your electronics cannot know a separate amount for leeway) and is only as accurate as your distance / speed log and your electronic compass, both of which need calibration and are subject to error.

Velocity made good

Every GPS that I have seen displays a figure for VMG Ė velocity made good. This figure is only relevant when heading towards a waypoint, and it is a measure of your speed towards this point. If you are heading down the line from start to waypoint (the track) and therefore your cross track error is zero then your VMG will be the same as your SOG (speed over ground). But as a sailor your waypoint may be upwind and your only way to the point is by using a series of tacks (Fig 3). In this case, the SOG will measure your speed over the ground along the course steered which is not really the course you want but the VMG will show the speed you are making towards the waypoint (which often can be despairingly small). By comparing VMG on opposite tacks you will be able to see which tack is the most favorable in making best progress towards your destination. But there are two points to note here. Firstly, the chosen tack will not remain the chosen tack for ever and secondly, and perhaps more importantly, the GPS is often programmed to give you the VMG from your start point to the next waypoint. Once off this track, it will not necessarily be telling you the VMG from your present position to the waypoint.

Setup features

All GPS receivers come setup with a standard set of parameters but some of these may need to be changed for your particular use and others you may choose to alter to suit your individual needs. Of the many features that can be changed, here are the ones to pay closest attention to:

  • Date / Time offset When you initially turn on your GPS you can enter a GMT time or the receiver can obtain its own time from the satellites. But until you put in a time offset, this time will be GMT. If you want your GPS to show local time here in Texas then you need to put in an offset of minus 6 hours as, daylight savings time apart, Central time is 6 hours behind Greenwich time.

  • Position format Most receivers will allow you to show a latitude longitude position in a number of different formats, the main ones being: degrees and decimal parts of a degree; degrees, minutes and decimal parts of a minute; degrees minutes and seconds. I usually chose one of the last two to coincide with the way parts of a minute are shown on my chart (either seconds or decimal parts of a minute)

  • Map Datum For your GPS to give you accurate positions on your chart, the GPS and the map have to use the same (or similar) charting system. Most US coastal charts use the WGS 84 map datum so select this unless you are using other charts compiled using a different datum

  • Units A GPS will show distances in meters, statute miles or knots. Select either statute miles (to coincide with distances as shown along the Intracoastal Waterway) or more usually, knots.

  • Headings Typically a GPS will show headings (bearings) as Magnetic bearings (True adjusted for magnetic variation) using its own internal program for magnetic variation. Check to make sure that the figure being used in your location is correct or turn this feature to show true bearings if you prefer.

Have fun figuring out how to use your GPS and then enjoy your time out on the water. But remember, your GPS can fail so donít set off with this as your only method of navigation!

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