SAFETY and TRAINING
The Global Positioning System (GPS)
Your map and compass are more important
by Roger Wild
Over the last two years I have been using a GPS receiver in Scotland and the Alps in both summer and winter conditions. The technology is very impressive and can provide some very accurate information but there is no doubt that, for mountain navigation, GPS is only an aid and that traditional map and compass skills remain the backbone of sound navigation.
The Global Positioning System uses radio signals that are beamed from satellites orbiting 12,000 miles above the earth. The GPS receiver picks up the signals, calculates your position and displays it in a way that can be identified on a map (grid reference, latitude & longitude etc.). The receiver needs to pick up signals from at least three satellites (more satellites will provide greater accuracy). GPS was developed by the US Defence Department and originally had a built-in inaccuracy or "Selective Availability" for civilian users, which meant that the accuracy was only 100 metres. This selective availability was "switched off" in the year 2000 and civilians with hand-held receivers may now achieve GPS accuracies in the range of 6 - 12 metres. This built-in inaccuracy could be switched back on again but for the time being this seems unlikely. There is a possibility that a dedicated European GPS system may be created at some future date.
So what can GPS receivers do and what are the pitfalls? For the mountain navigator, some of the main benefits that a GPS receiver might provide are:-
- A grid reference of your present location.
- The facility to return to a location that you (or your friends) have been to previously.
- "Spotting" – entering the grid reference of a specific point you want to go to.
- Entering waypoints prior to a journey so that you can use them as an aid to navigation during your trip.
Before we talk more about these potential benefits, let's have a look at some factors that might prevent your GPS receiver from providing you with accurate information:-
- Loss of battery power (or a fault in the receiver). Battery life varies considerably and cold conditions can drastically reduce power. It's a good idea to carry a spare set of batteries but even this does not guarantee success – changing batteries in a storm may not be possible.
- Your location may not have a clear line of sight to sufficient satellites. Cliffs, crags, deep-sided valleys and forests can prevent your GPS receiver from acquiring a fix.
- Reflection of the signal. Large areas of rock can reflect the signal so that it travels further and causes an error in the calculation of the distance between the satellite and your receiver.
- Severe weather conditions can prevent you from operating your GPS - pressing small buttons while wearing gloves or mitts can be difficult. It can also be difficult to read the details on the screen in stormy conditions.
- In remote areas of the globe, only four satellites may be available and your GPS receiver may not be sensitive enough to pick up enough signals.
- Technical factors. GPS works by measuring the time taken by signals to travel from the satellites to your receiver. The atmosphere slows up the signals and so an average, but not exact, calculation is used. The position that individual satellites give themselves can be inaccurate. The satellites available from your location may be in a line or grouped together.
- Subjective factors. You might have your GPS set up with the wrong Position Format, the wrong Map Datum, the wrong Grid Letters, or be following the wrong Waypoint.
So now we have discussed the pros and cons, let's examine the potential benefits for the mountain navigator:-
- Grid Reference
The most tangible and immediately obvious benefit is the possibility of acquiring a very accurate Grid Reference of your position. Most GPS receivers provide a reference that refers to a unique 1-metre square on the map. On the British Grid this will usually be two Grid Letters followed by a ten-figure Grid Reference. The specified accuracy for my GPS receiver is 15 metres (although it is usually more accurate). For the purposes of mountain navigation this is still pretty accurate and identifying your position down to the nearest 15 metres on a 1:50,000 or 1:25,000 scale map requires an element of estimation anyway.
- Returning to a location that you (or your friends) have been to previously.
This is done by marking a "waypoint" at the location when you are there, storing it in the GPS and then using the waypoint to return to the location later (this could be particularly useful for finding your tent, hut, car or the top of a descent route). Your friends can also provide waypoints but you have to be sure that they have given you accurate information.Marking the waypoint when you are actually "on the spot" enables the location to be stored in your GPS far more accurately than if you entered the Grid Reference from the map. Using GPS in this way can be very accurate and your receiver will usually take you to within a few metres of a previously visited location. This is clearly a very useful facility but it is important to remember that this accuracy is not guaranteed. I have had several anomalies in the mountains where my GPS receiver has definitely provided an incorrect location and other users have told me the same.
- "Spotting" – entering the grid reference of a specific point you want to go to.
This is the GPS version of traditional "point to point" map and compass navigation.
Using GPS for mountain navigation still requires the ability to read the map and to identify features along a route that is both safe and practical to follow. Remember that the information you receive from your GPS receiver can only be as good as the information you have keyed in i.e. if you enter a six-figure grid reference you can only expect to be within 100 metres of your target. The six-figure grid reference of the summit of Ben Nevis includes part of the north face. To be more accurate it will be necessary to enter an eight-figure grid reference (which refers to a 10-metre square). On a 1:25,000 scale map, 10 metres is less than half of a millimetre and so you will have to make an approximate estimation. A map of the Ben Nevis summit area is now available on a scale of 1:12,500 which has additional grid lines to assist in measuring eight-figure grid references. Even on this super-scale, 10 metres are represented by only 0.8 of a millimetre. Irrespective of the accuracy of your grid reference, if you simply follow your GPS you could walk over a cliff or cornice, end up on steep or loose terrain, or lose height that has to be regained - using up valuable time and energy. Another important factor to consider is the time it takes to key in the Grid Reference compared with the time taken to take a bearing and measure the distance in the traditional way. The traditional way wins easily. In a winter storm, it may not be possible to key the Grid Reference into your GPS. Nevertheless, it can be very useful to be able to key in a specific location and to follow your GPS to the spot.
- Entering waypoints prior to a journey
This method of using GPS overcomes the difficulty of keying in Grid References while "on the hill". However, it may not be practical to sit at home (or in the pub) and key in all the waypoints that you think you will need for your trip. This is time consuming and may restrict your choice of route. It may be better to study your proposed route on the map and just key in a few crucial landmarks along the way e.g. saddles and cols, knolls, featureless summits, tops of descent routes. A few carefully selected features that you may need to find along the way will probably be more useful than just having a list of points to follow and you will be able to maintain your traditional navigational awareness.
- Interfacing with your computer (maybe we're losing the plot here?)
Many GPS users have digital maps on their computers that can be connected to a GPS receiver. They can:-
- click on dozens of new waypoints and upload them to a GPS in a couple of minutes.
- store hundreds of waypoints and upload them to a GPS in a few seconds.
- email waypoints to each other and upload them without having to key them in manually.
- download waypoints from the world wide web.
- download the track from a recent journey and view it on a map.
- upload a track from a map to a GPS receiver to be followed later.
Avoid the quick fix
We've looked at the technical pros and cons but there any some other issues to think about. There is a danger that using GPS will erode our navigation skill. Navigating in the mountains is a craft that can be developed and enjoyed over a lifetime. When following a "GO TO" on a GPS receiver, it is very easy to ignore the ground under your feet, to leave the map in your pocket and to forget about the contours. You can easily get into a situation where you are lost, relying solely on only your GPS, and at the mercy of technology.
Map and Compass
Despite the increase in GPS technology, map and compass skills are still the essential requirements for sound mountain navigation. GPS receivers have their place (along with mobile phones) but over-reliance on the technology can lead you into hazardous situations.
Good navigating.
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