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GPS: Use it (or get lost)

Posted on 30 Jun 2004 at 14:26

So long as there's a clear view of the sky through the windscreen, the GPS should work. But with the weak transmissions from GPS satellites (see How GPS works), you may find that athermic glass, fitted in some newer cars, will block the signal. This isn't a terminal problem, however, as many of the latest devices have sockets for an external antenna, which can be placed outside the car.

A final consideration is memory requirements. Some bundles include a memory card with the maps pre-installed, but if not, then purchasing at least a 128MB module for your PDA is an absolute necessity. For Navman products, this will just about fit the entire UK map set, although some European countries are larger and some GPS products take up more space for their mapping data, making 256MB more realistic.

On your bike: GPS outside the car
An in-car navigation system based on a PDA will help prevent you having to sit outside a Little Chef scratching your head over a Great Britain road atlas, but for tougher environments where getting lost could be a serious hazard, a PDA is fatally flawed. In these circumstances a dedicated handheld unit is a far better option.

The main design considerations for all-in-one dedicated units are that they have long battery life, can keep track of where you are at all times, don't randomly hang up and need resetting and don't stop working as soon as the first drop of rain falls. Current PDAs are consequently not an option. And despite their better battery lives, often extending beyond 12 hours, separate Bluetooth GPS units usually switch themselves off if the Bluetooth connection is lost - in other words, if you switch the PDA off to save juice. Consequently, the unit can't keep track of where you've been, making retracing your steps via the device impossible.

Because of this, the market for standalone, dedicated GPS units is as healthy as ever, although they do still tend to be a little chunky compared to the CompactFlash-sized receivers that slot into PDAs. That said, the smallest units around at the moment, Garmin's ForeTrex and ForeRunner series, weigh less than 75g.

Although GPS only offers the ability to determine your position, a little bit of device intelligence can translate that into your distance travelled, your current and average speed, and direction of travel. This makes the system useful for more things than simply working out where you are at a given moment. Almost all handheld devices have the ability to remember your track over time and sport a data output that will allow you to upload or download tracks and routes to PC. You can use this in conjunction with third-party software, such as Memory-Map (www.memory-map.co.uk) or Anquet Map (www.anquet.co.uk), to overlay the results of your wanderings on an Ordnance Survey map of the area, or pre-calculate a route and upload it to the GPS as a series of waypoints; the GPS will then guide you along your route - all you need to do is follow the arrow. As long as your batteries don't die and the satellites are available (and there's no guarantee of this - see The history and future of satellite navigation), it's essentially impossible to get lost.

If you're a walker though, remember that for GPS to determine your direction of travel you actually need to be moving. A receiver calculates your direction by taking two or more successive position plots; if you've stopped this can't happen, and when moving very slowly the limits of accuracy of the plots can lead to false direction data. To combat this, anyone needing to know the right direction when they're stationary - when stuck on a mountain plateau in a blizzard, for instance - can buy a unit with a separate integrated electronic compass. These operate in the same way as a traditional compass, by sensing the earth's magnetic field lines. Models like Garmin's eTrex Summit (www.garmin.com) and Magellan's Meridian Platinum (www.magellangps.com) sport this feature. The eTrex Summit also has a barometric altimeter, which is accurate down to about three metres: better than the altitude fixes given by GPS alone. Accurate altitude helps you to use the contour lines on Ordnance Survey maps to find your position with more precision.