Buyers' Guide - GPS

24 (and a few spares for redundancy) satellites constantly circle the earth in 12 hour orbit patterns that give complete coverage of the earths surface. These satellites transmit signals to receivers on the ground giving the satellites location and the time. All the satellites transmissions are synchronised to ensure they send at the same time. As the signals travel at the speed of light they reach a GPS receiver at slightly different times depending on their distance from the receiver. The receiver determines the distance from the satellite from the time the signal takes to reach it. When the receiver has calculated the distance from at least four GPS satellites, it can calculate its position in three dimensions. This enables the receiver to give an accurate longitude, latitude and altitude.For a receiver to find its position it has to have a line of sight to the satellites, and this is where problems can occur. If you’re in woodland or amongst tall buildings it may be difficult to get an accurate signal or even any signal at all, though the receivers have improved in this area over recent years. Obviously the more satellites a receiver can obtain signals from the more accurate the “fix” will be.

Waypoints
A Waypoint is simply a place whose position you wish to record. You can use waypoints to record points of interest, places you took photographs from or important deviations in a route. Entire walking routes can be made up of a series of waypoints, and advanced GPS receivers and PDA based systems can often store thousands of waypoints. Basic units will be able to store waypoints internally, and a recall facility to view the waypoint positions. Advanced units will expand on this by allowing the export and import of waypoints between the receiver and digital mapping software on a computer.

Route and track storage
One of the best features of having a GPS is the ability to store a record of exactly where you’ve been. At it’s most basic level this means that should the weather close in you can use the record to retrace your route in safety. At the other end of the scale connecting a GPS to digital mapping software lets you check out routes on your computer once you get home, or even plan a route at home and transfer the route to your GPS for following once on the hill. Different manufacturers use different terminology, but essentially a route is a series of connected waypoints, whereas a track is a recording in real time of the route you take.

Altimeter
Although not particularly useful in the UK, an altimeter comes into its own when in the Alps or similar mountain environments. Many alpine routes require direction changes at known altitudes, and in reduced visibility an altimeter can literally be a life saver.

Other functions
Most GPS receivers will include additional features such as the local time and the speed of travel. While occasionally useful for working out how much longer you have to go before the end of a route these are minor features and pretty much universal.

Digital Mapping

The ability to combine a GPS with digital mapping software is where GPS really comes into its own. Using products from Memory Map, Anquet, Tracklogs and Fugawi you can display the output from GPS recorded routes using high quality digital OS maps on your PC (Mac versions due some time). This feature isn’t available in basic GPS units, which don’t have an import/export facility. Depending on the software (eg. MM, Anquet etc) you can overlay your routes on standard, aerial or 3D maps, or do the process in reverse and plan the route on your PC then export the route to the GPS.

PDA based GPS has the ability to show the same OS maps on screen on the PDA, with your GPS located position marked on the map. As with advanced stand alone GPS units, you can transfer routes and tracks in either direction.

Accessories

Like most modern gadgets, buying the unit is often only the start of things. Once you’ve got your GPS you find there’s a whole catalogue of extras you can buy. One of the most useful accessories is undoubtedly a connection cable for linking the GPS to a PC. Beyond that you need to think about waterproofing and spare batteries. In general a stand alone GPS should be able to cope with a weekend away without needing a battery change, but if you’re looking at PDA based systems don’t expect your batteries to last more than 6-8 hours of constant use.

How To Chooses your GPS

So how do you choose which GPS to buy? Well with the wide range on offer it’s sometimes hard to differentiate. The secret is deciding what you want to use it for. If you just want it as an additional safety device in case you need to check your position, but most of the time it will sit in your pack, then you don’t really need to look beyond the basic models. If you want the ability to transfer routes to a computer and use digital mapping the choice comes down to a combination of factors including weight/battery life and the number of routes and tracks you need to store.If you prefer to have an on-screen OS map then you need to look at the high end stand alone GPS units or a PDA based solution.