GPS units are becoming a vital tool for many outdoor activities from wildlife surveys to sports (some specifically invented for use with the units) as well as their obvious use for walkers. The choice is somewhat mind-boggling so we have put together a selection that covers the range from basic point plotting to complicated route planning and interaction with PCs.

GPS stands for Global Positioning System. A GPS unit receives information from satellites around the globe. From this, a GPS can calculate its geographic location anywhere in the world.

Once the GPS has located itself it will display your coordinates on the LCD screen. The format of the coordinates are selected by you on setting up the system. Most people choose latitude/longitude or OS grid references. You can then look these up on a map to see where you are. Most units will also show your bearing, heading and distance already travelled on the main screen.

In addition to giving your position, a GPS lets you record locations (waypoints) into memory, plan routes and guide you to pre-programmed destinations. Most GPS units can be pre-programmed with reversible routes and waypoints. A variety of map datums can be used, including Ordnance Survey.

A GPS unit picks up information from up to 12 satellites to calculate its position. Normal accuracy is approximately 7-15 metres. WAAS (Wide Area Augmentation System) increases accuracy to 3 metres or closer. WAAS is an American system of satellites and ground stations which provide corrections for GPS satellite signal inaccuracy caused by ionospheric disturbances, timing, and satellite orbit errors. There is an equivalent system in Europe called EGNOS (Euro Geostationary Navigation Overlay Service), which became fully operational recently. WAAS enabled GPS units are compatible with EGNOS and users should notice improved accuracy with their units in Europe. The latest models are equipped with an addition SiRF chip which improves satellite lock-on and accuracy even under dense tree canopies.

The basic units do not have any form of map. These should be used in conjunction with a paper map. Other units have differing degrees of mapping depending on the model and other software purchased. These can allow you to see your position on the map as you travel. Link the GPS to a handheld PC with more detailed mapping loaded on it for real time route tracking.

Yes, on the Satmap Active 10 GPS. On Garmin units, the actual OS maps are not available but TOPO maps - which use topographical data from the Ordnance Survey - are available for compatible units. Some GPS can take microSD memory cards onto which you can load your own choice of mapping or buy pre-programmed with detailed mapping of a single region.

You can use GPS for surveys - click here for further details. Some units also have an electronic compass and barometric altimeter built in, for extra navigation capability. Additional features can include sun, moon and tide times. Some units are compatible for Geocaching, a gaming activity specifically designed for GPS users. The latest models can include wireless connectivity for exchanging routes, locations etc.