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A world-wide treasure hunt, a game of satellite-guided lucky dip, putting a message in a bottle and tracking it on the internet, a walk in the woods, a harmless pastime and, for some, an obsession. A use for one of the coolest gadgets you can own, or an excuse to buy one. It's healthy, it's outdoors and lots of people are doing it. We are talking about Geocaching...

What is Geocaching?

Geo - of the earth, cache - a hidden store

It all started on Dartmoor in 1854 when James Perrott left his visiting card in an empty jar hidden in a bank at Cranmere Pool. Visitors could leave their own cards, and would return the card they found there to its owner. In time this developed into a network of what came to be known as 'Letterboxes' all over the moor, each containing a notebook and a rubber stamp. Letterboxing enthusiasts make their own rubber stamps to carry with them, leaving their unique mark and a message in each book they find, while collecting the Letterbox stamps in their own notebook. Collections of clues to Letterbox locations are published and the whole thing has grown, so that today there are many hundreds of boxes to be found. Read more at Letterboxing on Dartmoor.

Geocaching brings modern navigation technology and the power of the internet to the Letterboxing concept. Individuals create caches for others to find and they look for other people's caches. But instead of books of clues, map and compass, they use a GPS 1 satellite navigation device to find their way, into which they load the cache locations from the official Geocaching website. Try this article about the GPS system for a detailed explanation of the technology employed.

OK, but what do you actually do?

The Geocaching website has the latitude and longitude of each cache, and you enter this into your GPS receiver. Often there will also be encrypted clues about the precise location of the cache, in case your GPS does only get you to within 10 metres, when you would be left with a pretty big area to search. It is helpful to have the clue in your pocket, only to be read if you need it, telling you for example that the cache is under one end of a fallen tree next to a large holly bush. So, park your car (the cache owners will often have suggested the nearest carpark in their notes on the website), take your GPS, enter the cache location, hit the GO TO button and follow the arrow on the display. It's that simple.

When you find a cache you will immediately note one significant difference to Letterboxing, in that Geocaches tend not to have rubber stamps. Instead they contain a collection of 'trinkets', low value but interesting objects that are exchanged for items that visitors bring with them, like keyrings, small toys, 'novelty items'. Some people put disposable cameras in their caches; visitors take a photo of themselves and when the film is finished the cache owner publishes the pictures.There will usually be a notebook so that you can leave a message, but you will also want to log your find on the Geocaching website. To do that you need to register, following which the website will track all of the caches that you log, so if you wish there can be a competitive element to the game, comparing scores with other Geocachers.

What was that about a message in a bottle?

There is a variation of the Geocaching game, using 'Travel Bugs'. These are tokens with a unique reference number that you attach to an item that you place in a cache, such as a small teddy bear. The bear thus becomes a hitchhiker, and the token is an invitation to other Geocachers to take the bear and place him in another cache. The token is registered on the website and the bear's movement can be tracked. More than one teddy has travelled around the world in this way.

Another variation is the multi-cache, when one cache gives a clue to the location of another. Caches are linked together and it may require a considerable effort to solve the problems set by the cache owner.

Why would I want to take up Geocaching?

If you are wondering why you need to use satellite navigation technology to go for a walk, or if you need to justify the investment in a GPS receiver to yourself or your household's purchasing committee 2, here are a few ideas to help you along.

Geocaching can give a purpose to recreational walking. At the very least it has to be cheaper than buying a dog, and there is a lot less dirt involved. If you have children who need to be encouraged to take exercise, this is a great incentive - it's not a walk, it's a treasure hunt. It is a collaborative game, as a player you become part of a worldwide community, you get to see places that you might otherwise never have visited (there are caches in 149 countries), there is scope for competition and, in the case of multi-caches, genuine problem solving is required.

What equipment do I need?

Before we think about the GPS receiver, lets talk about where you are going to be walking. There are caches in cities, including many in London, but the majority are in the countryside. Sensible clothing and footwear is essential, bearing in mind that the cache might be under a holly bush or bramble thicket.

It is also worth taking a map. The GPS will get you there, but unless you give it some clues it will simply take you in a straight line to the cache, and that could be through a bog or over a cliff, either of which might spoil your day. A 1:50000 or 1:25000 Ordnance Survey map is ideal to avoid these kinds of hazards. A compass is good too, but only if you know how to use it.

The GPS receiver itself is not actually essential. You could convert the latitude and longitude to an Ordnance Survey grid reference and get there with map and compass alone, but that method will only be accurate to 100 metres, a limitation imposed by the way the grid reference works and the scale of the map. In any case, a perfectly good GPS can be bought for £120 or so. Garmin and Magellan have very small handheld models, no bigger than a mobile phone, in this price range, and the Garmin Etrex range is highly regarded in Geocaching circles. You will find them in most high street camping and outdoor shops and at various websites, but it is worth shopping around.

Finally, don't forget to take things with you to swap, a pen and spare batteries.

Great, where do I start?

Go to . It is all pretty self explanatory, but if you really get stuck there is a help forum. Have fun!

1GPS stands for Global Positioning System. It is a network of 24 satellites launched by the US military that transmit radio waves to the ground. With an appropriate receiver, which anyone can buy for little more than £100, the system allows you to pinpoint your location anywhere on Earth to within 10 metres or less.2for example, your wife

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