It is possible to define your position with only a few meters of tolerance. Closeness depends on the receiver’s intensity. Steep valleys or canyons, densely wooded areas or an unpropitious position of the satellites could interrupt the satellite’s signals. But that is not a big problem because you change your position frequently therefore failures are seldom and short.
If it happens that an interrupt lasts longer than a few seconds (e.g. driving through a tunnel) the magnetic compass - integrated in most GPS receivers - can be used for orientation. Therefore you should keep an eye on the functions of GPS receivers when you want to buy one. It should have an integrated magnetic compass as well as a barometric altimeter.
Because of common use of GPS (at sea, on the road,...) it’s proved that GPS is dead reliable as virtual tour guide. But anyway, never forget to check the material before you start a tour. You find a short list of things which should be checked before starting a tour in our online support ("Help").
GPS offers no certainty about atmospherec conditions which could put you to your limits. Therefore sport fans should always keep personal responsibility and attentiveness when they are on tour.
GPS could be used anywhere on our planet as you can see to your left: Reinhold Messner with GPS receiver on polar expidition.