a gps unit will recieve signals from the numerous satelites in order to triangulate your position (lat., long, and altitude) so a smaller view of the sky ie:inside a building, against the face of a cliff , in the forrest ect... will result in a smaller amount of satellites that the gps unit can recieve signals from. no view of the sky no signal to the gps.
thats what they say anyways. i have used mine in old growth forrest ,against the side of a cliff ect and i find it still works although it might take a bit longer to get a fix and the altitude is usually inconsistent under those conditions.
also you really should read the manual to get the best perfomance from you gps .there are different types of mapping systems out there and you will have to set your gps to the types of maps that you are using. they usually put all that information on the bottom of the map.
hope that helps. merry x-mas and dont get lost.
