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The mine is surveyed conventionally with lasers/collimators and benchmarks installed.

the surface survey was probably performed with GPS locator and transit to determine overlay of underground passages. But that is not necessary, surface laser distance measurement equipment used underground would have been more than accurate enough to hit a 12 ft wide passage at only 200 ft.

As long as your GPS can see stars at night and the sun in the daytime it can calculate your position using a chronometer and a sextant!

David Mellon wrote:As long as your GPS can see stars at night and the sun in the daytime it can calculate your position using a chronometer and a sextant!

Sorry, I don´t understand that , can the GPS look through the cotton of the bimini? . I have never seen my GPS using a sextant .

Rainer

Re. "the metal roof on your car ".... I have a GPS that did not work well inside my Chevy Venture minivan, because the windshield had an antenna embedded in it - the entire windshield except for a strip around the edge and near the mirror, was evidently GPS opaque, as well as the metal roof. When I use the same GPS in a rental car, and in my present hatchback, the GPS works fine, not because it is going thru the metal roof, but because it is going thru the unobstructed glass windshield.

David Mellon wrote:As long as your GPS can see stars at night and the sun in the daytime it can calculate your position using a chronometer and a sextant!

WHAT kind of tent?

GPS receivers are very specialized receivers. They look for mutliple satellites to triangulate your position. The receiver portion can be best described like being in a room full of people talking.....all mumbo jumbo, and then somebody on the other side of the room says"Macgregor" your ears pick that up because your brain is predisposed to hear it. The time signals transmitted from the satellites are very low power, so really anything can block the signal, but the denser it is, the more the blockage. Remember these satellites are literally in a constellation around the earth, constantly changing above and below the horizon based on EXTREMELY SMALL time measurement(quite a few zeroes right of the decimel point). So clear sky is what you want, but minimize the blockage.