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Basalt Formations and Rock


tjpratt
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I don't understand gpsrs, but we seem to have problems with rocks, too, especially basalt columns and canyons and such. We have a cache that is placed at the base of a basalt cliff, and we can't get the coords down to anything less than 100 feet. No one can ever find the darn cache, so I don't know what the point of it is. :rolleyes:

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Basalt has no minerals with which to interfere with a compass.

 

Iron ore, in its' many forms including magnetite and hematite, can lead (magnetic) compasses astray.

 

 

EDIT to add: That was how the iron ore deposits were discovered in Michigan and Minnesota. The original surveyors had much difficulty with their compasses.

Edited by Gitchee-Gummee
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Just to clarify, anything which gets between your GPS and the sky will affect the GPS reception, whether it's basalt, chalk, sandstone, or the roof of your house. So if you're standing at the foot of a cliff, just look around you and your GPS can't get a signal from the satellites in that part of the sky which you can't see because the cliffs are in the way, so I would guess the cliffs are obscuring ~40% of the satellites. If you're in a canyon then there's probably ~%80 of the sky obscured. The makeup of the cliffs in these situatuations is irrelevant, but they will affect the reception.

 

For the OP question I was taking him to mean he was walking around on top of Basalt rocks, which won't affect the GPS signal.

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Just to clarify, anything which gets between your GPS and the sky will affect the GPS reception, whether it's basalt, chalk, sandstone, or the roof of your house. So if you're standing at the foot of a cliff, just look around you and your GPS can't get a signal from the satellites in that part of the sky which you can't see because the cliffs are in the way, so I would guess the cliffs are obscuring ~40% of the satellites. If you're in a canyon then there's probably ~%80 of the sky obscured. The makeup of the cliffs in these situatuations is irrelevant, but they will affect the reception.

 

For the OP question I was taking him to mean he was walking around on top of Basalt rocks, which won't affect the GPS signal.

Sure, that makes sense. You have that problem being around buildings, too, like downtown Manhattan.

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When you're near a cliff, near a big building, in a canyon, etc., another thing that can happen is that your GPS receiver may pick up a satellite signal that has been reflected off the rock surface or off the side of a building. These multipath errors can introduce fairly large errors into your device's calculations.

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Thank you ALL for taking the time and the info.As I mentioned I am using...GarminGPSmapsc this GPS.

Should I just assume that if it gets me within lets say 10' I should start using my EYES, along with the hints and Logs.

Edited by tjpratt
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You should start using your eyes within about 30 feet. If the device placing the cache was 10-15 feet different from your device reading, that would allow for a up to a 20-30 foot variance. Variance in the satellite patterns and signals at different times can cause this much of difference in readings.

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What you are describing is common in rock areas even who they are all on the ground and not in any way obscuring the signal. It is called multi path and here is a good explanation. http://www.trimble.com/gps_tutorial/howgps-error2.aspx

 

That's the same thing that Nira mentioned in post #9 and explains a lot of what I've noticed. The only other way that basalt can interfere with a compass is from a gravity induced impact.

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What you are describing is common in rock areas even who they are all on the ground and not in any way obscuring the signal. It is called multi path and here is a good explanation. http://www.trimble.com/gps_tutorial/howgps-error2.aspx

 

That's the same thing that Nira mentioned in post #9 and explains a lot of what I've noticed. The only other way that basalt can interfere with a compass is from a gravity induced impact.

I love the gravity induced impact as a method of interference.

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