Theres GNSS as well as GPS. If your tunnel has underground GNSS repeaters then it knows where it is. At least that’s how I understand things to be. Next time you’re there see if the lat and long are still working.
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The next phase is to take several readings I.e. is the -100' reading at the midpoint and what are the entry and exit altitude readings?
Would be fun to get a reading in Death Valley.
Do you have a reference altimeter to verify? I don't think the Gren altimeter is ever spot on. Rather it confirms you are on the planet!
I believe it is stated somewhere that it's accuracy is +/- 150ft
Helpful, falling 3ft off a step ladder is one thing but if the drop transpires to be 153ft that is going
to be irksome
Result of this week’s Hurricane Helene?
I have not been that deep yet! So far I have only gone down to -78 feet.
No, I just happen to know the elevation of the places I roam, living near the coast in northern California, Baja California Sur, and Florida the risk of flooding and promise / threat of beach front property is real.
