General > General Technical Chat
How to locate an underwater object with a compass?
Chriss:
Hi!
Do somebkdy have any suggestion how can I locate the gps coordinates of an object in the water with a compass?
These objects are close to each other. Something like from 30cm to 2.5m.
From tools I have only a compass.
Thanks.
Ian.M:
With a *really* good magnetic hand-bearing compass, up-to-date charts and a detailed survey of any local magnetic anomalies, you could triangulate off three well spread out, distinctly visible landmarks at accurately known latitude and longitude coordinates to get a position on the surface at best accurate to approx. +/-10 meters per Km of distance from the landmarks. You'd do better with DGPS in the boat which could reduce the surface position uncertainty by an order of magnitude or two. Transferring that position to the seabed in anything except shallow water is problematic due to currents and tides. In a still lake you *MIGHT* be able to maintain reasonable accuracy dropping a plumb-line.
The pros will be using something like a network of DGPS equipped sonar buoys or a towed hydrophone array with surface GPS transponders at its head and tail.
Chriss:
These object are in a river, around 3-6m deep.
Some wood pillars which circle are 70cm.
I shoukd as a diver go down and locate them and if any
possibility to record the coordinates.
The visibility is really poor, aroind 30cm.
2N3055:
How do you expect to detect wooden (nonmagnetic) object with a compass that works on magnetic principle.... :-//
NivagSwerdna:
tape measure... actually two tape measures
anchor end of each at known different point... and then measure distance to each to point to be measured
plot result from triangulations
doesn't sound like a fun job in that viz
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