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I'm using an InvenSense MPU9150 as a tilt-compensated compass in an autonomous sailboat. I'm trying to decide where to put the the control board, with options being in a dry box on top of the "deck," or inside the hull, potentially below the waterline, encased in urethane foam.

My question is - will the proximity of the magnetometer to the water level impact the readings / soft/hard iron offsets in an appreciable way?

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    \$\begingroup\$ Water is diamagnetic, but I don't think the small effect would change the compass. (I don't understand about the soft/hard iron offsets?) \$\endgroup\$ – George Herold Oct 30 '14 at 18:25
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    \$\begingroup\$ @GeorgeHerold the soft/hard iron effects are from magnetic elements in the environment (such as the circuit board etc.) and and em interference. It's usually solved for by mapping a series of raw readings from the magnetometer onto an ellipsoid, and then computing an offset and transformation matrix that realigns the axes with the body and distorts the ellipsoid back to a sphere. \$\endgroup\$ – kolosy Oct 30 '14 at 18:29
  • \$\begingroup\$ Well all the pieces of magnetic material (iron etal.) will be much more important than the water. Water diamagnetism is the floating frog in a SC magnet. Iron is a few turns of wire and a D size battery. \$\endgroup\$ – George Herold Oct 31 '14 at 1:03
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No. Magnetic fields and water interaction are not your issues. Water and electronics we know don't mix, but your objective is to place the sensor as close to the center of the boat. Human sized boats frequently install fluxgate compasses below the waterline, but you want to avoid large ferrous masses like the engine, stove etc.

There is a nice diagram in this owners manual. (No endorsement its just the first one that came up with a decent picture) http://www.raymarine.com/WorkArea/DownloadAsset.aspx?id=1841

Fair winds

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