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I read here and elsewhere that

Exploratory and reconnaissance surveys are frequently made with the aid of a magnetic compass [...] Early references to the application of the magnetic compass to surveying indicate that the instrument was found to be particularly suitable for use in mines. This object is an Italian miner's compass of the seventeenth century. The use of 24 scale divisions is derived from the mariner's compass. Agricola referred to this feature in his 'De Re Metallica', the foremost work on mining and metallurgy of the Renaissance period. In translation, his words read:

"Now miners reckon as many points as the sailors do in reckoning up the number of winds... Any miner who please may therefore call the direction of the veins by the names if the winds."

Since mineral mines are often interested in non-magnetic minerals, I am skeptical that a compass would have much utility.

What useful information can a compass yield, in the context of mining?

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    $\begingroup$ I think it's all in the part you didn't quote: measuring horizontal angles to determine the direction of mineral veins. $\endgroup$ Commented May 3 at 15:59

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Geologists still use geological compasses in both open pit and underground mining

to measure orientation of geological structures, as they map in the field, to analyze and document the geometry of bedding planes, joints, and/or metamorphic foliations and lineations.

Prior to the development of the theodolite, which measure both vertical and horizontal angles, underground surveying of tunnels and chambers was done using compass and measuring tape or chains, such as the Gunther's chain. When used for surveying, compasses were mounted on a tripod.

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Compass surveying is the branch of surveying in which directions of survey lines are determined with compass and the lengths of the lines are measured with a tape or a chain.

Whether used by geologists or surveyors, the compass referenced the Earth's magnetic field. If the mineral deposit was magnetic this would influence the compass and could produce erroneous results.

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