It is the duty of the county surveyor and the surveyor's deputies to test their instruments by the meridian line in the counties in which such lines may be established, and to note the variation of the compass, that is, the difference between the magnetic meridian and the true or astronomical meridian, at least once every six (6) months.
Testing of instruments — Notation of magnetic declination
Tenn. Code Ann. § 8-12-111, under Surveyors.
Tenn. Code Ann. § 8-12-111