When any surveyor is required to survey lands which have been surveyed before, he shall make such survey by the magnetic meridian, but shall also return and certify in his plat the degree of the variation of the magnetic needle from the true meridian, at the time of the resurvey; and also (if the same can be done) the degree of such variation at the time of the original survey; and every survey, whether original or not, shall be made by horizontal measurement, unless otherwise provided by deed or contract.