372 MAGNETIC METHOD [Chap. 8 



Geodetic Survey. Approximate variation rates are also given on the 

 charts of magnetic elements. This variation does not have to be con- 

 sidered in magnetic surveying except when absolute values in relation to 

 government stations are desired. For calculations of the planetary correc- 

 tion and for adjustments of a magnetometer to a different locality, charts 

 for any year may be used. 



4. "Artificiar' variations are due to both power lines and power plants, 

 surges in A.C. lines, magnetic separators, electric tramways, their return 

 circuit through ground, and anticorrosion currents in pipe lines. A magne- 

 tometer should be set up at least 1,500-3,000 feet away from such sources 

 of interference. Nea,r tramways a magnetic station outside the return 

 circuit loop is generally much less disturbed than one inside. There are 

 exceptions to this rule if stray currents are carried to the outside by pipe 

 lines and wires.^'^ Abrupt changes in the base value of a magnetometer 

 may occur because of the demagnetizing effect of power lines. Koenigs- 

 berger reports displacements as large as 140y when working near power 

 lines. Transportation of magnetometers on electric tramways caused 

 changes of from — 5 to — 357, while transportation on railroads and auto- 

 mobiles produced little, if any, effect. 



C. Planetary Variation 



Since the vertical intensity increases and the horizontal intensity de- 

 creases toward the magnetic pole, the planetary correction is negative for 

 the former and positive for the latter. The rate of change of magnetic 

 elements with latitude and longitude may be taken from U. S. Coast and 

 Geodetic or other world charts. For a given area an average value may be 

 assumed, or the magnetic lines may be transferred from such charts to the 

 base map of the survey. For small areas and large anomalies (magnetic 

 mine exploration) the planetary correction is generally neglected. 



D. Base Change 



This correction is applied to allow for changes in the base value of 

 magnetometers between checks on base stations. Before the correction is 

 calculated, base station readings must be corrected for temperature and 

 diurnal variation. If the difference does not exceed 15-207 in one day, 

 it may be uniformly distributed among stations occupied between base 

 checks. Larger differences indicate abrupt changes and must be corrected 

 for accordingly. The observer is generally in a position to tell froB(i his 



^" Much information has been published on leakage dnrents in connection with 

 corrosion problems. See, for instance, U. S. Bureau of Standards Technologic 

 Papers, No. 63, 75, 127, and so forth. ' 



