1. © NERC All rights reserved Absolute Magnetic Measurements The Calculations

2. © NERC All rights reserved True North 090° 180° 270° CZ 000° Theodolite Horizontal Circle RO Mean Reference Object True North Circle Reading 1.Calculate RO mean : the average of the four RO circle readings 2.TNCR = 090° + RO mean – RO true (or the reciprocal)

3. © NERC All rights reserved True North Circle Reading

4. © NERC All rights reserved True North 090° 180° 270° CZ 000° Theodolite Horizontal Circle H WU, WD EU, ED D Mean 1.Calculate D mean : the average of the four declination circle readings 2.Declination = D mean – TNCR (or the reciprocal) Declination

5. © NERC All rights reserved Declination

6. © NERC All rights reserved Northern Hemisphere: I SU = 180° - Ө SU I ND = 360° - Ө ND I SD = Ө SD - 180° I NU = Ө NU Inclination is the mean of the four I values Inclination F F Theodolite Vertical Circle Magnetic North 000° 180° 270° 090° SD NU ND SU

7. © NERC All rights reserved Inclination

8. © NERC All rights reserved Total Field 1.Calculate the F site difference between the D/I site and the F site: F SD = F DI – F F 2.Apply the site difference to the four F readings measured during the inclination measurement: F I1 = F F1 + F SD 3.Calculate the mean of the four corrected F readings: F I 4.Calculate the horizontal component: H = F I cos I 5.Calculate the vertical component: Z = F I sin I

9. © NERC All rights reserved Total Field

10. © NERC All rights reserved Check Results Always double check the results with the expected values. The field values will not change much year to year and mistakes are more easily spotted and rectified at the time.