Examples

Ex19-1 The magnetic bearing of a line was recorded as N34°00'E on an old survey plan dated August 15, 1947. It is required to re-establish this direction on the site in 1997. In 1970 isogonic chart shows a declination 9° W for the area, with an annual change of l' eastward. Determine the

(i) Azimuth of the line;

(ii) magnetic bearing that must be used to relocate the direction of the line in the field.

Solution :

Declination in 1970 = 9° W

change in (1970 - 1947) = 23 yrs (23 x 1') = 23' Eastwards

Therefore

Declination in 1947 was 9° + 23' = 9° 23' W

Magnetic Bearing of the line on 1947 = N 34° E

(i) True bearing of the line = 34° - 9° 23' = 24° 37'

= N 24° 37' E

Declination in 1997 = (9° - 27 x 1' ) W = 8° 33' W

Therefore Magnetic bearing in 1997 = 24° 37' + 8° 33' W

= N 33° 10' E

Figure Ex19-1