Azimuth References

Azimuth, (AZ) used in directional drilling, may be defined as the direction of the wellbore (at a given point) projected into the horizontal plane measured in a clockwise direction from Magnetic North, True North or Grid North after applying a North Reference system.

Azimuth should be expressed as a value on a 0°-360° compass system. Quadrant or bearing systems (i.e. N45° 20'E) may be easier to visualize, but make the probability of convergence mistakes higher than in an azimuth system. It is therefore recommended to have all survey printouts converted to an azimuth system when making initial convergence corrections.

For directional drilling and borehole surveying, there are three primary azimuth references. They are Magnetic North (MN), True (Geographic) North (TN), and Grid North (GN).

Magnetic North is the direction of the horizontal component of the earth's magnetic field lines at a particular point on the earth's surface pointing to the magnetic pole. A magnetic compass will align itself to these lines with the positive pole of the compass indicating North. Magnetic North is usually symbolized on maps by a half arrowhead or the letters MN.

True or Geographic North is the horizontal direction from a point on the earth's surface to the geographic North Pole, which lien on the earths axis of rotation. The direction is shown on a globe by meridians of longitude. True North i.e. normally symbolized on maps by a star at the tip of the arrow or the letters TN.

Grid North is a reference system devised by map markers in "the complicated task of transferring the curved surface of the earth onto a flat sheet. The meridians of longitude on a globe converge toward the North Pole and therefore do not produce a rectangular grid system. A map can be drawn such that

Half Convergence Meridians

the grid lines are rectangular, for some specified area of the earth, the northerly direction of which is determined by one specified meridian of longitude. This direction is called Grid North and is identical to True North only for that specified central meridian. It is normally symbolized on a map by the letters "GN" at the tip of the arrow.

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