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1. True North

Given a Magnetic North (MN) direction survey and declination value (DEC), convert the

Magnetic North direction to the equivalent True North (TN) direction.

Since the dawn of time, man has had to describe a location in one way or another. The

directional driller must be able to use learned skills to read surveys and well proposals in locating

the position of a survey point along a well. The well proposal is usually derived from coordinates

in a grid system; therefore, directions are referenced to Grid North. However, the well surveys

use sensors that reference either Magnetic North or True North, and the driller must be able to

convert from one to the other.

True North

True North (TN) is the direction of a line from any geographical location on the earths surface to

the North Geometric Pole.

Calculate True North with this formula:

True North (TN) = Magnetic North (MN) + Magnetic Declination value (DEC).

Magnetic North is the direction of a line from any geographical location on the earths surface to the North

Magnetic Pole.

Magnetic Declination value is the angular difference, east or west, at any geographical location, between True North (Geographic North) and Magnetic North.

True North

Values of Magnetic Declination (DEC) change with time and location. As the movement of Magnetic North

(MN) is constant and predictable, Magnetic Declination can be calculated for any given point on the earth

at any given time.

Charts of the various magnetic declinations and rate of change are widely used.

Easterly Magnetic Declination values are expressed as a positive value and Westerly Magnetic Declination values are expressed as a negative value. Although converting from Grid (GN) to True North (TN) appears a simple task, it requires considerable careful attention, depending on the relative directions of Grid Convergence and the Magnetic Declination value. Grid convergence is the angle between a True North direction and Grid North direction. Before magnetic survey data can be converted to True North, the data at each survey point must indicate the direction of the wellbore at that point. Calculating the wellbore azimuth for each point provides the direction.

1.1 Converting Magnetic North to True North

Work through the example problem to learn how to calculate True North from a magnetic survey.

Example problem: If the observed magnetic survey point is located at N40E and the Magnetic Declination is 3.4 West, convert the Magnetic North direction to the equivalent True North (TN) direction.

Use the following steps to solve the problem.

First: Change the magnetic survey (N40E) to magnetic azimuth.

Second: Calculate True North (MN). (True North = True Azimuth), (Magnetic North =

Magnetic Azimuth)

As a result of working through this example problem you should have learned how to convert the

Magnetic North direction to the equivalent True North direction.

Study Questions

1) If an observed magnetic survey point is located at N38W and the Magnetic Declination is 3.4 West, correct the direction to True North.

Select the correct answer.

a) 45 AZM

b) 226.72 AZM

c) 235.3 AZM

d) 318.6 AZM

2 Grid NorthGiven a True North (TN) survey and Grid Convergence (GRID), convert the True North direction to the

equivalent Grid North (GN) direction.

The well proposal is usually derived from coordinates in a grid system; therefore, directions are referenced to Grid North.

However, the well surveys use sensors that reference either Magnetic or True North, and the driller must be able to convert from one to the other.

A Local Coordinate System is one requiring accurate plotting of Grid North. In many cases, government reports require the use of Grid North.

2.1 Converting True North to Grid North

Grid North is the direction of a line from any geographical location within a grid system paralleling the Universal True Meridian as determined by observation of Polaris.

Calculate Grid North with this formula: Grid North (GN) = True North (TN) Grid Convergence (GRID).

Grid Convergence is the angle between a True North direction and Grid North direction.

A magnetic survey is a directional survey in which the direction of the wellbore at the survey point is determined by a magnetic compass detecting the earths magnetic field, indicating a Magnetic North direction.

Calculate True North with this formula: True North (TN) = Magnetic North (MN) + Magnetic Declination value (DEC).

Magnetic North is the direction of a line from any geographical location on the earths surface to the North Magnetic Pole.

Magnetic Declination value is the angular difference, east or west, at any geographical location, between True North (Geographic North) and Magnetic North.

Converting True North to Grid North

Work through the example problem to learn how to convert a True North direction to a Grid North direction.

Example problem: The direction of a wellbore at a magnetic survey point is S23E, the Magnetic

Declination is 3.4 West, and the Grid Convergence is 8 West. Correct the direction to Grid North.

Use the following steps to solve problem.

First: Change the magnetic survey (S23E) to magnetic azimuth.

Second: Calculate the Grid North (GN) direction. (True North = True Azimuth), (Magnetic

North = Magnetic Azimuth)

As a result of working through this example problem you should have learned how to convert the

True North direction to the equivalent Grid North direction.

Study Questions

2) If the observed magnetic survey direction is N38W and the Magnetic Declination is 5

West, and the Grid Convergence is 3 East, correct the direction to Grid North.

Select the correct answer.

a) 326.72 AZM

b) 314 AZM

c) 136 AZM

d) 120 AZM3) If the observed magnetic survey direction is S40W and the Magnetic Declination is 8

East, and the Grid Convergence is 10 East, correct the direction to Grid North.

Select the correct answer

.

a) 120.91 AZM

b) 218 AZM

c) 13634 AZM

a) 26.72 AZM