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Land Nav Task 10 - Orient a Map Using a Lensatic Compass

Standards: Oriented the map to the ground using a compass so the north-seeking arrow of the compass was within 3 degrees (20 mils) of the angle shown in the grid-magnetic (G-M) angle of the declination diagram shown on the map.

Conditions: Given a field table, a standard 1:50,000-scale military map, a protector, a pencil, paper, and a compass in daylight.

Standards: Oriented the map to the ground using a compass so the north-seeking arrow of the compass was within 3 degrees (20 mils) of the angle shown in the grid-magnetic (G-M) angle of the declination diagram shown on the map.

 

Performance Steps

1.   With the map level, place the compass parallel to a north-south grid line with the cover side of the compass pointing towards the top of the map. This will place the black index line on the dial of the compass parallel to grid north. Because the needle on the compass points to magnetic north, a declination diagram on the face of the compass is formed by the index line and the compass needle.

2.   Rotate map and compass until the directions of the declination diagram formed by the black index line and the compass needle match the direction shown the declination diagram printed on the margin of the map. The map is then oriented.

3.   If the magnetic north arrow on the map is to the left of the grid north, the compass reading will equal the G-M angle (given the declination diagram). If the magnetic north is the right of the grid north, the compass reading will equal 360 degrees (6400 mils) minus the G-M angle.

4.   Remember to point the compass north arrow in the same direction as the magnetic north arrow (step 2 above), and the compass reading (equal to the G-M angle or the 360 degrees (6400 mils) minus G-M angle) will be quite apparent.

Note. If the G-M angle is less then 3 degrees (50 mils), do not line up the north arrow.

5.   Some maps have a built-in protractor considering of a pivot point "P" on the south neat line of the map and several degrees of arc along the north neat line of the map. The G-M line is obtained by connecting pivot "P" with the appropriate value of the G-M angle (taken from the declination diagram) on the arc. The map may then be oriented by placing the compass parallel on this line and rotating the map and compass until the needle point is aligned with the continuous line formed by the index line and the sighting wire. The map is then oriented.

6.   An alternate method is to draw a magnetic north line on the map from any N-S and E-W grid line intersection using the protractor. Align the straightedge of the compass along this magnetic north line and rotate the map and compass together until the north arrow falls beneath the fixed black index line on the compass.

Evaluation Preparation: 

Setup: Select an area that is free of magnetic interference (parlance, vehicles). Provide a field table, 1:50,000-scale military map, a protractor, a pencil, paper, and a compass.

Brief Soldier: Tell the soldier to orient the map to the ground using the compass. Tell the soldier he must use the G-M angle shown by the declination diagram if it exceeds 3 degrees (50 mils).

Performance Measures

GO

NO GO

1.   Determined whether G-M angle exceeds 3 degrees (50 mils)

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2.   Aligned the side of the compass with one of the north-south grid lines.

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3.   Positioned the cover of the compass toward the top of the map.

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4.   Oriented the map.

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5.   Corrected the orientations of the map when the G-M angle exceeded 3 degrees (50 mils) using one of the following:

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a. Used the G-M angle formed by the compass' black index line and the compass needle.

 

 

b. Used the pivot point "P" on the south neat line and the degrees of arc along the north neat line and placed the compass along this line.

 

 

c.  Drew a magnetic north line from any N-S and E-W grid the intersection using a protractor and placed the compass along this line.

 

 

Note. Step 5 is required only when G-M angle exceeds 3 degrees or 50 mils. Step 5b is tested only if the map has the built-in protractor.

 

 

Evaluation Guidance:  Score the soldier GO if all performance measures are passed. Score the soldier NO GO if any performance measure is failed. If the soldier scores NO GO, show what was done wrong and how to do it correctly.

References

 

Required

Related

 

FM 3-25.26