Field compasses The Brunton compass. Field compasses Tow major types of compasses are current in use...

Field compasses

The Brunton compass

Field compasses

Tow major types of compasses are current in use by the geologic field work. – The Brunton compass or pocket transit – The orienteering-type compasses

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Brunton compass

The bearing scale fixed to the outside body of the compass.

The east and west bearing marks appear to backward.

Brunton compass Orienteering-type compass

Less versatile in measuring versatile angles.

Less precise in bearing measurements.

Much cheaper than Brunton-type compass and is lightweight.

Rapid for bearing and dip measurements but however less precise than Brunton compass.

It lacks a leveling system for sighting-bearings and thus is not as precise as the Brunton compass.

Compasses work because the earth acts like a giant bar magnet.

Motions in the liquid nickel-iron core of the earth induce a magnetic field with a north and south pole.

Magnetic lines of force connect the earth's north and south magnetic poles.

Compasses work because a magnetized compass needle will align itself with the earth's magnetic lines of force and point approximately north.

I said approximately because the north and south magnetic poles don't exactly align with the earth's axis of rotation which defines the north and south geographic poles.

• The magnetic needle points freely to the magnetic North Pole of the earth.

• Make sure that: – The compass needle swings freely.– The hinges of the sighting arm and lid are firm.– The point of the sighting arm touches the axial line of

the mirror when the arm and lid are turned together. – The clinometer level should be centered when the

compass set at vertical angle index of 0˚.• Place the compass on a smooth surface that has been leveled with

a long carpenter's level. • If the tube bubble dose not move to center, the glass cover must be

removed and level-mount rotated gradually until the bubble is centered.

Instructions of Brunton compass

– The needle of the compass is induced by the Magnetic North Pole which changes usually and sometimes differs from the True North Pole that is used in mapping since it never changes.

– The angle between Magnetic North Pole and True North is called magnetic declination.

Instructions of Brunton compass, cont.

– Compass needle does not usually point due north, but at some angle east or west of north.

– This is because the earth's geographic pole (the axis about which it rotates) is not in the same place as its magnetic pole (the place where the magnetic lines of force emerge from the earth). The direction to the earth's geographic pole is called true north and the direction to the earth's magnetic pole is called magnetic north.


– if you're using a compass somewhere on the surface of the earth, you have to account for this difference between magnetic north (where your compass is pointing) and true north (which you need to know).

– The angle between true north and magnetic north is called the magnetic declination and changes with your location and, at any one location, with time.


– The compass graduated circle should be corrected for the declination angle before usage.

• Rotate the compass graduated circle with the slotted screw that extends through the side of the case until zero degree of the rotated circle aligned to the magnetic north. Then the declination angle is the difference between the rotated zero degree and the index pen that is located at the base of the sighting arm. The correction is done by adding or subtracting the magnetic declination from all bearings taken with a magnetic compass.


• For example, a magnetic declination 15deg; east of true north, you would turn the compass ring such that the index pin was over 15deg; (i.e. 15deg; to the east side of north).


• For a 15deg; west declination, you would turn the compass ring such that the index pin was over 345deg; (i.e. 15deg; to the west side of north).


– The compass circle is numbered in reverse sense where the east and west are therefore interchanged and the numbers run from 0˚ to 360˚ counterclockwise . This allowed the magnetic needle's north seeking directly to the angle on the compass circle to which the needed direction is truly pointing.

– The direction measurements:• Bearing: for example N30˚W, S20˚E ….etc• Azimuth: for example 030˚, 120˚, 320˚ ….etc


- Effect of magnetic anomalies during taking bearing- Magnetic anomalies may be cause by any nearby iron-bearing

objects or by magnetic rocks. - Any suspect object can be tested by bringing it close to the

leveled compass when the needle is at rest. Local magnetic rock bodies oscillates gradually the needle.

- The actual magnetic anomaly can be determined by locating the point precisely on a topographic map and taking a compass bearing to another point that can also be located on the map.

- The compass bearing is then compared with the true bearing.- The angle between the true bearing and compass bearings is

determined by a protractor. - In areas where magnetic anomalies are too large even it covers vast

areas, the sun compass is used where it measures bearing by means of a linear shadow cast by the sun. Such as areas of dominated by mafic and ultramafic rocks


• The following points are applied for accurate bearing for a visible object is between 45˚ above and 15˚ below the observer, the compass is held horizontally at the waist height:

• Open the compass and held it a horizontal position, waist height, be careful that your belt buckle does not affect the magnetism. The laid opened toward the field worker at approximately 45˚.

• Sighting arm is set vertically that is perpendicular to the bottom case and on the side away from the observer. Be sure that the compass is cupped firmly in both hands with both forearms pressed firmly against the body.

• The instrument is correctly sighted on the object when the user, looking down into the mirror, sees the black centerline of the mirror bisecting both sighting arm and the object.

• At this time, the bull's-eye level is centered.

• The north seeking end of the needle, the white end, then points to the bearing of the object on the graduated scale.

• Note, some field workers prefer to have the front sight laid-out horizontal.

• In this case the centering bull's-eye level is followed by adjusting the lid until the sighted tip and the point sighted appear in the mirror.

• The sighting arm may be turned upward as necessary.

• Rotate the leveled compass on a vertical axis until the mirror images of the point sighted and the sighting tip coincide with the axial line of the mirror.

• Check the bull's-eye level and read the bearing.

Taking a compass bearing

• Object is between 45˚ above and 15˚ below the observer, the compass is held

horizontally at the waist height:

Taking a compass bearing

• If the sighted object is located with more than 15˚ below the observer, the compass is led also at waist height with a new setting as the followings:

• The sighting arm is positioned toward the body of the observer and the mirror is away.

• The sighting arm is tilted over the bottom case at 45˚ and the mirror is tilted back at the same angle.

• Sight over the sighting arm and throw the window opening in the cover (mirror) and near the hinge.

• The mirror and the sighting arm are then adjusted so that the image of the sighting arm can be seen in the mirror bisecting the centerline of the mirror.

• The tip of the sighting arm and the mirror centerline are then aligned with the object.

• The azimuth or bearing is read at south (black) end of the needle.

Taking a compass bearing, cont.

• If the sighted object is located with more than 15˚ below the observer, the compass

is led also at waist height with a new setting.


• Sometimes some circumstances may be encountered between the point of observation and the object being sighted; in this case the compass should be held at eye level:

• The compass is held horizontally and the sighting arm is set vertically and slightly away from the face.

• The sighting tip of the sighting arm is positioned straight up.

• Some field workers prefer to have the sighting arm laid horizontally, pointing toward their eye.

• In either case, the mirror is angled about 45˚ toward the observer, over the compass body.

• The object is sighted through the sighting arm and over the mirror or through the window.

• Centering of the bull's-eye level at this time by observing it in the mirror. The needle can be seen in the mirror and the azimuth or bearing determined.

• Sighting should be read from the south end (black) of the needle.


• If object located at eye level or something may obstacles sighting in waist height. In this case the compass should be held at

eye level.


• A clinometer is a sighting instrument with a tube level mounted on the movable graduated arc. It is used to measure the vertical angle between the horizontal and any line of sight. It is built into the box of the Brunton compass which is used to measure a vertical angle as follows:

1. Setting the clinometer exactly at 0˚, opening the lid 45˚ and extending the sighting arm with the sighting point. Sighting tip should be lifted vertical to the sighting arm.

2. Hold the compass in a vertical plane with the sighting arm pointing toward the field worker's eye, but approximately 30-35 cm away so that the point sighted and the axial line in the sighting window can be focused clearly.

3. Rotate the clinometer by the lever on the base of the compass box until the tube bubble, as observed in the mirror, is centered.

4. Recheck the alignment, and finally bring the compass down in order to read the angle indicated by the vertical angle index.

5. If a horizontal distance is measured or is scaled from the map, a vertical angle can be used to calculate a vertical distance by relation: vertical distance = horizontal distance x tangent of the vertical angle.

6. The height of one's eye above the ground is then added for uphill and subtracted for downhill sights.

Measuring vertical angle (vertical distance)

Measuring vertical angle (vertical distance)

Hand leveling

A hand level has a tube level set parallel to the sighting axis and is designed so that the level bubble can be observed while sighting. The clinometer of the compass is converted to a hand level by bringing the clinometer to exactly zero degree. The compass is then held as for taking vertical angles and is brought to level by observing the level bubble in the mirror. All sighted points coincide with the axial line and the sighting tip will be at the observer's eye level.

The vertical distance between two points can be measured with a hand level by:1. Determining the height of one's eye above the ground.2. Standing at the lower of the two points and sighting a level line to select any convenient intermediate point at eye level.3. Walking to that point and repeating the procedure.4. Continuing similarly until the last measure before the second point which will generally be less than eye height and can be estimated or measured with a roll- up tape.5. The difference in elevation is the number of intermediate points multiplied by the height of one's eye plus the last fractional measure.

Hand leveling

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Field compasses The Brunton compass. Field compasses Tow major types of compasses are current in use...

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