Earth's Magnetic Field and Clinometer

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Questions and Answers

What is the primary purpose of a Brunton compass?

To calculate the rotation of the Earth

To obtain clinometer readings and measure azimuth angles (correct)

To measure ground distances accurately

To determine magnetic declination

What does a Brunton compass provide a resolution of in terms of vertical angles?

15 arcminutes

30 arcminutes (correct)

45 arcminutes

1 degree

What does magnetic declination represent?

The deviation of the Earth's axis from its original position

The difference between true north and magnetic north (correct)

The rotation angle of the Earth

The angle between the Brunton compass needle and the true north

In what year did D.W. Brunton receive a patent for his compass design?

1894 Signup and view all the answers

Which feature of the Brunton compass makes it convenient for field geologists?

Short sight with peephole Signup and view all the answers

What aspect of measurements is a Brunton compass accurate to within azimuth?

1/2 degree (30 minutes) Signup and view all the answers

What is the back bearing when the compass bearing to a destination is 90 degrees?

270 degrees Signup and view all the answers

If the azimuth notation mentions N25oE, what would be the equivalent in quadrant notation?

N25oW Signup and view all the answers

When measuring a ground distance, what is the importance of distinguishing between horizontal distance and slope distance?

Slope distance accounts for elevation changes Signup and view all the answers

What is the azimuth notation for a compass bearing of 300 degrees?

30o Signup and view all the answers

In map notation, what does the term 'strike' refer to?

Direction of a planar structure on a map Signup and view all the answers

If a dip is measured as 45 degrees SE, in which quadrant does this angle fall?

SE Signup and view all the answers

How can you calculate the height of an object using a clinometer?

Height = distance * tan(angle) Signup and view all the answers

What is the purpose of a Brunton compass in geology?

Determining inclination angles Signup and view all the answers

What is the correct formula to calculate height from a distance and angle?

Height = x / tan q Signup and view all the answers

In azimuth bearing, which direction does 0° refer to?

North Signup and view all the answers

What is the key function of an orienteering compass used by geologists?

Determining azimuth bearings Signup and view all the answers

When measuring vertical angles with a clinometer, what should be done to ensure accurate readings?

$20^o$ should be subtracted from the reading Signup and view all the answers

Study Notes

Brunton Compass

The Brunton compass was designed by Colorado mining engineer D.W. Brunton in 1894 and became widely used throughout the world.
It combines the principles of a surveyor's compass, prismatic compass, clinometer, hand level, and plumb.
It is accurate to 1/2 degree (30 minutes) in azimuth and has a 10 arcminute resolution for vertical angles.

Parts of a Brunton Compass

Short sight with peephole
Compass box with vertical line
Long sighting arm
Magnetic declination adjustment screw
Parts of a compass face front peephole

Magnetic Declination

Magnetic declination is the difference between true north and magnetic north.
Example: a bearing of 42° would be northeast and a bearing of 200° would be southwest.

Back-Bearing or Back-Azimuth

A back-bearing or back-azimuth is a projection of the azimuth or bearing from the origin to the opposite side of the azimuth circle.
To calculate the back bearing, add 180 degrees to the compass bearing to your destination if the bearing is less than 180 degrees.

Measuring Ground Distances

Horizontal distance vs. slope distance

Map Directions

Azimuth notation: 0°
Quadrant notation: N72°E
Example: N40°W
180°
Azimuth notation: 072°
Quadrant notation: N40°W

Map Notation of Planar Structures

Strike: 040°
Dip: 30° SE or 120° 30°

Navigation involves the determination of position and direction.

Clinometer Scale

A clinometer is an instrument used to measure an angle of inclination or elevation.
The clinometer of the compass gives the capability of measuring vertical angles both below and above your line of sight.
Formula: Height = x tan q, where "x" is equal to the distance and "q" is the angle in relation to the ground and the top of the object.

Getting the Inclination Angle

Using the compass clinometer, inclination values can be positive (+) or negative (-) relative to horizontal.

Orienting Compass

Another field compass used by geologists is called an orienteering compass.

Bearing

A bearing is a measurement of direction between two points, measured in degrees, from the reference line of North.
Bearings are generally given in one of two formats, an azimuth bearing or a quadrant bearing.

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Description

Test your knowledge on Earth's magnetic field generation and the use of a clinometer in measuring vertical angles. Learn about convection in the outer core and how it leads to the creation of a magnetic field, as well as the practical application of a clinometer in surveying.