Surveying Compass

Table of Contents
1. Surveying Compass
2. Main components of a compass
3. Surveyor's compass (box compass)
4. Prismatic compass
5. Comparison - Surveyor's compass versus Prismatic compass
6. Bearing systems
7. Procedure for taking a bearing with a prismatic compass
8. Temporary adjustments and precautions
9. Sources of error and limitations
10. Care, maintenance and handling
11. Applications
12. Meridian
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Surveying Compass

Surveying compass is an instrument used to determine the direction of survey lines with reference to the magnetic meridian. Two principal types are used in field surveying:

• Surveyor's compass (also called box compass).
• Prismatic compass.

Main components of a compass

• Magnetic needle - a pivoted needle that indicates the magnetic north-south direction.
• Graduated ring - circular scale graduated in degrees used to read bearing values.
• Sighting vanes - object vane and eye vane (or an optical slit and prism in prismatic type) used for accurate line of sight.
• Reading system - arrangement (glass top, prism or vernier) that permits reading the graduated ring while sighting.
• Tripod or handle - a tripod may be used for stability; some prismatic compasses can be hand-held.

Surveyor's compass (box compass)

• Needle: Edge-bar type magnetic needle which often acts as the index pointer for reading.
• Graduated ring: Attached to the compass box and does not rotate with the needle; graduations are normally marked for the Quadrantal Bearing system.
• Graduation system: Quadrantal Bearing (Q.B.) system is commonly used with this instrument; bearings are expressed as an angle from the nearest north or south towards east or west and range from 0° to 90°.
• Sighting vanes: Object vane with a vertical hair and eye vane with a fine slit for aligning the line of sight.
• Reading: Readings are taken through the top glass of the box; sighting and reading cannot be performed exactly simultaneously from the same eye position.
• Support: Requires a tripod or firm support for accurate work; not convenient for hand-held use.

Prismatic compass

• Needle: Broad needle type; the needle does not itself act as the index.
• Graduated ring: Usually attached to the needle assembly so that the ring rotates with the needle pointer.
• Graduation system: Whole Circle Bearing (WCB) graduations are commonly provided; readings are taken in degrees of a full circle (0°-360°).
• Sighting arrangement: Object vane with vertical hair and an eye slit with an optical prism located at the eye slit to allow simultaneous sighting and reading.
• Reading: Readings are observed by looking through the eye slit; prism brings the scale into view so the observer can sight and read at the same instant.
• Support: May be used on a tripod or hand-held; portable and convenient for traverse surveys.

Comparison - Surveyor's compass versus Prismatic compass

• Needle: Surveyor's compass uses an edge-bar needle which often serves as an index; prismatic compass uses a broad needle and a separate index.
• Graduated ring attachment: In surveyor's compass the ring is fixed to the box; in prismatic compass the ring is usually attached to the needle assembly.
• Reading method: Surveyor's compass readings are taken through the top glass and sighting and reading are separate operations; prismatic compass allows simultaneous sighting and reading through a prism at the eye slit.
• Graduation system: Surveyor's compass commonly uses Quadrantal Bearing (Q.B.); prismatic compass commonly provides Whole Circle Bearing (WCB).
• Use support: Surveyor's compass normally requires a tripod; prismatic compass can be used on a tripod or held in the hand.

Bearing systems

• Magnetic meridian: The direction of the magnetic needle (magnetic north-south line) at a place; bearings measured with respect to this line are magnetic bearings.
• Quadrantal Bearing (Q.B.): Bearing expressed as the angle (0°-90°) measured from the north or south direction towards east or west and written in the form N θ E, N θ W, S θ E or S θ W where θ is between 0° and 90°.
• Whole Circle Bearing (WCB): Bearing expressed as an angle measured clockwise from the north direction between 0° and 360° (for example, 0°, 90°, 180°, 270° correspond to N, E, S, W respectively).

Conversion between Quadrantal Bearing (Q.B.) and Whole Circle Bearing (WCB)

Rules for conversion:

• If Q.B. is N θ E, then WCB = θ.
• If Q.B. is N θ W, then WCB = 360° - θ.
• If Q.B. is S θ E, then WCB = 180° - θ.
• If Q.B. is S θ W, then WCB = 180° + θ.

Example: Convert N 30° E to WCB and convert S 25° W to WCB.

The worked conversions are given step-wise below for clarity:

N 30° E →

WCB = 30°

S 25° W →

WCB = 180° + 25°

WCB = 205°

Procedure for taking a bearing with a prismatic compass

1. Level the instrument on tripod (or hold steadily if hand-held).
2. Centre the compass over the station point and ensure the compass is approximately level.
3. Look through the sight slit and bring the object into line with the vertical hair on the object vane.
4. While keeping the object in the line of sight, read the bearing from the graduated ring using the prism; record the reading as the magnetic bearing of the line.
5. Repeat the reading by reversing the instrument or by taking reciprocal bearing to check errors due to local attraction.

Temporary adjustments and precautions

• Ensure the compass is approximately level before taking readings; excessive tilt causes errors.
• Centre the compass exactly over the survey station mark to avoid parallax and centring errors.
• Keep the instrument away from metallic objects, iron structures, rechargeable batteries and magnetic sources to avoid local attraction.
• Check the pivot and jewel for free movement of the needle; friction at the pivot causes large errors.
• Take repeated readings or reciprocal bearings to identify and reduce systematic errors.

Sources of error and limitations

• Local attraction: Presence of nearby iron or magnetic materials disturbs the needle and gives incorrect bearings.
• Pivotal friction: Worn or dirty pivot/jewel increases friction and prevents the needle from settling accurately.
• Parallax and miscentring: Failure to centre the instrument over the station or incorrect eye position for top readings causes parallax errors.
• Instrument not level: Tilt of the compass changes the effective direction of the needle and introduces error.
• Magnetic declination variation: The difference between true north and magnetic north (declination) causes magnetic bearings to differ from true bearings; must be allowed for when converting to true bearings.
• Sensitivity and accuracy: Compasses are less accurate than theodolites or total stations; suitable for rapid traversing and approximate work but not for high-precision surveys.

Care, maintenance and handling

• Keep the compass dry and store in a non-magnetic case.
• Avoid dropping or striking the instrument; shocks may damage the pivot and jewel.
• Regularly clean the pivot and jewel; lightly oil moving parts as recommended by manufacturer.
• Periodically check for local attraction and compare instrument readings with a known reference line.

Applications

• Rapid traverse surveys where speed and portability are important.
• Reconnaissance surveys and preliminary alignment work.
• Field work in remote areas where sophisticated instruments are unavailable.

Meridian

Meridian - a meridian in surveying denotes a fixed reference direction on the surface of the earth with respect to which the bearings of survey lines are expressed. When the magnetic needle is used as the reference, the direction is called the magnetic meridian; when the true geographic north is used, the direction is called the true meridian.