Civil Engineering: Compass Surveying

• Lecture acts as a crash course for civil engineering students, specifically in surveying, with compass surveying as a new chapter.
• Compass survey questions are common in the SSC JE exam.
• Topics include problems on included angles, declination calculations, and determining declination using the sun's position.
• Previous lectures covered errors and corrections, offset length limits, and the minimum error detectable by the human eye on paper (0.025 cm) from linear.

Key Topics in Compass Survey

• Declination, including magnetic vs. true bearing is crucial.
• Local attraction and its effects.
• Included angles and their calculation

Compass Survey Introduction

• Compass and chain/tape are used to measure distances and bearings.
• Compass determines the direction of magnetic north and south.
• Chain or tape measures distances.
• Bearings (angles) are measured using the compass

Meridians (Reference Directions)

• Meridian defines a direction.
• True meridian: Connects true north and south based on the Earth's rotational axis.
• Magnetic meridian: Connects magnetic north and south, influenced by the Earth's magnetic field and changing over time.
• Arbitrary meridian: Assumed direction based on local landmarks when true north is unknown.
• Grid meridian: Used for surveying large areas and are based on the true meridian through the area's center.

Bearings (Horizontal Angles)

• Bearing is a horizontal angle measured from a reference direction.
• True bearing: Uses true meridian as reference.
• Magnetic bearing: Uses magnetic meridian as reference.
• Grid bearing: Uses grid meridian as reference.
• Arbitrary bearing: Uses arbitrary meridian as reference.

Representation of Bearings

Whole Circle Bearing (WCB)

• Abbreviated as WCB.
• Uses North as the reference direction.
• Angles are measured clockwise.
• Values range from 0 to 360 degrees.
• Typically measured using a prismatic compass.

Quadrant Bearing (QB)

• Abbreviated as QB.
• Uses North or South as the reference, whichever is nearer to the survey line.
• Direction is clockwise or counterclockwise.
• Angle values are between 0 and 90 degrees.
• Expressed as Reference Direction - Angle - Direction (e.g., South 30 degrees East).
• A WCB of 150 degrees is equivalent to a QB of South 30 degrees East.

Declination Concept

• Declination is the horizontal angle between true north and magnetic north.
• Eastern declination: Magnetic north is east of true north.
• Western declination: Magnetic north is west of true north (negative declination).
• True bearing = Magnetic bearing - Declination (for Western declination).
• True bearing = Magnetic bearing + Declination (for Eastern declination).
• Isogonic lines: Connect points with the same declination on a map.
• Agonic line: Connects points with zero declination.
• Diagrams help to visualize and solve declination problems.

Declination problem example:

Scenario Analysis

• Two magnetic bearings of lines are measured on a map.
• Old map data includes a magnetic bearing and declination.
• Present data provides an updated declination; true north remains constant.

Process to Solve

• Calculate the true bearing using the old data.
• Use the calculated true bearing with the present data to find the new magnetic bearing.

Additional Notes

• Accuracy is increased by drawing a clear diagram.
• True bearing remains constant between datasets in "old map/new map" problems.

Noon Time Declination Problems

• At noon, the sun is directly north or south.
• Southern Hemisphere: Sun is at the South (true bearing is 180 degrees).
• Northern Hemisphere: Sun is at the North (true bearing is 360 degrees).

Declination Calculation at Noon

• If magnetic bearing is greater than true bearing, declination is westward.
• If magnetic bearing is less than true bearing, declination is eastward.

Differentiating Differences in Location

• Find the difference between magnetic bearing and true bearing to determine the declination angle.
• True North is 0/360 degrees, and True South is 180 degrees.

More on Practical Declination

• Declination varies by location.
• Always draw a diagram and double-check east or west.

Dip Angle

• Dip angle is the vertical angle between Earth's surface and magnetic field lines.
• Results from Earth's magnetism and core.
• Affects compass needles.

Variation in Value

• 90 degrees at magnetic poles.
• 0 degrees at magnetic equator.

Related Terms

• Isoclinic lines: Connect equal dip angles.
• Acinic line: Zero dip along equator.

Compass Types

• Surveyors use Prismatic and Surveyor's compasses.

Prismatic Compass

• Broad needle shape.
• Needle and graduated circle are attached.
• Used for WCB.
• Graduations marked with 0° at South, increasing clockwise.
• Sighting and reading are simultaneous due to the prism

Surveyor's Compass

• Has an edge bar needle.
• Needle moves independently of the graduated circle.
• Used for QB.
• Graduations: 0° at North and South, 90° at East and West.
• Sighting and reading are not simultaneous.

Local Attraction

• Local attraction is a disturbance affecting compass needle.
• Causes: magnetic objects (steel, iron), vehicles, current-carrying wires.
• Assess and correct for errors.
• The difference between the four and back bearings should be 180.

Related Terms

• Fore bearing: Bearing measured in the direction of the survey.
• Back bearing: Bearing measured from the opposite end of the line.
• Calculate: Back bearing from known fore bearing.
• Compare: Measured bearings to calculated.
• Identify: Station with local attraction.
• Correct: Affected bearings.
• Find: Station without issue.

Calculating

• If fore bearing < 180°, add 180°.
• If fore bearing > 180°, subtract 180°.
• Reverse (quadrant bearing): Switch North/South, East/West.

Correction Steps

• If a measured tape value is wrong, deduct this from what actually happened.
• Fix the station.

Included Angles

• Include internal and external angles.
• Measured from previous line to next line.
• Measurement is done clockwise.

Formulas

• Sum of exterior angles: (2n + 4) * 90 degrees.
• Sum of interior angles: (2n - 4) * 90 degrees.

Angle Calculation

• Included Angle = Fore Bearing (Next Line) - Back Bearing (Previous Line)
• Result Options:
• If negative, add 360.
• If > 360, do not use (adjust).
• Check: Use angles for bearing calculations.

Problem Summary:

• Solve "Bearing of Initial Line Given" problems.
• Remember: angle = FB (next line) - BB (previous line).

Notes on Review and Learning Style

• Students are encouraged to solve previous year questions for practice.
• Students are encouraged to apply unique methods.