Prismatic Compass Survey

Prismatic Compass Surveying: A Comprehensive Guide

Prismatic compass surveying is a fundamental surveying technique used to determine the bearings of lines and angles between points. This method, employing a prismatic compass, provides a relatively simple and efficient way to conduct surveys, particularly in areas where high accuracy isn't paramount. This comprehensive guide will delve into the intricacies of prismatic compass surveying, covering its principles, procedures, applications, advantages, disadvantages, and frequently asked questions. Understanding this method is crucial for anyone involved in land surveying, construction, or geographical mapping.

Introduction to Prismatic Compass Surveying

Prismatic compass surveying relies on the principle of measuring magnetic bearings. A prismatic compass is a specialized instrument used to measure the horizontal angle between a magnetic north and a target point. Unlike the theodolite which measures angles directly between two points, the compass measures the angle relative to magnetic north. This magnetic bearing, along with measured distances, enables surveyors to create detailed maps and plans. The simplicity and portability of the prismatic compass make it ideal for various applications, especially in areas with challenging terrain or where electronic equipment is impractical or unavailable.

Components of a Prismatic Compass

Before delving into the surveying process, let's understand the key components of a prismatic compass:

• Magnetic Needle: This is the heart of the compass, a magnetized needle that freely pivots and aligns itself with the Earth's magnetic field, indicating magnetic north.

• Compass Ring: The graduated ring surrounding the needle displays the bearing values, typically in degrees from 0 to 360.

• Prism: This optical device allows for comfortable reading of the bearing directly from the compass without needing to bend over. The prism reflects the image of the graduated ring.

• Sighting Vane: The sighting vane is used to align the compass with the target point. It typically includes a narrow slit or a fine wire for precise sighting.

• Object Glass: This lens helps to magnify the graduated scale, improving accuracy of reading.

• Leveling Screws: These screws allow for leveling the compass, ensuring accurate readings.

Procedure for Prismatic Compass Surveying

The procedure for prismatic compass surveying can be broken down into several key steps:

1. Setting up the Compass: The compass should be leveled using the leveling screws, ensuring the magnetic needle swings freely and settles at its equilibrium position.

2. Taking a Bearing: Aim the sighting vane at the target point, ensuring the observer's eye is aligned correctly with the slit or wire. The bearing is read directly from the graduated ring via the prism.

3. Reading the Bearing: The reading on the prismatic compass is the magnetic bearing of the line of sight. It is essential to record this bearing accurately in the field book.

4. Fore and Back Bearings: For every line surveyed, both fore and back bearings should be taken. The fore bearing is the bearing taken from the instrument station to the target point, while the back bearing is the bearing taken from the target point back to the instrument station.

5. Local Attraction: The presence of ferrous metals or electrical equipment near the compass can significantly affect readings, causing local attraction. To account for this, both fore and back bearings are measured. The difference between the fore and back bearings should ideally be 180 degrees (or 0 degrees). Significant deviations indicate the presence of local attraction.

6. Field Book Recording: All measurements, including bearings, distances, and relevant observations, should be meticulously recorded in a field book. This ensures data integrity and facilitates easy calculations later on.

7. Traverse Surveying: Prismatic compass surveying is often used to create a traverse, a series of connected lines whose bearings and lengths are measured to create a map. The traverse method is widely used for mapping and boundary surveys.

8. Plotting the Traverse: After collecting all field data, the traverse is plotted on a map using the measured bearings and distances. Geometric constructions or computer-aided drafting (CAD) software can be used for this purpose.

Calculating Angles and Coordinates

While the prismatic compass primarily measures bearings, it's crucial to calculate angles between lines for accurate mapping and analysis. This involves:

• Interior Angles: These are the angles formed within a closed traverse. Their sum should ideally equal (n-2) x 180 degrees, where 'n' is the number of sides of the traverse. Any discrepancy points to potential errors in measurements.

• Correction for Local Attraction: If local attraction is detected, corrections need to be applied to the bearings to eliminate the systematic error caused by the magnetic influence of nearby metallic objects. A systematic approach involves adjusting the observed bearings to minimize discrepancies.

• Coordinate Calculation: After correcting for local attraction and computing interior angles, the coordinates of each point in the traverse can be computed using trigonometry or coordinate geometry techniques. This process transforms the angular and distance measurements into a set of (x, y) coordinates for each point.

Advantages and Disadvantages of Prismatic Compass Surveying

Advantages:

• Simplicity and Ease of Use: The prismatic compass is relatively simple to use, requiring minimal training.

• Portability and Lightweight: It's highly portable and lightweight, making it suitable for surveying in difficult terrains.

• Cost-Effectiveness: Prismatic compasses are generally less expensive than electronic surveying instruments.

• Suitable for Reconnaissance Surveys: Its quick measurement capability makes it ideal for initial reconnaissance surveys and preliminary mapping.

Disadvantages:

• Lower Accuracy Compared to Theodolites: The accuracy of prismatic compass surveying is lower than that achieved using theodolites or electronic total stations.

• Susceptibility to Magnetic Disturbances: Local attraction significantly affects readings, demanding careful observation and correction procedures.

• Not Suitable for High-Precision Surveys: Due to inherent limitations, it is not suitable for surveys demanding high precision, such as those needed in large-scale construction projects or cadastral surveys.

• Dependence on Magnetic North: Variations in the Earth's magnetic field can introduce errors in measurements. The use of magnetic declination correction is essential for accurate results.

Applications of Prismatic Compass Surveying

Prismatic compass surveying finds applications in several areas, including:

• Topographic Mapping: Creating maps of land surfaces, including contours and features.

• Route Surveys: Planning routes for roads, railways, and pipelines.

• Preliminary Surveys: Conducting initial site surveys for construction projects.

• Boundary Surveys: Establishing and defining property boundaries, particularly in less demanding scenarios.

• Mine Surveying: Used in underground mining operations for mapping tunnels and galleries.

• Hydrographic Surveying: Surveying bodies of water and determining depths and shoreline characteristics.

Frequently Asked Questions (FAQ)

Q1: What is the difference between a prismatic compass and a surveyor's compass?

A1: While both measure magnetic bearings, a prismatic compass utilizes a prism for direct reading, providing a more convenient and comfortable viewing experience. Surveyor's compasses often require the user to look directly down at the needle and graduated ring.

Q2: How can I correct for magnetic declination?

A2: Magnetic declination is the angle between true north and magnetic north. To correct for this, the declination angle must be added to or subtracted from the observed magnetic bearing, depending on the direction of declination (east or west). This information is typically obtained from local magnetic charts or surveys.

Q3: What are the common sources of errors in prismatic compass surveying?

A3: Common sources of error include incorrect leveling, local attraction, faulty sighting, reading errors, and variations in the Earth's magnetic field.

Q4: How can I improve the accuracy of prismatic compass surveying?

A4: Accuracy can be improved by ensuring proper leveling, minimizing the impact of local attraction, employing careful sighting techniques, taking multiple readings, and applying appropriate corrections for declination and other systematic errors.

Conclusion

Prismatic compass surveying, despite its limitations compared to more advanced techniques, remains a valuable surveying method for many applications. Its simplicity, portability, and cost-effectiveness make it an ideal tool for various types of surveys, particularly those where high precision is not the primary concern. By understanding its principles, procedures, and potential sources of error, surveyors can effectively utilize this technique to conduct accurate and efficient surveys across a wide range of environments and projects. The information provided here aims to equip readers with a comprehensive understanding of this fundamental surveying method, enhancing their capabilities in land surveying and related fields. Further study and practical experience will solidify this knowledge and build proficiency in this essential surveying technique.