Prismatic Compass Survey

Prismatic Compass Surveying: A Comprehensive Guide

Prismatic compass surveying is a fundamental surveying technique used to measure horizontal angles and bearings. It's a relatively simple yet effective method, particularly useful in areas where electronic instruments might be impractical or unavailable. This comprehensive guide will delve into the intricacies of prismatic compass surveying, covering its principles, procedures, errors, and applications. Understanding prismatic compass surveying is crucial for anyone involved in land surveying, mapping, or related fields.

Introduction to Prismatic Compass Surveying

The prismatic compass is a lightweight, portable instrument primarily used for determining the magnetic bearing of a line. Unlike the more advanced theodolites, it directly measures the magnetic bearing, making it quicker and easier to use in straightforward surveying tasks. The instrument consists of a magnetic needle freely suspended within a circular box, which is graduated in degrees (typically 0-360°). A prism is incorporated to allow for easy reading of the graduated scale, without requiring the user to shift their position. This ease of use makes it a popular choice for preliminary surveys, route surveys, and traversing. The key advantage of the prismatic compass lies in its simplicity and portability, making it ideal for fieldwork in challenging terrains.

Components of a Prismatic Compass

A standard prismatic compass comprises several key components:

• Magnetic Needle: The heart of the compass, it aligns itself with the Earth's magnetic field, indicating magnetic north.
• Circular Box: Houses the magnetic needle and the graduated scale. The scale is typically graduated in degrees (0-360°) or in gon.
• Prism: A small optical device that allows the user to read the scale while sighting along the line of sight. It also magnifies the scale for greater accuracy.
• Sighting Vane: Located at the top of the compass, it aids in aligning the compass with the survey line.
• Object Vane: Helps to align the compass with the target point.
• Leveling Screws: Used to level the compass before taking measurements.
• Clamp: Secures the compass to prevent accidental movement.

Understanding the function of each component is essential for accurate operation and maintenance of the prismatic compass.

Procedures for Prismatic Compass Surveying

The process of prismatic compass surveying involves a series of steps to ensure accurate measurement of bearings. Here’s a detailed breakdown of the procedure:

1. Setting up the Compass:

• Leveling: Carefully level the compass using the leveling screws until the air bubble in the spirit level is centered.
• Orientation: Hold the compass steady, ensuring it's not influenced by nearby metallic objects that could affect the magnetic needle.

2. Sighting:

• Object Vane: Align the object vane with the target point.
• Prism: Look through the prism, making sure the graduated scale is clearly visible.

3. Reading the Bearing:

• Magnetic Bearing: The reading you observe through the prism represents the magnetic bearing of the line. Remember that this is a magnetic bearing, not a true north bearing. Conversion to true north requires knowledge of the local magnetic declination.

4. Recording the Bearing:

• Data Recording: Carefully record the bearing in your field book, along with details like station names, date, time, and any relevant observations.

5. Repeating Measurements:

• Accuracy: For improved accuracy, repeat the measurement several times, averaging the readings to minimize errors.

6. Traversing:

• Closed Traverse: When conducting a traverse, close the traverse by returning to the starting point. The closing error should be minimal and within acceptable limits.

7. Corrections and Adjustments:

• Local Attraction: Account for local attraction (interference from metallic objects). Methods for this will be discussed later.

Accurate record-keeping is crucial for error detection and analysis in prismatic compass surveying. A detailed field book is the cornerstone of reliable survey data.

Types of Prismatic Compass Surveys

Prismatic compass surveys are employed in various surveying tasks, each demanding specific procedures and considerations.

• Traversing: This technique involves measuring a series of connected lines (legs) to determine the position of points relative to each other. Closed traverses return to the starting point, allowing for error checking. Open traverses do not return to the starting point and require additional control points for accurate positioning.
• Bearing Measurements: Simple bearing measurements are used to determine the direction of a line between two points. This is useful for establishing the alignment of features, such as roads or pipelines.
• Location Surveys: Prismatic compasses are used to determine the position of points relative to a known baseline. This helps to create a map or plan of a specific area.

The selection of the appropriate survey method depends upon the project's requirements and the available resources.

Errors in Prismatic Compass Surveying

Several sources of error can affect the accuracy of prismatic compass surveys. Understanding these errors and their mitigation is crucial for reliable results.

• Instrumental Errors: These include errors due to faulty graduation, magnetic needle not being perfectly balanced, or the sighting vanes not being perfectly aligned. Regular calibration and maintenance of the instrument can minimize these errors.
• Personal Errors: These errors arise from the surveyor's actions, such as incorrect leveling, inaccurate sighting, or misreadings. Careful observation and multiple measurements help reduce these errors.
• Magnetic Errors: These arise from local attraction (the influence of nearby magnetic objects), variations in the Earth's magnetic field (magnetic declination), and diurnal variation (daily changes in the Earth's magnetic field). These errors can be mitigated through careful site selection, correction for magnetic declination, and proper timing of measurements.
• Natural Errors: These include errors caused by atmospheric conditions such as wind or temperature changes. Careful selection of suitable weather conditions can reduce these errors.

Careful planning, meticulous execution, and proper data analysis are essential to minimize the impact of these errors on the survey’s accuracy.

Local Attraction and its Correction

Local attraction refers to the deflection of the magnetic needle caused by the presence of metallic objects or magnetic materials near the compass. This significantly affects the accuracy of bearing measurements. Identification and correction of local attraction are crucial for reliable compass surveying. Methods for detecting local attraction include:

• Back Bearing: Take readings of both the fore bearing and back bearing along the same line. Ideally, they should differ by 180°. A significant deviation indicates the presence of local attraction.
• Multiple Observations: Take several readings at different locations along the line to check for consistency. Inconsistent readings could signify local attraction.
• Careful Site Selection: Avoid areas with known sources of magnetic interference whenever possible.

If local attraction is detected, it can often be corrected by using the following method. If the back bearing and the fore bearing differ by more than 180 degrees, the mean difference is divided by 2 and then added or subtracted accordingly to the bearings obtained.

Magnetic Declination and its Correction

Magnetic declination (also known as magnetic variation) is the angle between the magnetic north and the true north. This angle varies geographically and with time. It’s crucial to correct for magnetic declination to obtain true bearings. This correction can be applied either graphically using a magnetic declination diagram or mathematically. Information on the local magnetic declination can typically be found on survey maps or online databases.

The correction involves adding or subtracting the declination angle to the magnetic bearing to obtain the true bearing. The sign (positive or negative) of the declination depends on whether the magnetic north is east or west of the true north. This should be explicitly stated on reference material.

Advantages and Disadvantages of Prismatic Compass Surveying

Like any surveying technique, prismatic compass surveying has its own advantages and disadvantages.

Advantages:

• Simplicity and Portability: It is simple to operate and highly portable, making it ideal for fieldwork in diverse terrains.
• Cost-Effectiveness: The instrument is relatively inexpensive compared to electronic instruments.
• Speed of Operation: Measurements can be obtained quickly.
• Suitable for Preliminary Surveys: It's well-suited for preliminary surveys where high precision is not crucial.

Disadvantages:

• Lower Accuracy Compared to Electronic Instruments: The accuracy is lower than that achieved with electronic theodolites.
• Affected by Local Attraction and Magnetic Variations: It is susceptible to local attraction and variations in the Earth’s magnetic field, requiring careful handling and correction.
• Not Suitable for High-Precision Work: It's not suitable for tasks requiring extremely high accuracy.

Frequently Asked Questions (FAQ)

Q1: What is the difference between a prismatic compass and a theodolite?

A1: A prismatic compass measures magnetic bearings directly, while a theodolite measures horizontal angles independently of magnetic north. Theodolites offer much higher accuracy but are more complex and expensive.

Q2: How often should a prismatic compass be calibrated?

A2: Regular calibration is recommended, ideally before and after each survey or at least once a year, depending on usage.

Q3: What are some common mistakes to avoid when using a prismatic compass?

A3: Common mistakes include incorrect leveling, inaccurate sighting, neglecting local attraction and magnetic declination, and insufficient repetition of measurements.

Q4: Can I use a prismatic compass in all types of weather?

A4: It's best to avoid using the prismatic compass in severe weather conditions, such as heavy rain or strong winds, which could affect the accuracy of readings.

Q5: How do I clean and maintain my prismatic compass?

A5: Keep the compass clean and dry. Avoid dropping it or exposing it to excessive heat. Regularly inspect the magnetic needle for any damage.

Conclusion

Prismatic compass surveying, despite the advent of sophisticated electronic surveying equipment, remains a valuable and practical surveying method. Its simplicity, portability, and cost-effectiveness make it ideal for various applications, particularly preliminary surveys and situations where electronic instruments might be unsuitable. However, it's crucial to understand the potential errors, sources of error, and corrective procedures to ensure the accuracy and reliability of the survey results. With proper training, careful observation, and meticulous data handling, prismatic compass surveying provides a reliable and efficient tool for a range of surveying tasks. Remember that accuracy depends not only on the instrument itself but also on the skill and precision of the surveyor.