Prismatic compass survey

                                   

Introduction: 

A prismatic compass survey is a field surveying technique used to measure horizontal angles and bearings with respect to magnetic north. The prismatic compass, an essential instrument in this method, consists of a magnetic needle, a prism for reading, and a sighting system. It allows surveyors to accurately determine directions by aligning the compass with specific points or landmarks. By observing and recording bearings, the surveyor can plot the positions of objects and map out the land. This method is widely used in reconnaissance surveys, preliminary explorations, and for determining boundaries, particularly in areas where high precision is not critical.

Procedure of prismatic compass survey: 

Radiation methods: 

The radiation method in a prismatic compass survey involves plotting points by drawing radial lines from a station point. Here are the steps for using the radiation method in a prismatic compass survey:

 1)  Set the compass over a station point and make temporary adjustments

 2)   Observe the bearings to all other station points in a clockwise direction

 3)    Shift the compass to another station and repeat the process

 4)   Measure the distance between the two stations

 5)   Plot the bearings and project rays from the station points

 6)   Find the location of each station by where the rays intersect

 7)  Measure the distance between one side of the field and compare it to the plotted length

 8)    Divide the field into triangles and calculate the total area 

Intersection method: 

The intersection method in a prismatic compass survey involves determining the location of an unknown point by taking bearings to it from two known points. The surveyor sets up the prismatic compass at both known points, takes bearings of the unknown points , and plots these bearings on a map. The intersection of the two lines indicates the position of the unknown points. This method is useful for locating inaccessible points in the field.

 The advantages and disadvantages of using a prismatic compass:

Advantages:

1. Portability: The prismatic compass is lightweight and easy to carry, making it suitable for fieldwork.

2. Simplicity of Use: It is relatively simple to operate, requiring minimal training for basic use.

3. Cost-Effective: Prismatic compasses are generally less expensive than more advanced surveying instruments like theodolites or total stations.

4. Quick Measurements: It allows for rapid measurement of bearings and angles, making it efficient for reconnaissance surveys.

5. Durability: Designed for outdoor use, prismatic compasses are often robust and can withstand harsh field conditions.

6. No Power Requirement: They do not require batteries or external power sources, making them reliable in remote locations.

Disadvantages:

1. Limited Precision: The accuracy is lower compared to modern instruments such as theodolites, especially over long distances.

2. Magnetic Interference: Being a magnetic instrument, it can be affected by nearby metallic objects, causing erroneous readings.

3. Limited Functions: It primarily measures angles and bearings, lacking advanced features like distance measurement.

4. Requires Skill: Accurate readings depend on the skill and experience of the user, especially in sighting and reading the compass.

5. Weather Sensitivity: Extreme weather conditions (like heavy rain or fog) can affect visibility and the reliability of readings.

6. Calibration Needs: Regular calibration may be necessary to ensure accuracy, especially in varying magnetic fields.