Introduction
Compass surveying is an essential aspect of civil engineering, involving the use of compasses to measure bearings and directions accurately. Two primary types of compasses used in this field are the Prismatic Compass and the Surveyor’s Compass. Understanding the differences between these instruments can significantly impact the precision and efficiency of surveying tasks.
Prismatic Compass
The prismatic compass is a convenient, lightweight, and straightforward instrument, ideal for rough surveys where high accuracy is not crucial. It can be handheld or mounted on a tripod.
Components of Prismatic Compass
- Magnetic Needle: A broad-type needle that does not act as an index while taking readings.
- Graduated Ring: Fixed to the broad-type needle, it does not rotate with the line of sight.
- Right-Angled Prism: Allows simultaneous sighting of objects and reading of bearings.
- Agate Cap: A hard glass piece on the pivot to reduce wear and tear.
- Eye Vane & Object Vane: Used for ranging, bisecting, and reading bearings. The eye vane has a slit for clear visibility.
Functionality and Features
- Graduations: Marked in Whole Circle Bearing (WCB) and inverted for prism reflection.
- Reading Direction: Clockwise, with 0° at the South.
- Least Count: 30 minutes (30’).
Surveyor’s Compass
The surveyor’s compass is another simple instrument comprising a graduated circle in a box, an eye vane, and an object vane, typically mounted on a tripod.
Components of Surveyor’s Compass
- Magnetic Needle: An edge-type needle that acts as an index while taking readings.
- Graduated Circle: Fixed to the box, with the needle freely suspended, always indicating magnetic north.
- No Prism: Sighting and reading processes cannot be done simultaneously and must be done by looking vertically into the box.
- Bubble Tubes: Two perpendicular tubes for leveling the compass.
Functionality and Features
- Graduations: In Reduced Bearing (RB) with interchanged east and west graduations for accurate readings.
- Reading Direction: Based on the orientation of the box.
- Least Count: 15 minutes (15’).
Adjustments in Compasses
Both compasses require temporary and permanent adjustments to maintain accuracy.
Temporary Adjustments
- Centering: Positioning the compass over the station using a plumb bob.
- Levelling: Ensuring the graduated circle or edge bar needle does not touch the bottom of the box.
- Focusing the Prism (Prismatic Compass): Adjusting the prism for clear visibility of readings.
Permanent Adjustments
- Needle Sensitivity: Checking and remagnetizing if necessary.
- Pivot Adjustment: Ensuring the pivot point is sharp and centered.
- Level Adjustment (Surveyor’s Compass): Ensuring bubble tubes are perpendicular to the vertical axis.
- Sight Vane Adjustment: Ensuring vertical alignment using a plumb bob.
Comparison: Prismatic Compass vs Surveyor’s Compass
Here are the key differences between the prismatic compass and the surveyor’s compass:
| Feature | Prismatic Compass | Surveyor’s Compass |
|---|---|---|
| Needle Type | Broad type, does not act as an index | Edge bar type, acts as an index |
| Graduated Circle | Fixed to the broad-type needle, marked in Whole Circle Bearing (WCB), inverted graduations | Fixed to the box, marked in Reduced Bearing (RB), normal graduations |
| Reading Mechanism | Through the prism, simultaneous sighting and reading | Directly through the top of the glass, sighting and reading not simultaneous |
| Usage | Can be handheld or tripod-mounted | Requires a tripod |
| Least Count | 30 minutes (30’) | 15 minutes (15’) |
Conclusion
Understanding the distinctions between the prismatic compass and the surveyor’s compass is vital for selecting the right instrument for specific surveying tasks. The prismatic compass is suitable for rough, less accurate surveys, while the surveyor’s compass provides greater precision and is indispensable for more detailed surveys. By mastering the use of these tools, surveyors can enhance the accuracy and efficiency of their projects.