Compass Survey – Instrument and Its Setting Up
In compass surveying, accurate measurement of directions is achieved using a magnetic compass. Proper knowledge of the instrument and its correct setup in the field is essential for obtaining reliable results. In ITI Draughtsman Civil, students must understand the parts of the compass and the correct procedure for setting it up.
1. Compass Instrument
A compass is an instrument used to measure the direction of survey lines with respect to the magnetic north. It works on the principle that a freely suspended magnetic needle always points towards the north-south direction.
Types of Compass
- Prismatic Compass: Provides more accurate readings and allows sighting and reading at the same time.
- Surveyor’s Compass: Less precise and commonly used for rough work.
2. Parts of Prismatic Compass
- Magnetic Needle: Indicates the north-south direction.
- Graduated Circle: Marked from 0° to 360° for measuring bearings.
- Prism: Used to read the graduations while sighting.
- Object Vane: Used for sighting the object.
- Eye Vane: Used for viewing through the prism.
- Glass Cover: Protects internal parts.
- Base Plate: Supports the instrument.
- Tripod Stand: Used to hold the compass steady.
3. Setting Up the Compass
Proper setup of the compass is necessary to obtain correct readings. The following steps are followed:
Step 1: Fixing the Instrument
- Mount the compass on a tripod stand.
- Ensure it is firmly fixed and stable.
Step 2: Centering
- Place the compass exactly over the survey station.
- Use a plumb bob to ensure accurate positioning.
Step 3: Levelling
- Adjust the tripod legs to make the compass horizontal.
- Ensure the magnetic needle swings freely.
Step 4: Focusing and Sighting
- Adjust the prism for clear reading of graduations.
- Align the object vane with the target point.
Step 5: Taking Reading
- Allow the magnetic needle to settle.
- Read the bearing through the prism.
- Record the reading in the field book.
4. Precautions While Setting Up
- Keep the compass away from magnetic materials like iron objects.
- Ensure proper levelling for accurate readings.
- Avoid working near electric lines or vehicles.
- Handle the instrument carefully to prevent damage.
5. Importance of Proper Setup
- Ensures accurate measurement of bearings.
- Reduces errors in survey work.
- Improves reliability of data.
Role of Draughtsman Civil
A Draughtsman Civil uses compass survey data to prepare plans and layouts. Proper understanding of the instrument and its setup ensures accurate plotting and design.
Conclusion
The compass is an important instrument in surveying used for measuring directions. Proper setting up of the instrument, including centering, levelling, and sighting, is essential for accurate results.
For ITI Draughtsman Civil students, mastering the use and setup of the compass is important for effective surveying and construction planning.