Short Answer:

The main difference between a prismatic compass and a surveyor’s compass lies in how the readings are taken. A prismatic compass allows the user to read bearings while sighting the object, using a prism. In a surveyor’s compass, the readings are taken by looking directly over the needle after sighting.

Prismatic compass provides more accurate and quicker readings because it combines sighting and reading in one step. Surveyor’s compass requires two separate actions—first aligning to the object, then reading the scale—so it takes slightly more time and skill to use correctly.

Detailed Explanation:

Difference between prismatic compass and surveyor’s compass

Both prismatic compass and surveyor’s compass are important instruments used in compass surveying. They help in measuring horizontal angles or bearings of lines based on the Earth’s magnetic field. While their basic purpose is the same—to determine directions—their design, method of use, and accuracy level are different. Understanding their differences helps in selecting the right instrument for the type of survey being conducted.

1. Structure and design

The prismatic compass is designed with a prism attached near the eyepiece. This prism allows the user to see the compass card and the object at the same time, making it easy to take readings while sighting.

In contrast, the surveyor’s compass does not have a prism. It has a graduated ring fixed with the needle and open sights at both ends for aligning the instrument with the object. The user sights the object first, then looks vertically down to read the angle from the needle’s position.

Because of this design, the prismatic compass is more suitable when quick and continuous readings are needed in the field.

2. Reading method

• Prismatic compass: The reading is taken simultaneously while sighting through the prism. This saves time and gives better accuracy because the instrument doesn’t need to be moved during the process.
• Surveyor’s compass: The reading is taken after sighting, by looking from above. This can lead to minor errors if the compass moves between sighting and reading.

In short, the prismatic compass is easier for beginners and is better in situations where speed and precision are needed.

3. Type of graduated ring

The prismatic compass has a graduated ring fixed to the box, and the compass card rotates with the magnetic needle. The graduations are usually from 0° to 360°.

The surveyor’s compass has a fixed needle, and the graduated ring rotates. The ring is marked in quadrantal bearings, meaning 0° to 90° from each cardinal direction (N, S, E, W).

This difference in graduation makes the prismatic compass more suitable for full-circle readings.

4. Accuracy and usability

Prismatic compass is generally considered more accurate because:

• It has direct reading through the prism
• It uses full-circle bearings
• It reduces human error due to movement

The surveyor’s compass is simpler but requires more care and experience. It is often used in traditional or educational settings where low to moderate accuracy is acceptable.

5. Application and field use

• Prismatic compass is used in military, topographical, and quick field surveys.
• Surveyor’s compass is used in basic land measurement, teaching, and small survey tasks.

Both are valuable tools, but the choice depends on the accuracy, time, and simplicity required for the survey.

Conclusion:

The difference between prismatic compass and surveyor’s compass lies mainly in the method of taking readings, the type of graduation, and the ease of use. The prismatic compass allows direct reading through a prism and uses 0°–360° bearings, making it more accurate and faster. The surveyor’s compass uses quadrantal bearings and requires separate sighting and reading steps. Both are useful in surveying, but the prismatic compass is preferred for precision and efficiency in modern fieldwork.