Change point surveying is a common task in leveling and layout work. This guide explains change point surveying instrument staff turn plate procedures and rules in clear, simple language. You will learn how to use the instrument, staff, and turn plate, and how to perform level transfer and change point handling safely and accurately.
Basics of Change Point Surveying
Before you start, know the key parts and terms. This short intro helps you get ready. Then the subsections explain each role.
What is a change point?
A change point is a temporary reference point used when you cannot see the previous benchmark directly. It lets you transfer elevation or position from one location to another. Change point surveying keeps your work accurate across gaps.
Why use a turn plate and staff?
The turn plate and instrument staff help keep the line of sight true and stable. The staff provides a reading. The turn plate supports the staff and helps you reset position when moving between points.
Equipment and Roles
Here are the main items you use in change point surveying. Each has a clear role. Read these notes before procedure steps.
Surveying instrument
The instrument is usually a level or total station. Place it on a stable tripod. Check bubble or electronic leveling. The instrument staff turn plate and the instrument must work together for level transfer method to be correct.
Instrument staff
The staff is a graduated rod used for readings. The instrument staff position must be steady and vertical. The instrument staff often moves with the turn plate during change point handling.
Turn plate
The turn plate sits under the staff foot. It allows small lateral moves without carrying the staff. Use the turn plate when you need to shift the staff to a new position while keeping the same approximate point.
| Item | Purpose |
| Instrument | Measure and read levels or angles |
| Staff | Provide a reference for the instrument |
| Turn plate | Support and shift staff position smoothly |
Procedure Rules for Change Point Surveying
Follow clear rules for safety and accuracy. These short rules guide instrument staff position, use of the turn plate, and level transfer method. Read the rules before doing the work.
Instrument setup rules
- Set the instrument on firm ground.
- Level the instrument precisely.
- Check the optical or electronic calibration.
- Keep the instrument aligned to the work line.
Staff and turn plate rules
- Place the staff vertically on the turn plate.
- Keep the staff plumb using a bubble or plumb bob.
- Use the turn plate to slide the staff a small distance.
- Record the position and reading before and after each move.
Change point procedure rules
- Always note the original benchmark or reference reading.
- Use a change point when the benchmark is out of sight.
- Perform level transfer method carefully to avoid cumulative error.
- Double-check readings after moving the staff or instrument.
Step-by-Step Level Transfer Method
This section gives a clear sequence for level transfer using the instrument, staff, and turn plate. Follow these steps slowly and check each reading.
1. Initial setup
Set the instrument over a stable spot. Level it. Take a backsight reading on a known benchmark or reference staff.
2. Place staff on turn plate
Put the staff on the turn plate at the first change point. Make sure the staff is vertical. Note the backsight reading on the staff.
3. Move instrument or staff
If you need to advance, either move the instrument forward or move the staff on its turn plate. When moving the staff, slide the turn plate carefully to the new position. Keep the staff plumb.
4. Take foresight and compute
Take a foresight reading at the new staff position. Calculate the new elevation using standard leveling formulas. Record both backsight and foresight to check for consistency.
5. Repeat until you reach target
Repeat the process for each change point. Each time, set the staff on the turn plate, move, and read. Keep notes to track any small errors.
Change Point Handling: Tips and Common Issues
Handling change points can be tricky. These tips help you avoid common mistakes and keep accuracy high.
Common problems
- Staff not vertical. This gives wrong readings.
- Turn plate slips on soft ground.
- Instrument not level after moving.
- Large gaps cause cumulative error.
Best practices
- Check verticality of staff each time.
- Use a stake under the turn plate on loose soil.
- Re-level the instrument after moving it.
- Reduce steps or take checkshots to limit error.
| Issue | Quick fix |
| Turn plate slips | Press a small stake or flat board under it |
| Staff not plumb | Use a plumb bob or adjust till bubble is centered |
| Instrument out of level | Re-level and re-check reference readings |
Practical Checklist Before You Start
Use this short checklist to make sure you follow the procedure rule for instrument staff position and turn plate method.
- Instrument calibrated and leveled
- Staff clean and marked clearly
- Turn plate present and stable
- Reference benchmark confirmed
- Notebook ready for readings
Frequently Asked Questions
What is the correct instrument staff position for change point surveying?
The staff should be vertical and placed on a stable turn plate. Use a bubble or plumb bob to check vertical. Mark the staff foot if you must return to the same spot.
How does the turn plate level transfer method work?
You place the staff on the turn plate, take a reading, then move the staff or instrument in small steps. Each time you record backsight and foresight readings and compute elevations. The turn plate allows small lateral moves without lifting the staff foot entirely.
Conclusion
Change point surveying is a reliable method when done with care. Follow the procedure rule for instrument setup, instrument staff position, and use of the turn plate. Use the level transfer method step-by-step, check readings, and handle change points with care to keep errors low.
