How to Check Auto Level Accuracy with a Two-Peg Test Malaysia

 

How do you know whether an auto level is still accurate?

 

One of the most useful field checks is the two-peg test.

 

The two-peg test is commonly used to check whether an auto level has excessive line-of-sight or collimation error.

 

It can be particularly useful when:

 

Readings suddenly become inconsistent

 

The auto level has been dropped

 

Benchmark results do not agree

 

The instrument has been transported frequently

 

You want to check the equipment before an important project

 

You suspect the instrument requires calibration

 

A two-peg test is a useful field check, but it should not be confused with professional calibration.

 

What Is a Two-Peg Test?

 

The principle is straightforward.

 

Two stable points are established some distance apart.

 

Readings are first taken with the auto level positioned approximately midway between the two points.

 

The instrument is then moved much closer to one point and another set of readings is taken.

 

Comparing the two sets of observations helps identify whether the instrument's line of sight introduces excessive error.

 

Why Position the Auto Level in the Middle First?

 

When the instrument is positioned approximately halfway between the two staff positions, the sight distances are similar.

 

This is important because certain line-of-sight errors affect both observations in a similar way.

 

By comparing the two staff readings, you can establish a more reliable reference height difference between the points.

 

This becomes the basis for the second part of the test.

 

Equipment Required

 

A basic two-peg test requires:

 

Auto level

 

Stable survey tripod

 

Levelling staff

 

Two stable observation points

 

Suitable measuring distance

 

Field notes or recording method

 

Before beginning, make sure the tripod and levelling staff are in good condition.

 

A loose tripod or damaged staff can introduce additional errors and make the test less useful.

 

Step 1: Establish Two Stable Points

 

Select two stable points on reasonably level ground.

 

The exact test distance may depend on your procedure, instrument and site conditions.

 

The important point is to create sufficient separation for the test while maintaining clear staff visibility.

 

Label the points:

 

Point A and Point B

 

Step 2: Set the Auto Level Approximately Midway

 

Position the tripod approximately halfway between Point A and Point B.

 

Mount the auto level securely.

 

Centre the circular bubble and allow the compensator to settle.

 

Make sure:

 

Tripod is stable

 

Instrument is securely mounted

 

Staff is vertical

 

Image is properly focused

 

Parallax is minimised

 

Step 3: Take the First Two Readings

 

Place the levelling staff on Point A and record the reading.

 

Then place it on Point B and record the second reading.

 

For example:

 

Reading A1

 

Reading B1

 

The difference between these readings provides the reference height difference for the test.

 

Record everything carefully.

 

A recording mistake can make the entire test meaningless.

 

Step 4: Move the Auto Level Closer to One Point

 

Now relocate the instrument much closer to one of the two points, for example Point A.

 

Set up and level the instrument again.

 

Do not move the original Point A and Point B locations.

 

Step 5: Take the Second Set of Readings

 

Take another reading on Point A.

 

Then observe Point B.

 

Record these as:

 

Reading A2

 

Reading B2

 

Because the sight distances are now significantly unequal, line-of-sight error becomes easier to detect when the results are compared with the reference height difference.

 

Step 6: Compare the Results

 

Compare the height difference obtained from the first setup with the result from the second setup.

 

If the difference exceeds the acceptable tolerance for your instrument or work requirement, the auto level may require adjustment.

 

Do not use an arbitrary tolerance.

 

Refer to:

 

Manufacturer specifications

 

Company quality procedures

 

Project requirements

 

Applicable calibration requirements

 

Different auto levels and applications may have different acceptable limits.

 

What If the Auto Level Fails the Two-Peg Test?

 

Do not immediately assume the instrument is beyond repair.

 

A failed test may indicate that the instrument requires:

 

Inspection → Adjustment → Calibration

 

If there is internal damage, the process may instead be:

 

Inspection → Repair → Adjustment → Calibration → Verification

 

The exact solution depends on the instrument's condition.

 

Should I Adjust the Auto Level Myself?

 

Some auto levels provide adjustment points for trained users or technicians.

 

However, randomly turning adjustment screws is not recommended.

 

Incorrect adjustment can:

 

Increase the error

 

Affect instrument geometry

 

Make troubleshooting more difficult

 

Create additional calibration problems

 

If you are uncertain about the correct procedure, have the instrument checked professionally.

 

Two-Peg Test After Dropping an Auto Level

 

If an auto level has been dropped, a two-peg test can provide a useful initial indication of whether accuracy has changed.

 

However, impact can affect more than collimation.

 

A drop may also damage or disturb:

 

Compensator

 

Circular bubble

 

Focusing mechanism

 

Optical components

 

Mechanical components

 

Therefore, passing one field test does not necessarily prove that every part of the instrument is in perfect condition.

 

For important work, professional inspection may still be appropriate.

 

Two-Peg Test vs Calibration

 

These two terms should not be treated as interchangeable.

 

Two-Peg Test

 

A field procedure used to check for possible line-of-sight error.

 

Professional Calibration

 

A controlled process used to inspect and verify instrument performance according to the applicable service or calibration procedure.

 

If your company or project requires documented calibration, a field two-peg test alone may not satisfy that requirement.

 

How Often Should You Perform a Field Check?

 

The appropriate frequency depends on how the auto level is used.

 

Consider additional checking:

 

Before critical projects

 

After accidental impact

 

After rough transportation

 

When readings appear suspicious

 

After extended heavy use

 

Between scheduled calibration intervals

 

Regular field checks can help identify problems before they create expensive construction errors.

 

Don't Forget the Tripod and Staff

 

If a two-peg test produces strange results, also inspect the accessories.

 

Check the tripod for:

 

Loose legs

 

Worn clamps

 

Unstable head

 

Movement during observations

 

Check the levelling staff for:

 

Bent sections

 

Loose joints

 

Damaged locks

 

Difficulty remaining vertical

 

Surveying accuracy is produced by the complete measurement system.

 

Auto Level Calibration & Repair Malaysia

 

If your auto level fails an accuracy check, MTM Precision can inspect the instrument and advise on the appropriate next step.

 

Depending on its condition, this may involve:

 

Inspection

 

Adjustment

 

Calibration

 

Repair

 

Replacement

 

Upgrade

 

MTM Precision also supplies professional auto levels, tripods, levelling staffs and other surveying accessories.

 

Protect Your Surveying Accuracy

 

A two-peg test is a valuable part of preventive equipment management.

 

Instead of waiting until a construction level is obviously wrong, establish a routine:

 

Use → Field Check → Identify Error → Calibrate → Repair if Necessary → Verify → Return to Site

 

This helps contractors, engineers and surveyors reduce the risk of inaccurate levelling work.

 

MTM Precision Sdn Bhd

 

Surveying Equipment • Calibration • Repair • Accessories

 

Website: www.mtmpre.com.my

 

WhatsApp: +6016-660 7346

 

Email: mtmpre@yahoo.com

 

Showroom & Service Centre

 

No. 29-1 & 29-2, Jalan Bandar 18,

Pusat Bandar Puchong,

47160 Puchong, Selangor, Malaysia.