Two-Peg Test: How to Determine the Accuracy of Your Leveling Instruments

You should do this periodically to make sure your surveying equipment is correctly leveled so it provides accurate readings

Two-Peg Test: How to Determine the Accuracy of Your Leveling Instruments

Something I mentioned in passing in a previous series is how important it is for an installer to take good care of any surveying equipment: a surveying rod, hand level, transit or tripod level. 

I often see equipment bouncing around in the back of pickups or left exposed in truck cabs to extremes in heat and cold. These conditions are harmful to sensitive instruments. There is a way to check whether the line of sight of a surveying instrument is parallel to the axis of the bubble tube on the instrument so when the bubble is centered and the instrument is level, the axis is a level line.

Since it is not possible to sight along the axis of the bubble tube itself, a telescope is used. For a leveling instrument to be accurate, the telescope line of sight must be parallel to the axis of the bubble tube.

The two-peg test is used to make sure the line of sight provides an accurate reading and determine how much of an adjustment is necessary. This should be done by the installer periodically to make sure the instrument is correctly leveled so it provides accurate readings.

The following procedure can be used to determine the accuracy of any leveling instrument:

1. Set up the instrument and set stake “A” about 150 feet away. Set stake “B” the same distance in the opposite direction. Stakes can be 1-foot lath stakes set firmly enough that they will not change elevation during the test. The distance can be reduced if the telescope has a small magnifying power.

2. Hold the rod on top of stake “A” and take rod reading “a.” Then hold the rod on stake “B” and take rod reading “b.” If the instrument is exactly halfway between stakes “A” and “B,” the difference in rod readings “a” and “b” is the true elevation difference between stakes “A” and “B” regardless of the inclination of the line of sight.

3. Move the leveling instrument as close as possible to stake “A” so a rod reading “c” can be taken. How close the instrument can be to stake “A” will be determined by the magnification power of the instrument to focus on the numbers. It may be necessary to back through the objective lens at a pointed pencil held on the face of the rod. Since the instrument is so close to stake “A,” the rod reading “c” is assumed to be correct. 

4. To determine what rod reading should be found sighting to stake “B,” the elevation difference and rod reading “c” are combined. If stake “A” is higher than stake “B,” the true elevation from step 2 is added to the rod reading “c.”

This value is the rod reading “d” that should be sighted at stake “B” if the instrument is in adjustment. If the instrument is out of adjustment, it will read a different value than “d.” To adjust, move the horizontal crosshair to the correct reading by loosening one of the capstan screws and tightening the other. 

5. It may be desirable to run the test again to make sure the instrument is in adjustment.

About the author: Jim Anderson is connected with the University of Minnesota onsite wastewater treatment program and is an emeritus professor in the university’s Department of Soil, Water and Climate. Send him questions about septic system maintenance and operation by sending an email to

This post is part of a series on topography and site mapping:


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