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Two Common Level Errors – And How to Avoid Them

A Leica auto level on a construction site

Have you ever spent a few hours on the jobsite taking readings with your level, only to find out later that the readings weren't correct? This frustrating situation might be a result of cross-hair parallax, an error that affects optical and electro-optical instruments such as levels and total stations. It could also be due to a misaligned compensator. Fortunately, both errors can be easily avoided with the right setup procedures.

Cross-hair parallax occurs when the plane of the cross-hair reticle does not coincide with the image plane of the focused object (the level staff or prism, for example). This error can be easily recognized by moving your head slightly up or down – or left or right – in front of the eyepiece. The reticle appears to move and does not stay in line with the optical axis.

If this error is not corrected, the readings of the level staff or the aiming of the prism will be incorrect and will lead to the wrong results.

Before you start measuring, remember to check the parallax every time.

First, aim the telescope at a high-contrast or bright background like a piece of white paper and focus the cross-hairs by turning the eyepiece drive. Then focus on the staff or the prism. The image plane of both the cross-hairs and the object you have aimed at should coincide.

In newer levels, the compensator has been adjusted at room temperature so that the line of sight is horizontal even if the instrument is slightly tilted. This changes if the temperature fluctuates by more than 10-15 degrees, after a long journey, or if the instrument has been subjected to excessive vibration. A misaligned compensator will produce inaccurate level readings.

To inspect the line of sight:

1. Set up two staffs not more than 30 m (95 ft) apart on flat terrain.

2. Make sure the instrument is equidistant from the two staffs (it is fine to pace out the distance).

3. Read off both staffs and calculate the height difference. (In the illustration below, staff reading A = 1.549 and staff reading B = 1.404. A-B = 0.145.)
 

Checking a Level Line-of-Sight

4. Set up the instrument about 1 meter (3 ft) in front of staff A and take the staff reading. (Staff reading A = 1.496.) Calculate the required reading B. (Staff reading A – ∆H = 0.145. Required reading B = 1.351.)
 
checking a level line of sight
 
5. If staff reading B is different from the required reading by more than 3 mm (1/10 inch), adjust the line of sight by referring to the instruction manual.
 

Set up your level correctly and check for these common sources of error before you begin your work, and you'll be more efficient on every project. If you need help choosing the right level or have any questions about level setup or operation, please contact us.