Author: DADI Technical Team
Technical basis: DADI electronic theodolite production, calibration and service experience, with collimation-adjustment principles confirmed by Mr. Ronghua Zhou and production/service inspection details provided by Engineer Zhang of the DADI workshop engineering team.
Updated: September 2026
Quick Answer
Collimation error, commonly evaluated as 2C error, occurs when an electronic theodolite's line of sight is not exactly perpendicular to its horizontal trunnion axis. It mainly affects horizontal direction measurement and is checked by comparing Face Left and Face Right observations. At DADI, mechanical adjustment is performed first, followed by software correction of the small residual error. For the DE2A series, the difference between repeated 2C results should remain within −10″ to +10″.
Error Definition
Line of Sight vs. Horizontal Axis
Inspection Method
Face Left + Face Right
DE2A Repeatability
2C Difference Within ±10″
Correction Sequence
Mechanical First, Software Residual
Face Left observation →Face Right observation →Calculate 2C →Repeat observation sets →Compare 2C results →Mechanical adjustment if required →Residual software correction →Stability verification
The telescope of a theodolite contains a reticle that defines the aiming direction of the instrument. This direction is commonly called the line of sight or collimation axis.
Ideally, the collimation axis should be perpendicular to the horizontal trunnion axis. If this geometric relationship is not exact, the actual direction indicated by the telescope differs slightly from the theoretically correct direction.
This deviation is known as collimation error. In practical theodolite inspection and calibration, it is commonly evaluated through Face Left and Face Right observations and expressed as 2C.
2C error should not be confused with encoder error. An angle encoder may correctly report the rotational position of the instrument while the telescope's actual line of sight still contains a geometric deviation. Complete theodolite accuracy therefore depends on optical alignment, mechanical geometry and electronic angle measurement together.
A common source of collimation error is incorrect positioning of the telescope reticle. If the reticle is not properly installed or adjusted, the line of sight may not remain exactly perpendicular to the horizontal axis. Because the reticle defines the direction used by the operator to sight a target, even a small positional deviation can influence horizontal direction observations.
The focusing lens moves during telescope focusing. Its movement depends on the mechanical slide and guide structure inside the telescope. If the machining accuracy or fit of the slide and guide structure is insufficient, movement of the focusing mechanism can introduce a small change in the optical relationship. Temperature changes can also cause small deformation of mechanical components.
Electronic theodolites contain precisely aligned optical and mechanical components. Rough transportation, strong vibration, sudden temperature changes, impact or a drop can cause very small internal displacement or deformation. These changes may not be visible from outside the instrument.
After long periods of operation or storage, residual stress within mechanical components and assembled structures may gradually release. These small changes can alter the original geometric relationship and contribute to collimation deviation over time.
Collimation error mainly affects horizontal direction measurement.
For a target observed at a vertical angle α, the influence of collimation error on the horizontal direction can be expressed as:
Where:
C represents the collimation error.
α represents the vertical angle of the target.
ΔC represents the influence on the observed horizontal direction.
As the target moves farther away from the horizontal line of sight and the vertical angle increases, the influence of collimation error on horizontal direction becomes more significant.
The direct contribution of collimation error to the vertical-angle reading itself is relatively small. Its primary importance is the effect on horizontal direction and accurate target sighting.
Theodolite observation commonly uses two telescope positions: Face Left and Face Right.
One important reason for observing in both faces is that the influence of collimation error changes sign when the telescope is changed from one face to the other.
Face Left
+ΔC
Face Right
−ΔC
When properly observed Face Left and Face Right measurements are combined, the two error components have opposite signs and can cancel each other.
This is why two-face observation remains important in precision angular measurement even when a modern electronic theodolite uses an electronic angle-reading system.
The difference between Face Left and Face Right observations can be used to evaluate the instrument's collimation condition.
Where:
L is the Face Left horizontal reading.
R is the Face Right horizontal reading.
Whether 180° is added or subtracted depends on the angular reading range and calculation convention used by the instrument.
In a simplified calibration procedure, the horizontal circle can be set to zero while sighting the target in Face Left. After changing to Face Right and sighting the same target again, the horizontal reading should ideally correspond to 180°. The remaining difference can then be used to evaluate the 2C condition.
DADI uses a horizontal collimator calibration bench for 2C inspection during instrument production, adjustment and service work.
The collimator provides a stable optical target representing a target at effectively infinite distance. A typical inspection process includes:
Install the electronic theodolite on the calibration bench and level the instrument carefully.
In the Face Left position, align the telescope reticle with the collimator crosshair target.
Observe or record the corresponding horizontal angle.
Change the instrument to Face Right.
Sight the same collimator target again.
Obtain the corresponding 2C result from the Face Left and Face Right observations.
Repeat the observation sets and compare the results for stability.
During DADI service and adjustment work, the relevant 2C result can also be viewed through the instrument's service-program interface.
DE2A Series Repeatability Requirement
For the DE2A series, DADI compares 2C results obtained from repeated observation sets. The difference between repeated 2C results should remain within −10″ to +10″, equivalent to an absolute difference of no more than 10 arc-seconds.
This requirement refers to the difference between repeated 2C results. It should not be interpreted as a statement that the absolute 2C value from a single observation must always be less than 10″.
A single 2C measurement provides information about the instrument's collimation condition at that moment, but it does not fully describe the stability of the system.
If repeated Face Left and Face Right observation sets produce significantly different 2C results, the problem may involve more than a fixed collimation offset.
Possible causes may include instability in the focusing mechanism, small internal mechanical movement or another changing relationship within the optical or mechanical system.
For this reason, DADI does not evaluate only one isolated 2C result. Repeatability between observation sets is also used to judge whether the instrument remains mechanically and optically stable.
Collimation correction at DADI involves two different levels: mechanical adjustment and software correction.
When the collimation relationship requires correction, the telescope is first adjusted mechanically.
The position of the reticle is adjusted through its left and right correction screws so that the line of sight is physically brought as close as possible to the required perpendicular relationship with the horizontal axis.
This is the fundamental correction because it addresses the actual optical and mechanical geometry of the instrument.
After mechanical adjustment, a small residual collimation error may remain.
This residual amount can then be handled through the instrument's collimation-error setting function. The electronic correction compensates for the small remaining error, particularly during single-face observation.
Mechanical adjustment corrects the physical relationship.
Software correction compensates for the small residual error remaining after mechanical adjustment.
When properly performed Face Left and Face Right observations are combined, the primary influence of collimation error is already cancelled geometrically. Software correction and two-face observation therefore serve different purposes.
No. Electronic compensation should not be used as a substitute for proper mechanical adjustment.
If the physical collimation relationship has changed significantly, the first step is to restore the optical and mechanical geometry as closely as possible. Software correction should then be used only for the small residual amount that remains.
This distinction is especially important after strong vibration, impact or a drop, because the problem may involve physical movement or deformation rather than only a fixed electronic correction value.
A 2C check is particularly important after conditions that may affect the optical or mechanical relationship of the instrument.
After rough or prolonged transportation.
After a drop, impact or strong vibration.
After extended periods of use or storage.
When the operating temperature differs significantly from the previous calibration environment.
After repair or adjustment of the telescope or related mechanical components.
When Face Left and Face Right observations show unusual differences.
When angular measurement stability appears to have declined.
Before work requiring higher angular accuracy.
After a theodolite has been dropped or subjected to a significant impact, a 2C inspection is required. It should not be treated as only an optional precaution.
A single inspection is also not sufficient to determine whether the instrument has returned to a stable condition.
In DADI's service practice, an instrument that has experienced a drop or significant impact is checked repeatedly over a period of two to three days.
Multiple 2C inspections are performed each day.
Results from different observation sets and different times are compared.
The technician checks whether the 2C condition remains stable over time.
The instrument is not judged only by one apparently normal result immediately after adjustment.
This repeated inspection is necessary because internal deformation or stress caused by an impact may not stabilize immediately. A normal result from one measurement does not necessarily prove that the optical and mechanical system will remain stable later.
After a drop, the key question is not only:
"What is the current 2C value?"
It is also:
"Does the 2C result remain stable after repeated checks over the following two to three days?"
No. They involve different parts of the measurement system.
| Item | What It Relates To |
|---|---|
| Encoder reading | Determining the angular position of the rotating instrument axis. |
| 2C / collimation error | The geometric relationship between the telescope's line of sight and the horizontal trunnion axis. |
An electronic theodolite can therefore have a correctly functioning angle encoder while still requiring collimation adjustment. Diagnosing angular measurement problems requires the electronic reading system, optical alignment and mechanical axis relationships to be considered separately.
2C is a practical value used to evaluate horizontal collimation error through Face Left and Face Right observations. It reflects the deviation caused when the telescope's line of sight does not maintain the correct perpendicular relationship with the horizontal axis.
Its main influence is on horizontal direction measurement. The effect on horizontal direction becomes more significant as the vertical angle of the observed target increases.
Yes. The primary collimation-error component appears with opposite signs in the two telescope positions. When the two observations are correctly combined, these opposing components can cancel each other.
No. Mechanical adjustment should first restore the telescope geometry as closely as possible. Software correction is then used for the small residual error remaining after the physical adjustment.
Repeated measurements provide information about stability and repeatability rather than relying on one isolated reading. For the DE2A series, the difference between repeated 2C results should remain within −10″ to +10″.
Yes. After a significant drop or impact, 2C inspection is required. DADI does not rely on a single check. The instrument is inspected repeatedly over two to three days, with multiple measurements performed each day to confirm that the optical and mechanical system remains stable.
Yes. Internal optical or mechanical components can shift or deform by very small amounts that are not visible externally. These changes may still affect collimation and angular measurement stability.
For a broader explanation of how an electronic theodolite determines angles, see How Does an Electronic Theodolite Measure Angles?
For information about DADI's inspection of mechanical axis components before assembly, see Inside DADI Factory: Horizontal and Vertical Axis Inspection
For an overview of DADI's theodolite development, manufacturing, testing and customization capabilities, visit Electronic Theodolite Manufacturing at DADI
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