Author: DADI Technical Team
Technical basis: DADI electronic theodolite design, assembly, factory calibration, optical and mechanical inspection, angular accuracy verification, laser alignment testing, and pre-shipment inspection experience. The 2C measurement method, ±10″ factory acceptance limit and ±5″ internal control target were confirmed by Mr. Ronghua Zhou. Other model-specific procedures described here reflect DADI manufacturing and inspection practices.
Updated: October 2026
At DADI, an electronic theodolite is inspected after assembly to verify its mechanical operation, telescope optics, geometric relationships, electronic angle measurement, compensator and centering system. The factory process includes i-angle and 2C checks, compensator calibration, angular accuracy testing and laser-plummet verification, with additional tests for specialized models. For 2C, the factory acceptance limit is ±10″ and DADI targets a tighter ±5″ internally. Key measurements are checked repeatedly for stability. Before dispatch, DADI uses three inspection stages: an initial documented inspection after production, a recheck during shipment preparation and a final check on the shipment day. Later results are compared with the original qualified inspection record.
Dedicated resolving-power testing collimator
Measured 2C error; distinct from repeatability
Post-calibration Face Left / Face Right check difference
Horizontal-angle test with repeated observations and dedicated software
Factory acceptance specification with 360° rotation verification
Actual rated distance tested outdoors for every unit
DADI's tighter adjustment target, confirmed by Mr. Ronghua Zhou
Initial baseline record followed by two verification inspections
An electronic theodolite combines several interconnected mechanical, optical, and electronic systems. These include the telescope, horizontal and vertical axes, electronic angle encoder, compensator, leveling mechanism, mechanical motion system, and associated electronic controls.
Individual components may already have passed inspection before assembly. However, the geometric relationships between these components must still be verified after the complete instrument has been assembled.
For example, a telescope may produce a clear image without its line of sight being correctly aligned with the instrument's mechanical axes. Similarly, an angle encoder may respond normally while the finished instrument still requires geometric adjustment and final accuracy verification.
Component inspection and finished-instrument calibration therefore address different engineering requirements.
At DADI, horizontal and vertical axis components are inspected using dedicated equipment. These checks help establish the mechanical foundation for subsequent assembly and calibration.
For further information on axis inspection, see Horizontal and Vertical Axis Inspection.
The finished-instrument calibration process involves a sequence of mechanical, optical, angular, and functional inspections.
Mechanical movement and leveling inspection
Telescope and optical system inspection
i-angle inspection
Collimation-error (2C) inspection and adjustment
Compensator linearity and zero-point calibration
Horizontal and vertical angular accuracy verification
Laser or optical plummet inspection
Upper-laser or other specialized function inspection, where applicable
Final functional inspection and acceptance
The DE2A, DE2A-L, MPE-02L, DE2A-GF500, and DE2A-AC configurations share essentially the same basic angular calibration procedure.
Additional tests are performed according to the functions and engineering applications of individual models. For example, DE2A-GF500 requires long-distance laser verification beyond the standard angular calibration process.
Before precision angular calibration, the instrument's mechanical condition must be checked.
DADI inspects the following items on every instrument:
Horizontal rotation
Vertical rotation
Horizontal and vertical clamps
Horizontal and vertical tangent-motion mechanisms
Leveling mechanisms
Horizontal and vertical axis movement
Relationship between the plate level and circular level during leveling
The purpose is to confirm that the instrument rotates, clamps, adjusts, and levels normally before the optical and electronic measurement systems are calibrated.
The telescope provides the optical sighting reference used during angular measurements.
DADI checks telescope image clarity, focusing operation, and optical resolving power during finished-instrument inspection.
A dedicated resolving-power testing collimator is used to inspect telescope resolution. This inspection is performed on every instrument rather than only on randomly selected units.
The required resolving-power specification depends on the particular model and its technical documentation. Establishing a proper optical sighting reference is essential before evaluating the instrument's geometric errors and angular accuracy.
Collimation error concerns the geometric relationship between the telescope line of sight and the horizontal trunnion axis. DADI evaluates it by observing the same reference target in Face I and Face II on a horizontal collimator.
Securely mount and level the instrument on the inspection bench.
In Face I (Face Left), sight the crosshair target in the horizontal collimator and set the ordinary horizontal-circle reading to 0°.
Reverse the telescope to Face II (Face Right) and sight the same collimator target.
Record the ordinary Face II horizontal-circle reading and calculate the signed 2C result after removing the nominal 180° face difference.
Compare the 2C result with the factory acceptance limit and internal control target. Repeat the observation as required to check stability.
2C = L − (R ± 180°)
L = ordinary Face I horizontal reading; R = ordinary Face II horizontal reading. Select +180° or −180° so that the opposite-face reading is referred to the Face I direction, then normalize the signed difference around zero.
In DADI's zero-setting procedure, L = 0° and R is normally close to 180°. For this case the formula becomes 2C = L − (R − 180°). If R reads 180°00′03″, the calculated 2C is −3″; if R reads 179°59′56″, the result is +4″. These are illustrative calculations, not recorded results from an actual production instrument.
DADI 2C Factory Acceptance and Internal Control
Factory acceptance: measured 2C within −10″ to +10″ (±10″).
Stricter internal control target: measured 2C within −5″ to +5″ (±5″).
These criteria refer to the measured 2C error itself, not to the difference between repeated measurements. For example, a result of +8″ meets the stated factory acceptance limit but is outside DADI's tighter internal target; +12″ exceeds the factory limit. A separate numerical repeatability tolerance is not specified here.
In the production process described by DADI, two 2C observation sets are normally taken. Repeating the check helps the technician judge whether the measured condition remains stable after adjustment; it does not change the definition of the ±10″ acceptance limit.
The original documented acceptance requirement is described for DADI DE2A-series electronic theodolites, DADI total stations and the related angular inspection of DSD-IIIA. For the underlying geometry and detailed error explanation, see What Is Collimation Error (2C Error) in an Electronic Theodolite?.
If 2C is outside the acceptance range, or remains above the tighter internal adjustment target and further correction is practical, DADI's procedure prioritizes mechanical adjustment rather than relying on software to conceal a significant geometric error.
The mechanical adjustment corrects the actual geometric relationship of the instrument. Afterward, the instrument's internal software is used for fine adjustment of the remaining small error.
Software correction is not used as a substitute for necessary mechanical adjustment.
The instrument is then measured again to confirm the adjustment result.
Mechanical components may retain residual stress after manufacturing and assembly.
As the instrument is operated and time passes, some of this stress may be released, causing small changes in the previously adjusted geometric relationship.
Repeated 2C inspection helps verify that the instrument remains stable after adjustment rather than producing only one acceptable result.
The compensator uses a gravity reference to detect small residual instrument tilt after mechanical leveling and applies corresponding angular corrections according to the instrument's design.
At DADI, compensator calibration includes three principal stages:
Linearity calibration
Zero-point calibration
Face Left / Face Right verification
A fixed optical target is used as a reference while controlled changes in instrument tilt are introduced.
During adjustment, the telescope crosshair is returned to the same optical reference target, and the corresponding compensator response is recorded.
The process is used to calibrate and verify the relationship between detected tilt and the correction applied by the instrument.
After linearity calibration, zero-point calibration establishes the correct relationship between the electronic compensator zero and the instrument's mechanical and optical reference condition.
A compensator may respond normally to changing tilt while still having an incorrect zero reference. This is why the two calibration procedures serve different purposes.
After calibration, Face Left and Face Right inspection is performed.
For the specified DADI DE2A-series electronic theodolites, total stations, and DSD-IIIA system, the difference between the two check results must not exceed 30″.
This value is a post-calibration inspection criterion, not the compensator's accuracy or compensation range.
DADI DE2A-series instruments use single-axis compensation. Total-station compensator configurations depend on the particular model; for example, DTM 752R and DTM 952R use dual-axis compensation.
For a detailed explanation of the compensator, its operating principles, and calibration methods, see What Is a Compensator in a Total Station or Electronic Theodolite?
Completing optical, mechanical, collimation-error, and compensator adjustment does not automatically establish the finished instrument's angular accuracy.
The final measurement result must be verified using dedicated testing equipment and repeated observations.
DADI uses a precision multi-tooth indexing table, a computer, and dedicated angle-testing software to evaluate horizontal angular accuracy.
For a 2″ electronic theodolite, the horizontal-angle test includes:
12 angular test positions
Two repeated measurements at each test position
Face Left and Face Right observations
Calculation and evaluation using dedicated testing software
The indexing table provides the angular references used during the test. Measurements at different angular positions provide information about the instrument's performance around the horizontal circle.
The software processes the observations and calculates the results for comparison with the corresponding technical requirements.
Vertical angular accuracy is also evaluated using DADI's dedicated testing equipment and software.
The inspection includes Face Left and Face Right observations, with two measurements for each face.
The final results are calculated through the testing software and evaluated according to the instrument's requirements.
Although both horizontal and vertical measurements are software-evaluated, this does not mean their complete testing procedures or underlying calculations are identical.
The angular accuracy of a finished electronic theodolite is established through systematic testing, not by relying on a single angle reading.
For an explanation of the angle measurement system itself, see How Does an Electronic Theodolite Measure Angles?
The laser plummet is used to center the instrument over a survey point. It must not be confused with the upper pointing laser used for directional guidance on certain DE2A-series configurations.
The standard DE2A is equipped with an internal Class 2 laser plummet. An optical plummet is also available as an optional configuration upon request.
DADI uses a dedicated calibration plate to inspect the laser plummet.
The factory inspection process includes:
Level the instrument correctly.
Establish the reference center using the dedicated calibration plate.
Observe the position of the laser spot relative to the reference center.
Rotate the instrument through 360°.
Observe the movement of the laser spot during rotation.
Examine the maximum spot deviation and the diameter of the circular trace produced during rotation.
Evaluate the result according to the specified centering requirement.
For the DADI DE2A, the laser-plummet specification of 1.5 mm at a distance of 1.5 m is also used as the factory acceptance requirement.
The rotation test helps reveal centering deviations that may not be apparent when the instrument is checked in only one angular position.
When an optical plummet is supplied as an optional configuration, it is checked according to the corresponding product manual and technical requirements.
| Optical-Plummet Parameter | Specification |
|---|---|
| Image | Erect |
| Magnification | 3× |
| Focusing range | 0.3 m to infinity |
| Field of view | 5° |
Optical-plummet inspection is more time-consuming than inspection of the standard laser plummet.
The instrument must be checked at multiple angular positions to verify that the optical centering relationship remains consistent as the instrument is rotated.
Some DE2A-series configurations include an upper laser for projecting a visible directional reference.
Examples include the DE2A-L red-laser configuration, the MPE-02L green-laser configuration, and the long-range DE2A-GF500.
Unlike the downward-pointing laser plummet, the upper laser is intended to project the instrument's sighting direction into the working area.
A central factory calibration requirement is that the laser-beam axis and the telescope line of sight are aligned along the same straight line.
In practical terms, when the telescope crosshair center is accurately aligned with a target, the center of the upper-laser spot should correspond to the same sighting direction.
DADI checks both the coaxial relationship and the required focus relationship between the laser and optical sighting systems.
During the factory check, an enlarged cross target is positioned more than 50 m from the instrument.
The inspection includes:
Aim the telescope at the cross target.
Align the telescope reticle center with the target cross center.
Activate the upper laser.
Observe the position of the laser spot relative to the target center.
Adjust and verify the laser-to-telescope alignment according to the instrument's requirements.
Repeat the inspection to verify the stability of the result.
The objective is not simply to produce a visible laser spot. The laser direction must correspond to the optical reference established by the telescope.
Alignment at one observation distance is an important check, but does not alone establish the complete laser-to-telescope relationship at every working distance. Long-range configurations therefore require additional verification.
The DE2A-L and MPE-02L provide an upper-laser effective working distance of approximately 300 m under favorable conditions.
This working-distance figure is based on product verification. It should not be interpreted as a requirement that every standard DE2A-L or MPE-02L must undergo an individual 300 m outdoor test before shipment.
The DE2A-GF500 is a long-range green-laser configuration developed from DADI's DE2A electronic theodolite platform.
Its specialized application requires verification beyond ordinary angular measurement and short-distance laser alignment.
Every finished DE2A-GF500 unit must undergo additional testing. This is a mandatory unit-by-unit requirement, not merely a development-stage test or a sample inspection.
DADI combines indoor calibration with outdoor field testing.
Basic instrument parameters and related calibration items are checked indoors, while actual working-distance performance is verified outdoors.
Outdoor tests are conducted in real long-range environments, including road locations where sufficiently long lines of sight are available.
The inspection is not considered complete based on a single successful observation.
The DE2A-GF500 is offered in 1 km and 1.5 km long-range configurations.
| Configuration | Required Actual Outdoor Test Distance |
|---|---|
| DE2A-GF500 — 1 km | 1 km |
| DE2A-GF500 — 1.5 km | 1.5 km |
Each configuration must be tested at its corresponding actual working distance.
Short-distance testing followed by a theoretical estimate is not used as a substitute for this required outdoor verification.
DADI repeatedly checks the key laser characteristics relevant to GF500 performance, including:
Long-distance laser visibility
Laser directional stability
Consistency between the laser axis and telescope line of sight
Laser spot size
Stability during extended operation
Changes in laser direction during instrument rotation
All these key items require repeated verification. Depending on the inspection item, the checks are carried out indoors or under actual outdoor working conditions.
Repeated testing is used to confirm that the instrument's laser performance remains consistent rather than meeting the required condition only once.
Technical specifications define the acceptance limits that a finished electronic theodolite must satisfy.
However, meeting an acceptance limit does not necessarily mean that factory adjustment should immediately stop.
In DADI's production process, engineers use dedicated calibration equipment, mechanical adjustment methods, and internal software fine-tuning to reduce the remaining errors of adjustable items as far as reasonably achievable under the actual technical conditions.
The objective is not simply to bring an instrument just inside the permitted error boundary.
For example, the 2C factory acceptance range of ±10″ defines the permitted inspection result, while DADI adopts a stricter ±5″ internal control target. Meeting the factory limit is therefore not the same as reaching the preferred internal adjustment condition.
Where the mechanical and electronic adjustment conditions permit, engineers continue to reduce the residual error and verify the resulting condition.
A small measured error at one particular moment does not automatically establish that the instrument will remain stable.
Mechanical components can retain residual stress after machining and assembly. As the instrument is operated and time passes, stress release may cause small changes in certain geometric relationships.
This is one reason DADI places particular emphasis on repeated inspection of 2C and the compensator.
Repeated measurement helps distinguish an instrument that produces one acceptable result from an instrument whose adjustment remains consistently within its required range.
Use mechanical adjustment, dedicated equipment, and software fine-tuning to reduce the remaining adjustable errors as far as practical.
Use repeated inspection to confirm that the instrument continues to satisfy its technical requirements after adjustment and subsequent handling.
These two objectives complement each other.
An instrument with a small residual error still requires stability verification. Similarly, a stable measurement result must remain within the appropriate technical acceptance range.
DADI's factory calibration process therefore addresses both the magnitude of adjustable residual errors and the consistency of the final measurement condition.
An electronic theodolite is not considered fully verified for shipment simply because it passed inspection when it first left the production line.
DADI uses a three-stage inspection process to establish an initial qualified baseline and check whether the instrument's condition changes before shipment.
After the instrument comes off the production line, its adjustable inspection items are brought within the specified acceptance ranges.
A formal inspection is then performed, and the qualified condition of the individual instrument is documented.
The first inspection record is retained in paper form and serves as the baseline inspection record for that instrument.
This baseline provides the reference for subsequent checks.
When the instrument is being prepared for shipment, DADI measures the key inspection items again.
The new results are compared with the qualified baseline established during the first inspection.
This comparison helps identify any relevant changes that may have occurred during storage or subsequent handling.
On the shipment day, the instrument undergoes another inspection covering the same key items as the second inspection.
The results are again checked against the original qualified condition.
The purpose is to verify that the instrument still meets the applicable technical requirements at the time of dispatch.
A later measurement does not have to be numerically identical to the first recorded value.
The important requirement is that the current result remains within the specified acceptance range for that inspection item.
If a result exceeds the corresponding technical limit, the instrument must be readjusted or recalibrated as required.
First inspection: Adjust, verify, and establish the baseline record.
Second inspection: Recheck key measurements and compare with the baseline.
Third inspection: Repeat the key checks before dispatch.
Acceptance decision: Continue only when the current results meet the applicable inspection requirements.
DADI currently retains the initial formal inspection records primarily in paper form.
The company plans to gradually enter these records into an internal system to support more systematic retrieval and traceability. This is a planned improvement to record management rather than a claim that all historical records have already been digitized.
DADI's production workshop does not normally experience extreme temperature changes during routine assembly and factory calibration.
Temperature-related testing is carried out separately using dedicated low-temperature cooling equipment and high-temperature heating equipment.
Routine calibration conditions and dedicated temperature-performance testing serve different inspection purposes.
An electronic theodolite contains precision optical and mechanical components whose calibrated relationships may be affected by significant impact or severe transportation shock.
Even if the instrument powers on and displays angles normally, this alone does not confirm that its calibrated geometric condition remains unchanged.
After a significant impact, the relevant inspection items should be checked again, including:
2C collimation error
i-angle
Compensator condition
Horizontal angular measurement
Vertical angular measurement
Laser plummet
Upper-laser or autocollimation system, where applicable
The detailed definition, measurement procedure, and adjustment of i-angle will be addressed separately in a dedicated technical article.
In addition to the specialized angular and laser tests described above, DADI engineers identify several important conditions that must be verified before a finished instrument is accepted for shipment.
The plate level and circular level must function correctly and provide a consistent mechanical leveling condition.
The display must operate normally, and the instrument's control keys must respond properly.
Key adjustable items, including i-angle, 2C, compensator-related parameters, and other relevant index or angular conditions, must satisfy their specified requirements.
The finished instrument must pass the corresponding appearance and assembly-condition inspection.
Horizontal and vertical axis movement, clamps, and tangent-motion mechanisms must operate reliably.
These checks complement the dedicated optical, angular-accuracy, compensator, centering-system, and specialized-function inspections performed during factory calibration.
Because the final relationships between the telescope, mechanical axes, angle encoder, compensator, and centering system must be verified as a complete instrument. Passing individual component inspections does not automatically establish finished-instrument accuracy.
DADI uses a multi-tooth indexing table and dedicated measurement software. Horizontal angular accuracy is evaluated at 12 positions with repeated Face Left and Face Right observations. The software calculates the test results for evaluation against the corresponding requirements.
Not necessarily. DADI engineers aim to reduce adjustable residual errors as far as practical rather than merely reaching the acceptance boundary. Repeated inspection is then used to confirm that the result remains stable.
DADI first sights a horizontal collimator target in Face I and sets the ordinary horizontal reading to 0°. After sighting the same target in Face II, it calculates 2C = L − (R ± 180°), choosing the appropriate 180° adjustment for the circle readings. When L = 0° and R is near 180°, the calculation is 2C = L − (R − 180°).
The measured 2C result must satisfy the ±10″ factory acceptance limit. DADI also sets a tighter ±5″ internal control target. Neither number is a tolerance for the difference between repeated 2C observations.
Mechanical components may release residual stress after assembly, causing small changes in their previously adjusted condition. Repeated checks help verify the stability of important geometric and compensation parameters.
The laser plummet is used for centering the instrument over a survey point. An upper laser projects the telescope sighting direction toward a target or working area. Their functions and calibration procedures are different.
A cross target positioned more than 50 m away is observed through the telescope. After the reticle is aligned with the target center, the upper-laser spot is checked against the same reference. The laser axis and telescope line of sight are required to be aligned along the same straight line.
Yes. Every DE2A-GF500 unit must undergo additional outdoor long-range testing. The 1 km configuration is tested at an actual distance of 1 km, while the 1.5 km configuration is tested at an actual distance of 1.5 km. Key laser characteristics are checked repeatedly.
The initial inspection establishes the qualified baseline for the individual instrument. The second and third inspections verify that key measurement results remain within the required technical limits during shipment preparation and on the shipment day.
Not simply because the numerical result has changed. Recalibration or adjustment is required when the current result exceeds the applicable acceptance limit for that inspection item.
The performance of an electronic theodolite depends on the correct interaction of its optical, mechanical, electronic, and measurement systems.
Factory calibration therefore involves more than checking whether the instrument displays a correct angle once.
At DADI, dedicated inspection equipment is used to verify telescope resolution, geometric relationships, compensator behavior, and horizontal and vertical angular accuracy. Instruments equipped with special functions receive additional tests appropriate to their applications.
For example, every DE2A-GF500 undergoes repeated indoor and outdoor verification, including outdoor testing at its actual rated working distance.
DADI also aims to minimize the residual errors of adjustable items whenever practical. However, a small error observed once is not sufficient evidence of a stable instrument. Repeated testing is needed to verify that the adjusted condition is maintained.
The initial qualified inspection record provides a baseline for each finished instrument. Subsequent inspections during shipment preparation and on the shipment day confirm whether the instrument's critical conditions remain within their required limits. For 2C specifically, DADI distinguishes the ±10″ factory acceptance limit from its stricter ±5″ internal target and from separate repeatability observations.
The purpose of factory calibration is not only to demonstrate that an instrument has met its specifications, but also to minimize adjustable residual errors where practical and verify that the instrument remains in an acceptable, stable measurement condition before delivery.
The following technical resources provide further information on DADI's electronic theodolite measurement principles, compensator calibration, axis inspection, absolute encoder inspection, and manufacturing processes.
DADI's Face I / Face II collimator procedure, 2C calculation, mechanical adjustment and the distinction between acceptance and repeatability.
Read Technical Article →How optical sighting, absolute angle encoding, and electronic corrections contribute to the displayed angle.
Read Technical Article →Single-axis and dual-axis compensation, residual instrument tilt, linearity calibration, and zero-point adjustment.
Read Technical Article →How DADI checks critical axis components using dedicated precision inspection equipment.
Read Factory Article →Inspection of the absolute encoder disk and its optical code patterns before instrument assembly.
Read Factory Article →DADI's electronic theodolite product development, instrument manufacturing, inspection, and customization capabilities.
Explore Manufacturing →The standard DE2A electronic theodolite configuration, featuring 2″ angular accuracy and absolute encoder angle measurement.
View DE2A Product →
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