Sokkia Horizontal Collimation Error (C-Error): What It Means and How to Fix It

Applies to: Sokkia CX-50, CX-60, CX-101, CX-105, SET530, SET630R, SET730, iX-series

What Is the Horizontal Collimation Error?

The horizontal collimation error — commonly called the C-error or 2C value — is the angular misalignment between the instrument's line of sight (optical axis) and the plane perpendicular to the horizontal axis. In a perfectly adjusted total station, these are exactly 90 degrees. When they diverge, the measured horizontal angles in Face Left and Face Right observations disagree by twice the C-error value.

Sokkia total stations automatically check C-error during the two-face calibration routine. If the computed C-error exceeds the factory tolerance (typically ±20 arc-seconds for standard instruments), the instrument displays a warning and prompts recalibration. Uncorrected C-error causes equal-and-opposite angular offsets in Face 1 and Face 2 observations — it cancels when averaging both faces but introduces bias in single-face measurements.

Common Causes of C-Error Out of Tolerance

• Physical impact or rough handling — dropping the instrument or tripod shock shifts the reticle or telescope alignment
• Thermal cycling — repeated extreme temperature changes cause differential expansion in the telescope assembly
• Normal long-term wear — C-error drifts gradually over years of field use
• Transport vibration — extended vehicle travel over rough terrain without proper case padding
• Previous calibration performed incorrectly — improper target setup during the 2-face check
• Moisture ingress affecting the optical assembly

How to Fix Sokkia Horizontal Collimation Error — Step by Step

1. Set up the instrument on a stable tripod in a location with a clear, level sightline to a well-defined target at least 50 meters away (the farther the better for angular precision).
2. Level the instrument carefully — a poorly leveled instrument will corrupt the collimation check.
3. Navigate to Menu → Instrument Settings → Collimation Check (or Calibration → Horizontal Collimation depending on firmware version).
4. Aim accurately at the target in Face 1 (normal position) and confirm. The crosshair intersection must be precisely on the target — use the fine tangent screws.
5. Transit (plunge) the telescope to Face 2, re-aim at the same target, and confirm. The instrument computes 2C from the difference in readings.
6. If C-error is within tolerance, the instrument confirms calibration OK. If out of tolerance, follow the on-screen prompts to apply the correction electronically.
7. Repeat the two-face check to verify the correction was applied successfully. The residual C-error should now be near zero.
8. If C-error remains out of tolerance after electronic correction, the instrument requires physical adjustment by a Sokkia service center.

When to Send It In for Service

If the electronic collimation correction does not bring C-error within tolerance after multiple attempts, or if C-error returns within days of calibration, the telescope optical assembly or horizontal axis bearing requires physical realignment at an authorized Sokkia service center. Turnaround is typically 5–10 business days. Annual calibration checks are recommended for instruments in daily field use.

Preventing C-Error Drift

Always transport the instrument in its carry case with the tribrach properly secured. Perform a two-face collimation check at the start of every project. Avoid exposing the instrument to rapid temperature changes (e.g., moving directly from a cold truck to warm sunlit conditions).

Related Error Codes

Sokkia Total Station Vertical Index Error (i-Angle) | Sokkia Total Station Error E001 | Sokkia Total Station Error E003