When you're recording control points and the Distance Errors or Station Errors numbers come back high, it's tempting to nudge the reflector around on the control point until the numbers drop. Don't. Lowering the number this way doesn't fix your control — it hides a real problem and shifts your whole layout.
The short answer
The error numbers exist to warn you that a control point is bad. Moving the reflector to make the warning go away doesn't make the control good — it just stops the system from telling you it's bad. You end up with a station that looks clean and prints confidently wrong.
Why the numbers drop when you move the reflector — and why that's a trap
The laser tracker records the position of the reflector, not the mark on the floor. If the reflector isn't sitting on the true control point, the tracker records the wrong coordinate.
So when you slide the reflector off the mark and the error drops, you haven't corrected anything. You've fed the system a coordinate that happens to "agree" better with the math — a coordinate that doesn't match where the point actually is on the slab.
Stationing then takes that false coordinate and best-fits your entire layout to it. The system rotates and shifts the whole layout as one rigid piece to sit on the points you gave it. One bad point drags the whole fit with it.
⚠ A lower error number is not the goal. An accurate layout is. It is possible to drive the error numbers down while making the layout less accurate. That's exactly what moving the reflector off the mark does.
What the error numbers are actually telling you
High Distance Errors and Station Errors are an intentional warning system. They almost always mean one or more control points were mis-surveyed, mis-identified, or disturbed.
A single bad control point disagrees with every point it's compared against, so it inflates the numbers across the board. The fix is to find and remove that point, not to massage the reflector until the warning disappears.
What to do instead
- Re-seat the reflector on the true mark first. Confirm the Control Point Target is centered on the slab mark and the Blue Ring Reflector is fully seated, then re-scan. Clearing debris or fixing a wobble can legitimately lower the error — because you've made the placement more accurate, not less.
Use the Distance Errors table to find the bad point. The control point that shows up in multiple high-error rows is your suspect.
Remove bad points one at a time and re-check, until your errors are within your project tolerance (typically between 1/16" and 1/4", depending on the work).
- Re-survey, don't just delete, a point you need for coverage. If a point you need to surround the print area is genuinely bad, ask the surveyor to re-shoot it.
Keep your points spread out and around the print area. Don't drop a far, well-placed point just because it has a slightly higher (but in-tolerance) number — those points do the most to keep a large layout accurate.
For the full walkthrough, see Troubleshooting High Error Values During Stationing.
The one time moving the reflector is correct
There's a real difference between correcting placement and gaming the number:
| Correct | Not correct |
|---|---|
| Re-centering the reflector so it sits exactly on the true control point mark | Sliding the reflector off the mark until the error number looks better |
| Clearing debris, fixing a wobble, fully seating the reflector, then re-scanning | Re-scanning at a spot you know isn't the surveyed point |
| Goal: make the recorded position match reality | Goal: make the number look smaller |
If re-seating the reflector on the true mark lowers the error, great — that's an accurate placement. If you can only lower the error by moving off the mark, the number is telling you the point is bad. Listen to it.
Bottom line
Bad control is a control problem, not a robot problem, and it can't be fixed by moving the reflector. Re-seat on the true mark, remove or re-survey the bad point, and accept the station only when it's genuinely within your project tolerance — not just when the number looks low.
Related Articles
- Troubleshooting High Error Values During Stationing
- Ensuring the Reflector is Properly Seated on the FieldPrinter
- Understanding Control Point Error vs Difference Values in the FieldPrinter iPad App
- How to Use the Verification Reflector to Detect Laser Tracker Movement