A tilt sensor inside the tracker's rotating head measures how far the instrument is leaning away from gravity, in two directions at once. That single sensor is the source of the digital bubble, the L and T values, and the pass or fail on a levelness check.
The tracker does not infer level from the tripod, the floor, or the control points. It measures gravity directly, and everything downstream that depends on a true vertical — floor elevation, 2D stationing, the elevation half of your stationing solution — depends on that one reading being good.
What the sensor measures
The sensor reads tilt on two axes at the same time: lean along the direction the tracker is facing, and lean side to side. Those two readings are what the system reports as L and T. Zero on both means the instrument is sitting square to gravity. See What the L and T Values Mean (Longitudinal and Transverse Tilt) for what to do with the numbers.
Why the tracker needs to know which way is down
Two angles and a distance tell the tracker where the reflector sits relative to itself. Turning that into a position in your building's coordinate system requires knowing how the instrument is oriented, and gravity is the reference that fixes the vertical part of that orientation.
If the instrument is tilted and does not know it, every vertical measurement it reports is rotated by that same amount. The error is small nearby and grows with distance, which is why a tilt too small to see by eye can still put elevation readings out of tolerance at the far end of a bay.
What happens when you run a levelness check
The levelness check runs the tracker's orient-to-gravity routine: the head turns to face several directions in turn and measures its own tilt at each one. Passing means the instrument was level enough at every position, not just the one it started in. When you station with floor elevation enabled, that sequence is four positions and takes about 45 seconds.
The bubble on the instrument and the bubble on the screen
The circular bubble on the tracker body and the digital bubble on your screen do not read from the same thing. The digital bubble comes from the tilt sensor inside the instrument. The physical bubble is a mechanical part that drifts out of true over time with transport and vibration.
Use the physical bubble to get roughly level quickly, then finish with the digital bubble. In normal operation, where the two disagree, the digital bubble is right.
Do not adjust the circular level yourself
The circular level has adjustment screws behind it. Do not run it. If the physical and digital bubbles disagree, that is normal and no action is needed — level with the digital bubble and keep working. If the tracker will not level at all, gives readings that jump around, or fails levelness checks in setups that used to pass, contact Dusty Support. The fix may be a calibration issue inside the instrument, and a field adjustment of the outside bubble will hide the symptom without correcting the measurement.
When the levelness check fails
| What you see | What it usually means | What to do |
|---|---|---|
| Head moves to the first position, then returns | Not level enough for the routine to start | Re-level with the digital bubble and re-run |
| Levelness check fails after rotating | Level at the start position, out of range at another rotation | Re-level, then confirm the reading holds as the head turns |
| Readings drift or jump | Tripod moving — soft ground, a loose leg lock, vibration nearby | Lower the tripod, retighten every lock, confirm no rocking, re-run |
| Elevation values disagree with the file | Often a stationing-height problem rather than a tilt problem | See the tilted-tracker article linked below |
Exact messages you may see include "The tracker may be tilted" and "Recorded elevations do not match the file data."
A tracker that repeatedly fails to level in a spot where the tripod is solid is worth a support call rather than another hour of adjustment.
What the sensor can and cannot see
These figures are the same on every tracker Dusty deploys.
| Specification | Value |
|---|---|
| Setting accuracy | ±1 arc second (2σ), about ±0.0003° |
| Tilt range the instrument compensates for orient-to-gravity | ±227 arc seconds, about ±0.063° |
| Total tilt measuring range | ±616 arc seconds, about ±0.171° |
| Compensation | Two axes — lengthwise and crosswise |
Past the total measuring range the sensor reports nothing at all. A tracker that shows no tilt values is not level to within a fraction of a degree — it is leaning far enough that the sensor has run out of scale, and the tripod needs resetting rather than fine adjustment.