3.3 Transitions: Doors, Stairs, and Between Spaces

Why Transitions Are High-Risk

A transition is any point where the device's spatial context changes sharply over a short distance: through a doorway, up a stairwell, out of a confined space. The geometry on either side may share nothing, the walls of a stairwell look alike from many positions, and confined spaces offer few features to lock onto. Each creates conditions where SLAM can lose tracking or accumulate alignment error. Correct technique prevents most of these failures; incorrect technique cannot be fixed in processing.

Interior Doors: Pre-Opened

The cleanest doorway is already open when you approach. Where access permits, pre-open all interior doors along the route before startup.

1. Begin slowing 2 to 3 m (6 to 10 ft) before the doorway. Target 1.6 ft/s at the threshold.
2. Leave the door half open. A fully open door sits parallel to the wall and shows the LiDAR only one face; a half-open door at an angle lets it capture both faces and the frame.
3. Turn 90 degrees sideways before stepping through. One side of the scanner faces the room you leave, one faces the room you enter, so both stay in view through the crossing.
4. Pause on the threshold until the preview shows point cloud from both rooms. A few seconds, varying with how complex each space is. Do not use a fixed timer.
5. Step through and pause briefly with the scanner angled back through the doorway. This captures the doorway from both sides and sets the loop closure for the new room.
6. Resume normal speed 2 to 3 m (6 to 10 ft) past the doorway.

Interior Doors: Closed

If a door cannot be pre-opened, the challenge is that opening it during the scan creates a rapidly changing geometry SLAM must process alongside the transition.

1. Turn so the scanner faces away from the door as you approach. Your body sits between the scanner and the closed door.
2. Open the door with your back to it. Your body blocks the LiDAR's line of sight to the moving panel.
3. Stand sideways under the frame, one side facing each room. Your body no longer blocks either space.
4. Pause until point cloud from both rooms appears. This takes longer than a pre-opened door, since the panel just moved through the camera's view.
5. Enter slowly, resuming normal posture 2 to 3 m (6 to 10 ft) past the doorway.
6. If you must close the door behind you, stop the scan first. Closing it live produces the same changing geometry with no benefit.

Stairwells

Stairs look alike from flight to flight, stairwell walls repeat from many positions, and vertical movement compounds the similarity. Comprehensive coverage needs more than a single ascent.

Standard Procedure

1. Scan the base landing fully before the ascent. It is the reference geometry the next flights align against.
2. Walk up one side at 1.6 ft/s or slower, consistent posture, no stopping on the stairs.
3. Angle the scanner upward, within 30 degrees of vertical, to catch the underside of the flight above.
4. Scan each landing fully on the ascent. Walk the full perimeter before the next flight.
5. Pause at the top landing and capture it fully.
6. Return down the opposite side for bidirectional coverage of treads, risers, and walls.
7. Scan each landing again on the descent. These returning landings provide the loop closures that align the multi-floor scan.

Enhanced Coverage: The Sideways Pass

For maximum detail, include one pass walking sideways (crab-walking) up or down. It gives the LiDAR a different perspective on tread and riser geometry. Slow and requiring careful footing, but noticeably better stair detail.

Time Estimate

Budget 5 to 10 minutes for a standard three-flight stairwell with full coverage. Complex stairwells take longer. Do not rush them; they are time-consuming to rescan and contribute disproportionately to overall quality.

Elevators

The elevator car moves through space the scanner never sees, so SLAM cannot track it; it sees only the car interior during the ride.

• Do not scan inside elevator cars. The interior is small and repetitive, and the exterior geometry changes completely between floors, breaking SLAM tracking.
• Stop the scan before entering. Ride without scanning, start a new session when you exit.
• Join elevator-separated sessions with Map Fusion. Each must share overlap geometry with its neighbors.
• Where a stairwell also connects the two floors, use it for at least one round trip to give Map Fusion a continuous-trajectory reference.

Exiting Confined Spaces

When you finish a closet, mechanical chase, or other confined space and turn to leave, the view changes abruptly from close walls to open geometry. Abrupt reversal without LiDAR contact on structural features causes reference loss, because SLAM has no continuous geometry to bridge the transition.

• Keep feature-rich geometry in view as you exit. Doorframes, equipment, conduit, or wall geometry should stay in the LiDAR's view throughout.
• Walk backward out when possible. This keeps the scanner facing the geometry you just captured, providing continuous reference.
• If you cannot walk backward, turn slowly over several seconds and pivot at the entrance, where the doorframe bridges the rotation.
• Pause briefly at the threshold, confirming in the preview that the interior remains in the trajectory before walking away.