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.

Speed Through a Transition

Transitions are where the device-specific speed limits matter most, because a doorway or a corner is the published slow case on all three.

• L2 Pro and K2: 1.6 ft/s (0.5 m/s) at the threshold and through the turn.
• PortalCam: 1.0 ft/s (0.3 m/s). Doorway transitions and corners are named specifically in its slow-condition rule, and its 1.6 ft/s figure is a ceiling for open conditions rather than a transition speed.
• All devices: stay at least 1.6 ft (0.5 m) from the surface you are scanning, and 1.6 to 3.3 ft (0.5 to 1 m) on the K2. Doorframes are the most common place to end up closer than that.

Interior Doors: Pre-Opened

Where access permits, pre-open all interior doors along the route before startup. Every door you open in advance is a closed-door procedure you do not have to perform mid-scan.

1. Begin slowing 2 to 3 m (6 to 10 ft) before the doorway, down to the transition speed for your device from Section 1.
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. The technique is to keep the moving panel out of the LiDAR's view using your own body.

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, use the same method in reverse: turn so your body is between the scanner and the door, close it with your back to it, then resume. Do not stop the scan to close a door. Stopping ends the segment permanently and turns one session into two that must be fused.

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 your device's transition speed or slower, with consistent posture and no stopping on the stairs themselves.
3. Hold normal posture while climbing. Stay within the tilt limit for your device: not beyond 20 degrees on the L2 Pro, not exceeding 15 on the K2. Tilting up at 30 degrees for a whole flight to reach the underside above exceeds both limits and degrades the data you are climbing to collect.
4. Capture the underside of the flight above from the landings, where you can stand relatively still. This is where a brief steeper tilt is permitted: up to 30 degrees, and on the K2, in a genuinely constrained stairwell, slowly over 30 and under 60 degrees, for no more than 30 seconds, while relatively still, then back to normal posture.
5. Scan each landing fully on the ascent. Walk the full perimeter before the next flight.
6. Pause at the top landing and capture it fully.
7. Return down the opposite side for coverage of treads, risers, and walls from a second angle.
8. 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

Alpine's field measurement is 5 to 10 minutes for a standard three-flight stairwell with full coverage. Complex stairwells with wide landings or multiple branches 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. Follow the stop procedure on 3.2 Route Planning before you press stop, because this is a real session end rather than a pause.
• Join elevator-separated sessions with Map Fusion. Each must share overlap geometry with its neighbors, and each L2 Pro, K2, or PortalCam session must carry control points or RTK data. A session with neither is rejected at import and cannot be fused at all. The L3 does not require control points for Map Fusion: its sessions are matched by region selection.
• Where a stairwell also connects the two floors, use it for at least one round trip to give Map Fusion a continuous-trajectory reference.

Entering and 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.

Going In

• Approach slowly. Never push the scanner suddenly into a small enclosed space such as a cabinet, chase, or void under 1.6 ft (0.5 m) deep. The sudden change degrades the LiDAR solution and causes data drift that propagates beyond the space itself.
• Where the space is too tight to enter within the standoff distance, scan it from outside rather than reaching in. A space smaller than the minimum working distance cannot be captured well from inside it.

Coming Out

• 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.