XGRIDS Pro Guide™ / Module 3: Field Technique

3.3 Transitions: Doors, Stairs, and Between Spaces

Transitions are where scans fail most often. Doorways, stairwells, and floor connections all compress a lot of tracking challenge into a short distance. Each has a specific technique that keeps SLAM stable through the transition.

Section 1

Door Transitions

A doorway concentrates several problems at once: rapid change in the geometry visible to the scanner, potential lighting change, a narrow passage that restricts movement, and often a feature-poor zone immediately on either side. Passing through a doorway too quickly is one of the most reliable ways to introduce a tracking disruption.

Interior Doors

Slow to 1.5 ft/s as You Approach

Begin reducing speed at least 6 to 10 feet before the doorway. Do not slow abruptly, taper speed gradually.

Leave the Door Half Open

A fully open door leaves one side of the door frame area unscanned. A half-open door allows the scanner to capture door geometry on both sides of the opening while passing through. Scan the door from 2 to 3 feet away before stepping through to capture its surface and the surrounding frame.

Turn Sideways and Enter Slowly

Turn 90 degrees so that one side of the scanner faces out (back toward the room you are leaving) and one side faces in (toward the room you are entering). This keeps both visual cameras acquiring geometry from both spaces simultaneously during the transition rather than cutting abruptly from one environment to the other.

Pause 5 Seconds on Each Side

Before entering, pause briefly in the doorway. After passing through, pause again on the new side. These short pauses allow SLAM to accumulate reference geometry from each environment before the visual frame changes completely.

Resume Normal Speed After 6 to 10 Feet

Once you are fully through and have moved a few feet into the new space, taper back up to normal scanning speed. Do not accelerate immediately after the transition.

Entry Doors (Building Perimeter)

For main building entry doors, typically heavier, with self-closing mechanisms, keep the door closed and scan it thoroughly from 2 to 3 feet away before passing through. The door itself is geometry that many operators skip and discover missing in the final scan. Capture it deliberately.

The sideways entry technique applies to all XGRIDS devices, not just PortalCam. Turning sideways ensures the scanner captures geometry from both the room you are leaving and the room you are entering simultaneously during the transition. For PortalCam specifically, the priority is even higher: its front-facing camera assembly has a fixed orientation, and entering a doorway straight-on cuts the visual frame abruptly, producing tracking disruptions more often than the sideways technique.

Section 2

Lighting Transitions

The visual cameras in XGRIDS devices adapt to ambient light, but they cannot adjust instantaneously. A sudden transition from bright outdoor light to dark interior, or from a lit corridor to an unlit room, gives the cameras a frame or two of severely over- or under-exposed imagery. In that window, visual tracking is degraded or lost.

Before transitioning outdoor to indoor: ensure all indoor lights in the destination space are turned on. The goal is to minimize the brightness difference between the two environments, not eliminate it entirely
Use supplemental lighting in dark transition zones: a portable LED panel held by a second person significantly reduces the tracking disruption in dim lobbies, dark corridors, and unlit rooms
Slow down through any lighting transition, regardless of whether it is a doorway. The camera adapts more reliably when there is more time per unit of distance
Never point the scanner toward direct sunlight during outdoor-to-indoor transitions. Overexposed frames in the transition window disrupt visual tracking exactly when the SLAM system needs it most

Section 3

Stairwells

Stairwells are one of the most demanding scanning environments in any building. They are geometrically repetitive (each flight looks like the last), physically constrained, and require continuous changes in vertical position that challenge the IMU. They also connect floors, making a high-quality stairwell scan critical to maintaining a connected, accurate multi-floor dataset.

Stairwell Scanning Procedure

Scan the Landing Thoroughly Before Ascending

At the bottom landing, walk a small loop to capture the full landing geometry and give SLAM reference points it can use for loop closure when you return. Do not step onto the stairs until the landing is fully covered.

Ascend Very Slowly, Keeping the Device Vertical

Walk up the stairs at the slowest pace that is stable for you, slower than normal walking pace. Keep the device vertical throughout. Resist the natural tendency to tilt the device upward as you climb. Scan the underside of the flight above you as you ascend by angling slightly, but stay within 30 degrees of vertical.

Scan the Upper Landing

At the top landing, walk a loop to capture the landing geometry thoroughly. This landing connects the stairwell to the floor above, it needs full coverage on all sides.

Descend the Same Flight

Turn around and descend the same flight of stairs you just ascended. The return pass captures the surfaces that faced away from you on the way up (the opposite stair risers, the outer walls) and creates the loop closure between the two landings. A stairwell scanned only in one direction will have incomplete geometry and no loop closure between floors.

Repeat for Each Flight

If the stairwell has multiple flights, ascend each one and descend it before moving to the next. Do not scan multiple flights in one direction and return all at once, each flight needs its own up-down pair with landing loops at both ends.

Stairwells scan slowly by necessity. Budget significantly more time per vertical meter than any other building element. A standard three-flight stairwell properly scanned with landing loops and bidirectional passes takes 5 to 10 minutes for complete coverage. Complex stairwells with wide landings or branching flights take longer. Do not rush stairwells — they are a high-failure zone and are time-consuming to rescan.

Section 4

Multi-Floor Connections

Connecting multiple floors in a single scan session requires that SLAM maintain continuous tracking through the stairwell or other vertical transition. The stairwell scan procedure above addresses this. The key additional consideration is the order in which floors are scanned.

The most reliable approach is to complete one floor entirely before moving to the next. Scan all rooms and corridors on floor one, then pass through the stairwell to floor two, scan all of floor two, and so on. Mixing floors, scanning part of one, going to another, returning to the first, creates a more complex trajectory that is harder for SLAM to maintain without additional control points.

Complete each floor before moving to the next where possible
Use stairwells as planned transition points, not shortcuts between floors mid-scan
After arriving on a new floor, slow down and scan the area immediately at the top of the stairwell thoroughly before proceeding into the floor, this creates the connection geometry between floors
Return to the stairwell at the end of each floor scan to close the loop before moving on
For buildings taller than 4 to 5 floors in a single scan session, consider splitting by stairwell section and using Map Fusion, the cumulative trajectory length increases drift risk significantly in very tall structures

Section 5

Elevator Transitions

Elevators present a specific SLAM challenge: the interior of an elevator car is a small, featureless box, and the vertical motion during travel provides the IMU with a movement signal that has no visual geometry to anchor it. SLAM tracking through an elevator ride is unreliable.

The practical approach is to not rely on the elevator for vertical scan continuity. Scan each floor as a separate segment and connect floors in LixelStudio using Map Fusion, with the elevator lobby on each floor serving as the overlap zone. If you must use an elevator during a scan, scan the lobby area on both the departure and arrival floors thoroughly, this gives Map Fusion the reference geometry it needs to merge the segments accurately.

Treat elevator-connected floors as separate scan segments where possible
Scan elevator lobbies on every floor thoroughly, front, sides, door frame, ceiling indicators
If the elevator interior must be scanned (for a building documentation project), hold the device very still during transit and cover the car interior geometry at both the departure and arrival floors before and after riding
Plan Map Fusion connections through lobby geometry, not through the elevator car trajectory

Transitions covered. Next: the specific techniques for scanning environments that challenge SLAM, corridors, glass, dark spaces, and large open areas.

Challenging Environments →