3.4 Challenging Environments

Tunnels and Long Corridors

Tunnels combine repetitive geometry, low feature density, and length beyond a single uncorrected session. They need segmentation, target-based alignment, and a processing mode that defaults off.

Segment Length

• 500 m (1,600 ft) per segment maximum. Divide longer tunnels before starting. A longer continuous scan is not reliably recoverable.
• Place segment boundaries at structural features (cross-passages, recesses, alcoves) that distinguish one segment's end from another's start.

Speed and Posture

• 1.6 ft/s or below throughout. Slower in low light.
• Side shuffle in tunnels under 2.5 m (8 ft) wide to keep both walls in view.
• Forward walk in wider tunnels at the same speed limit.

Multi-Segment Overlap

• Plan a shared overlap point at every segment boundary. The same physical location appears at the end of one segment and the start of the next.
• Mark the shared point within 10 cm (4 in) and 10 degrees of its position in the first scan. Tape the spot on the ground during the first segment so you can return precisely.
• Minimum spacing between shared points: 5 m (15 ft). Closer placements confuse alignment.

Control Point Placement Across Segments

• First segment: two control points at the end only.
• Last segment: two at the start only.
• Middle segments: two at the start and two at the end, four total.
• Same physical location, same point ID in every segment. This is how Map Fusion matches the point across sessions.

Target Placement

• Place high-contrast SLAM anchor targets every 30 to 45 m (100 to 150 ft) in long, featureless runs.
• The L2 Pro ships with magnetic steel anchor targets for ferromagnetic surfaces (rebar, embedded steel, metal panels). The K2 uses adhesive reflective sticker targets for any flat surface.
• These are SLAM anchors, not GCPs. Give them surveyed coordinates in LixelStudio if you also need absolute position.

Processing

Reflective Surfaces: Glass, Mirrors, Polished Metal

Reflective surfaces are the most common cause of indoor artifacts. LiDAR pulses pass through glass and bounce off mirrors, creating false geometry, and the cameras can track reflections as features.

Distance and Speed

• Stay at least 1 m (3.3 ft) from highly reflective objects where the route allows. Closer increases reflection exposure.
• Move at 1.6 ft/s past any reflective surface. Never stop in front of glass or mirrors. A continuous walk produces fewer false observations and lets SLAM treat them as outliers.

Approach and Positioning

• Approach glass at an angle, 30 to 60 degrees, not head-on. Head-on maximizes reflection back into the sensor.
• Capture each reflective surface in a single sweep. Multiple passes add conflicting reflection data that degrades the surrounding cloud.
• Put your body between the scanner and the mirror where proximity is unavoidable. Useful in bathrooms with vanity mirrors and rooms with mirrored doors.
• Cross fully-glazed facades rather than walking parallel at close range. Parallel at 2 m (6 ft) for a long run is the worst case.

PortalCam Sensitivity

The PortalCam is more affected by reflective surfaces than the L2 Pro and K2 because its 3DGS reconstruction leans on visual data. In heavily reflective spaces, use an L2 Pro or K2 for point cloud deliverables and add PortalCam coverage of less reflective areas only if 3DGS is required.

Dark Environments

Darkness degrades two of the three SLAM inputs at once: the cameras lose feature tracking and the LiDAR loses ambient reflectivity. The IMU continues, but with two inputs weak, drift accumulates faster.

Pace and Lighting

• 1.6 ft/s or below in any dark area.
• Bring a portable light panel. Even 200 to 400 lumens of aimed fill light measurably improves tracking and color.
• Aim light forward into the area you are walking toward, not down at your feet. The upcoming area must be lit before you reach it.
• For static dark spaces, set up battery work lights before scanning. They give more even light than a handheld panel and free you to focus on posture.

Anchor Targets

• Place high-contrast anchor targets before scanning. L2 Pro magnetic steel targets or K2 reflective sticker targets both serve.
• Even without surveyed coordinates, they give SLAM features to match when visual texture is minimal.
• Place them at intersections, doorways, and along long blank walls, aiming for a reliable match every 10 to 15 m (30 to 50 ft).

3DGS in Dark Areas

• Color in dark spaces is poor regardless of technique. The cameras need light, and auto-exposure introduces noise and color shifts 3DGS cannot fully fix.
• Disable color for point-cloud-only deliverables, re-enabling for lit areas.
• For 3DGS in dark spaces, bring enough light for well-exposed images, or run the dark zones as separate no-color sessions. Joining a dark session to a lit one with Map Fusion gives clearly differentiated quality zones, which is often acceptable; mixing conditions in one session looks uniformly compromised.

Outdoor Scanning

Outdoor work adds variable weather, direct sun, large open areas with sparse vertical features, and wind.

Feature Density

• Keep fixed structures in the scan path. SLAM needs vertical geometry. An open plaza with no surrounding buildings is one of the hardest outdoor cases.
• Walk the edges of open areas where buildings, walls, or fences are visible, not diagonally across the middle with nothing in view.
• For very large sites, consider drone capture for the open areas and ground scanning for the perimeter. See Module 5: Aerial-Ground Fusion.

Sun and Direct Light

• Do not point cameras at the sun or scan into direct sunlight. Overexposure corrupts color for the entire scan, not just the bright frames.
• Keep the sun at your back where possible.
• Avoid scanning toward windows in bright sun from inside; the window overexposes against a darker interior.
• Prefer diffuse light: overcast, early morning, or late afternoon. Midday sun produces harsh shadows and overexposure.

Wind and Conditions

• Wind affects the PortalCam more than the heavier L2 Pro and K2. Walk with the wind where possible.
• Rain and snow degrade LiDAR. Airborne droplets return false pulses. Heavy precipitation makes outdoor scanning impractical.
• Cold cuts battery life. Carry spares and keep them warm in an interior pocket until use.

Range Selection for Outdoor Work

The L2 Pro 32-300 reaches 300 m (985 ft) and is required for tall structures and large sites. The 16-120 and 32-120 reach 120 m (400 ft), enough for most indoor and small outdoor work but not tall facades. The K2 reaches 40 m (130 ft) at 10% reflectivity and up to 70 m (230 ft) at 80%, which suits indoor and close-quarters outdoor work but not tall facades or large open exteriors.

Featureless Corridors and Repetitive Geometry

Uniform walls, identical doorways at regular intervals, and no distinguishing features are SLAM's weakest case. Every position looks like every other from the LiDAR's view.

Movement

• Side shuffle in narrow uniform corridors to keep both walls in view, which matters most where SLAM has few anchors.
• Use diagonal paths in wider featureless spaces for perspective variation a straight centerline walk lacks.
• Branch into any available room or recess. Even a brief detour resets accumulated drift.

Targets

• Place targets at irregular intervals. Regular spacing matches the repetitive geometry; irregular spacing gives SLAM distinguishable anchors.
• Vary target placement between similar corridors. On identical corridors across floors, change target height or wall position per floor so SLAM can tell them apart.

When Multiple Challenges Combine

A dark featureless corridor with reflective windows combines three challenges; a glass walkway in sun combines two. Apply the most conservative technique from each category: the slowest applicable speed, anchor targets even if only one category normally requires them, and extra time. A compound zone that takes 10 minutes in normal conditions may take 25 with correct technique. Plan these zones explicitly, with their own segment boundaries, targets, and time. Field surprises at compound zones are how unrecoverable scans happen.