corridor-alignment-pipeline

Corridor Alignment Pipeline with Drift Detection

Overview

Align large road corridor captures incrementally in batches, with automatic drift detection between every batch. The pipeline stops if alignment diverges from GPS, preventing hours of wasted processing.

When to Use

• Aligning road corridor captures (100+ cameras along a linear path)
• Any linear capture where drift is a concern (railways, pipelines, coastlines)
• When GPS reference data is available for cameras

Prerequisites

• Photos imported into a chunk with GPS reference data (EXIF or imported CSV)
• Sensors configured (fisheye, rolling shutter, axes) per the metashape-alignment skill
• GPU config: set_gpu_config(cpu_enable=True) for alignment

Pipeline Steps

For each batch of cameras (recommended ~200 per batch):

1. Enable batch cameras

enable_cameras(labels=batch_labels, enable=True)

2. Match and align

match_photos(
    generic_preselection=True,
    reference_preselection=True,
    keep_keypoints=True,          # ALWAYS True for incremental
    reset_matches=False            # True only for very first batch
)
align_cameras(
    reset_alignment=False          # True only for very first batch
)
save_project()

3. Check drift (CRITICAL — do this after EVERY batch)

get_camera_spatial_stats()

Evaluate the error_gradient_per_100m field:

Gradient	Assessment	Action
< 0.5 m/100m	PASS	Continue to next batch
0.5 - 2.0 m/100m	WARN	Alert user. Suggest placing GCPs in the drifting region before continuing.
> 2.0 m/100m	FAIL	STOP. Report the problem. Do NOT continue alignment.

4. Check continuity with previous batch

check_alignment_continuity(new_camera_labels=batch_labels)

If continuous is False:

• Report which cameras have position jumps or rotation breaks
• STOP and let the user investigate before continuing

5. Repeat for next batch

After all batches:

6. Full corridor assessment

get_corridor_drift_report(num_segments=10)

Review the segment-by-segment breakdown. If error increases along the corridor, GCPs should be placed in the high-error region.

7. If DEM is available

compare_alignment_to_dem(camera_height_offset=2.0)

This gives ground-truth vertical drift independent of GPS noise.

8. Optionally generate virtual checkpoints

generate_virtual_checkpoints(spacing_meters=200.0, camera_height_offset=2.0)

Creates evenly-spaced markers from DEM to measure alignment quality along the corridor. These are check points (for reporting), NOT control points (for optimization).

9. Proceed to USGS filtering

Only after drift assessment passes. See metashape-alignment skill for the filtering workflow.

Decision Tree

After each batch:
├── error_gradient < 0.5 AND continuous → next batch
├── error_gradient 0.5-2.0 → WARN user
│   ├── User says continue → next batch (with note)
│   └── User says stop → place GCPs, re-evaluate
├── error_gradient > 2.0 → STOP
│   └── Recommend: place GCPs, possibly re-align last batch
└── discontinuity detected → STOP
    └── Recommend: check camera labels, matching settings

Thresholds

These defaults work for vehicle-mounted cameras with consumer GPS (~3m accuracy):

Parameter	Default	Description
error_gradient PASS	< 0.5 m/100m	Normal GPS noise
error_gradient WARN	0.5 - 2.0 m/100m	Possible drift starting
error_gradient FAIL	> 2.0 m/100m	Alignment is diverging
max_position_jump	5.0 m	Continuity check for position
max_rotation_jump	15.0 degrees	Continuity check for rotation

For RTK GPS (~2cm accuracy), tighten thresholds by 10x.

Common Drift Causes

1. Insufficient overlap between batches — ensure 50+ cameras overlap between consecutive batches
2. Repetitive road surface — all asphalt looks the same. Enable guided_matching=True
3. Long straight corridor — no cross-track features to constrain. Place GCPs at curves
4. Mixed lighting — morning/evening batches don't match well across day boundaries
5. GPS offset — systematic GPS error in one direction. Not drift per se, but shows as gradient

Example Conversation

User: "Align these 2000 Z9 cameras in batches of 200"

Agent workflow:

1. Disable all cameras
2. Enable first 200, match (reset_matches=True), align (reset_alignment=True)
3. Check drift → PASS (0.1 m/100m, first batch baseline)
4. Enable next 200, match, align (reset_alignment=False)
5. Check drift → PASS (0.15 m/100m)
6. ... repeat batches 3-8 ...
7. Check drift → WARN (0.8 m/100m, gradient increasing)
8. Report to user: "Drift is increasing in cameras 1600-1800 area. Recommend placing GCPs near [coordinates] before continuing."
9. User places GCPs, says "continue"
10. Enable next 200, match, align
11. Check drift → PASS (0.3 m/100m, GCPs stabilized it)
12. Continue remaining batches
13. Run full corridor report → PASS
14. Proceed to USGS filtering