Map Matching Failure Modes in Production

False Matches
The algorithm matches to the wrong road. A highway runs parallel to a service road 20 meters away. GPS noise puts points closer to service road. Algorithm picks service road. Route shows vehicle on wrong road.
Consequences: wrong ETA (service road is slower), wrong distance (service road is longer), wrong turn instructions. User sees car on small road when they are on highway. Trust in the system erodes.
Path Discontinuities
Matched path jumps between disconnected roads. One point matches to Road A, next point matches to Road B, but A and B are not connected. The vehicle would have to teleport or drive through buildings.
Causes: insufficient consideration of road connectivity, sparse GPS points, algorithm prioritizing point-to-road distance over path continuity. Solutions: penalize disconnected transitions, require valid path through road network, fill gaps with shortest-path routing.
Stale Map Data
Road network changes: new roads built, old roads closed, turn restrictions added. If your map data is months old, matching fails for new roads. Vehicle appears to drive through fields because the road does not exist in your data.
Mitigation: regular map updates, detection of systematic mismatches (many vehicles show same error), fallback to raw GPS when no good match exists. Flag trips with low match confidence for review.
⚠️ Key Trade-off: Aggressive matching forces every point onto a road, even when GPS is in a parking lot or off-road. Conservative matching leaves points unmatched when uncertain. Balance based on use case: navigation needs continuity, analytics can tolerate gaps.

💡 Key Takeaways

✓False matches: wrong parallel road due to GPS noise

✓Path discontinuities: jumps between unconnected roads

✓Stale maps: new roads not in data cause systematic failures

✓Penalize disconnected transitions to maintain path continuity

✓Flag low-confidence matches; do not force every point onto a road

📌 Interview Tips

1Describe parallel road problem: highway and service road 20m apart, GPS noise picks wrong one

2When path jumps to disconnected road, explain connectivity constraint: valid path must exist through network

3For stale map detection, monitor systematic mismatches: many vehicles showing same impossible path indicates map issue

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