How a mileage app cut GPS errors from 50% to 95% accuracy by doubting bad data

A developer building a mileage-tracking app discovered that GPS signals are frequently unreliable in real-world conditions such as tunnels, urban canyons, and parking garages, causing absurd readings like a stationary car clocking 400 km/h. The team shifted their approach from trusting GPS as a source of truth to actively filtering out physically impossible data points before they could affect trip logs. When GPS drops out entirely, the app now uses dead reckoning — combining the last known speed and heading with accelerometer data — to estimate position during short blackouts like tunnel crossings. A weighted filtering system was also introduced so that jittery or inconsistent GPS signals have less influence on the final position estimate than clean, stable ones. These layered techniques collectively pushed trip-tracking accuracy from around 50 percent to 95 percent.