Understanding EVV and GPS accuracy

Why GPS or Location Data Can Be Inaccurate

How GPS Determines Location

GPS-enabled devices (like smartphones or tablets) determine your location by receiving signals from a network of satellites.

  • Minimum signals needed: At least 4 satellite signals are typically required for an accurate 3D fix (latitude, longitude, and altitude).
  • Accuracy range: Under ideal conditions, GPS can be accurate to within 5–10 meters.

Factors That Can Cause GPS Inaccuracy

  1. Obstructions
    • Tall buildings (urban canyons) can block or reflect signals, causing position shifts.
    • Dense tree cover or mountains can reduce the number of satellites the device “sees,” lowering accuracy.
    • Concrete walls, tile, or metal roofs (especially in apartments or first-floor units) can weaken GPS reception.
  2. Signal Reflection (Multipath Errors)
  • GPS signals may bounce off nearby structures before reaching your device, which can make your location appear far from your actual spot.
  1. Atmospheric Conditions
  1. Severe weather or ionospheric activity can slightly delay signals and reduce precision.

Indoor & Wi-Fi–Based Location

When your device is indoors, GPS signals can be too weak to use. Many apps and browsers then rely on Wi-Fi positioning instead.

How Wi-Fi Location Works:

  • Your device measures nearby Wi-Fi router signals and sends that data to a location service (such as Google Location Services).
  • The service looks up the router’s registered or “last known” location in its database.
  • If your device is connected to a Wi-Fi router provided by an Internet Service Provider (ISP), the location might reflect the ISP’s equipment hub — not your actual position.

Possible inaccuracies with Wi-Fi location:

  • Off by several miles if the router’s recorded location is outdated.
  • Off by 100+ miles (even 200 miles or more) if the ISP’s registered hub location is used instead of your current location.
  • Apartments, offices, or public buildings that share a central Wi-Fi system may all appear at the same coordinates.

Example: Why You Might Appear 200 Miles Away

If you’re indoors, on Wi-Fi, and your browser uses the ISP’s hub location instead of GPS, it may place you hundreds of miles from your true location.

  • Example scenario: You’re in Phoenix, AZ, but your ISP’s routing center is in Los Angeles, CA. Your location may show up as Los Angeles until GPS reception is restored.

Improving Accuracy

  • Step outside or near a window to improve GPS reception.
  • Temporarily disable Wi-Fi to force GPS-based location.
  • Keep device location settings enabled and up-to-date.
  • Give your device a few minutes to lock onto multiple satellites before using location-dependent features.