Why is my GPS position wrong?
Sometimes users are on site, but their recorded GPS location is some distance away. A common misunderstanding is that GPS technology is always accurate. Whenever the app requests a user's GPS location, the hardware on the device provides the user's latitude and longitude along with an estimated accuracy of the reading. GPS accuracy can be as precise as one meter or as imprecise as several kilometers. This article explores why this happens and how our software works around the limitations of GPS technology.
Here is some analysis based on over 30,000 GPS readings:
- 12% of GPS readings are accurate to within 10 meters (~30 feet).
- 50% of GPS readings are accurate to within 30 meters (~100 feet).
- 90% of GPS readings are accurate to within 150 meters (~500 feet).
- 93% of GPS readings are accurate to within 800 meters (~2600 feet or a half-mile).
This means that 7% of GPS readings are off by at least a half-mile.
What causes GPS readings to be inaccurate?
Here is an excerpt from a US government website (https://www.gps.gov/systems/gps/performance/accuracy/):
GPS satellites broadcast their signals in space with a certain accuracy, but what you receive depends on additional factors, including satellite geometry, signal blockage, atmospheric conditions, and receiver design features/quality.
For example, GPS-enabled smartphones are typically accurate to within a 4.9 m (16 ft.) radius under open sky (VIEW SOURCE AT ION.ORG). However, their accuracy worsens near buildings, bridges, and trees.
Many things can degrade GPS positioning accuracy. Common causes include:
- Satellite signal blockage due to buildings, bridges, trees, etc.
- Indoor or underground use
- Signals reflected off buildings or walls ("multipath")
Far less common causes may include:
- Radio interference or jamming
- Major solar storms
- Satellite maintenance/maneuvers creating temporary gaps in coverage
- Improperly designed devices that do not comply with GPS Interface Specifications
How can I get better GPS readings from the app?
Unfortunately, the Clean Smarts app is entirely dependent on the GPS readings provided by each user's device. Different devices may provide different levels of GPS precision.
What does the software do to work around these limitations?
When Clean Smarts checks to see whether a user is on-site, the software considers the accuracy of the reading. If the distance to the location is within the estimated accuracy, the user is allowed to check in.