GPS is convenient for recording where you’ve been, but there are some important things to understand about GPS accuracy in terms of measuring precise distances.
The GPS chip in your phone connects with anywhere between 3 to 20 GPS satellites orbiting 12,500 mi or 20,000 km above the surface of the Earth. These satellites send timestamp information down to your phone, and your phone is able to compare the timestamps sent to and triangulate your precise position and altitude.
If you’re connected to any cell towers or wifi, your phone can use your approximate location to acquire a GPS strong fix within 10 to 30 seconds or so. If you’re in airplane mode or if your using an Apple Watch without cellular capabilities, it can take up to two minutes to acquire a strong GPS fix. Once you’ve acquired a GPS fix, you’ll be able to use GPS even in airplane mode.
All in all, GPS is pretty amazing technology when you consider that it can position you to within a few meters, anywhere on Earth.
GPS is typically only accurate to, at best, around 10 feet or 3 meters in direct view of a clear sky.
If there are any obstructions to the sky, such as a dense forest canopy, heavy clouds, high canyon walls or tall downtown buildings, these can scatter the GPS signals or cause them to bounce and lead to extra error in the GPS positioning accuracy. This can lead to GPS accuracies typically around 100 feet or 30 meters, or sometimes even worse than 300 feet or 100 meters.
Even with great GPS accuracy (<5 meters), the error can accumulate as noise when you are standing still or moving slowly. This can cause your GPS track to appear to move around even when you are not moving, and can cause your speed and pace estimates to show incorrect values.
Footpath uses smoothing algorithms at slower speeds to try to reduce some of this noise, but it isn’t foolproof.
Over the course of many miles, all of these little errors and squiggles can start to over-estimate your actual distance, typically by around 1-3%. So your 26.2 mile marathon might end up showing as an extra 0.25 mile or 400 meters longer than reality.
Here’s are some detailed stats on the GPS accuracy as measured from a number of different GPS devices: https://fellrnr.com/wiki/GPS_Accuracy
Just as GPS noise can lead to overestimates of your actual distance, GPS sampling rates can sometimes lead to underestimates of distance on courses and hiking trails with tight turns and switchbacks.
GPS data is typically reported once per second. If you’re moving quickly on a course with tight corners, this can lead to some corners getting clipped or truncated.
When combined with GPS smoothing algorithms designed to combat GPS noise as described previously, an algorithm might mistake interpret tight switchbacks as GPS noise and smooth out the path.
All of this could lead to an underestimate of distance on technical trails and courses with frequent turns.
Manual route planning on the whole tends to lead to more accurate distance measurements than pure GPS, as paths can be drawn out in greater detail, and without the side-to-side noise of GPS.
Route planning is not without its own flaws: if you are using the “snap to map” feature in Footpath, your actual path may not travel exactly down the center of roads, or the path data on the map may not precisely match a path on the ground.
If you want the most accurate measure of GPS distance, you can record and save a GPS track in Footpath, and then tap the “More” tab to “Create Route” and load the raw path in the route editor.
Then you can use Footpath’s route editing tools to smooth out any noisy spots, or add more detail to certain areas using satellite imagery. With this, you can measure distances with greater accuracy.
GPS is an amazing technology, but it’s not without its flaws. With an understanding of how GPS works, you can better understand situations where GPS accuracy might suffer and affect your recordings and distance measurements.