Originally Posted by
FBinNY
For any of a number of technical reasons, the ONLY way to accurately and consistently measure speed and distance ridden is with a wheel mounted sensor.
GPS simply can't duplicate that because it it calculates what might be called "connect the dots" distance vs. Actual distance ridden. It's actually pretty good for most purposes, but will never satisfy data obsessed people who look too closely.
As long as you have a good GPS lock phone apps work fine for 95% of the population. Those dots that are being connected can be 60 seconds apart or 0.5 seconds apart (or any other number if you have a configurable app). The closer together in time the GPS refreshes, the more accurate your readings for speed and distance. (But the harder the app will be on your battery)
Now if you're phone has a crummy GPS antenna and you lose a lock every few minutes your speed and distance data is going to be haywire. (Once I lost GPS for about a mile and when it relocked it thought I had gone that mile in about 1 second so my average speed for the trip was about 100mph. (I ate my Wheaties that morning)
GPS is accurate to within 3 feet or less, even on a phone, if the phone's decent enough. On my LG G5 my GPS can tell which side of my living room I'm in, from inside the house, with the GPS signal going right through the roof. I can bring up my location on Google Maps and look at the satellite feed and I can watch my phone move around my house. My bike phone that I use for mileage and what not is an off brand cheaper Android and it's not nearly as good. It's fine when I'm outside but it can't get a lock on GPS without a direct line of sight (tree branches and stuff not withstanding)
Now it probably wouldn't notice if I was weaving back and forth across a bike path 2 or 3 feet at a time, thus adding distance traveled. But I also don't do that much. I pretty much ride in a straight line. It can easily see me doing circles in a parking lot or something like that though. I can look at the GPS tracking data and see where I did a loop around a parking.
So for the purposes of most people, a phone as a GPS works just fine.
If you're a competitive racer and need hyper accurate Swiss Timing Olympic style accuracy, the phone won't do. If you want to know if you biked 15 miles or 15.1 miles, a half decent phone works fine.
As for apps. Urban Biker. I love it. I don't use it for tracking much. I like it because it has a good dashboard display that tells me thins like distance and speed and can overlay on a map so it's right there if I'm in an unfamliar place.