i want to develop a geolocation based app. Currently i am reading the phones position and get the longitude and latitude values.
These are for example currently
Lat 50.89808359344173 Lon 4.482628385834053
I have 14 decimal places for the latitude an 15 for the longitude.
What is the meaning of these places?
A man is being shown around a natural history museum by a guide, sees a fossilised tyrannosaurus skeleton. He asks the guide how old it is, and the guide tells him it's 67million and 13 years old.
"Wow, how can you be so accurate?"
"Well, I was told it was 67million years old when I started here, and that was 13 years ago".
The precision of your result is similar to this. Or to calculating your taxes on a transaction and realising you need to pay USD1492.19482019331. The software has done some calcuations based on GPS and/or wifi location signals, and that's the answer it came up with, but it isn't really so precise as to be accurate to the nearest atom's width, it's false precision; precision in a result that is greater than the accuracy of that result.
Just how accurate it is will depend on which services are turned on (gps or wifi-based only), available distinctive wifi signals (makes the wifi-based better) whether you are indoors or in a built-up area (makes GPS less accurate) speed you are moving. It could be better than a few meters or even as bad as 1km. There's also a difference in absolute and relative accuracy, so it might be e.g. 30m out, but after the phone has moved 100m still be 30m out, and have tracked that relative movement of 100m to within a meter or so.
The phone might not only be inaccurate but also be inaccurate in trying to judge how inaccurate it is! This is a reason in itself not to round the figure; how much to round them? It's also a good general principle that you should do any rounding at the last possible point; better to have false precision greater than your accuracy than to introduce a rounding error by rounding early and actually lose accuracy further.
If you're storing the positions in a memory-conscious way, or presenting the figures directly, then 5 or 6 decimal places is probably more than enough to be sure you're being more precise than you can really claim to be.
If you're plotting a point on a map, you may as well just feed in the ridiculously over-precise figures you get; it won't hurt anything, and its simpler than trying to second-guess just how accurate you really are.
