Reduce array of coordinates - ios

Question

I track the users location in my app to a database with all the coordinates. I then do some stuff to select a range of coordinates in a time frame, but when I save it to the server is takes a long time due to the big amount of data. (15 minutes is 900 CLCoordinate2D's and that is quite a bit).
What I wanna do is to remove the coordinate which are intersected by the preceding and following coordinate. Using overly simple coordinates for illustration purposes, but imagine this being done on real coordinates in an array of a couple thousand objects.
Example:
0,0 //Keep
1,1 //Drop
2,2 //Drop
3,3 //Keep
3,4 //Keep
4,4 //Keep
5,3 //Keep
Or, shitty visualized:
I know I should probably use some vector stuff, but I am not good at maths.
How can I reduce this array to remove the obsolete points?

Answer 1

You could try something like this...
var coordTimes:[(coord: CLLocationCoordinate2D, time: Double)] = []
// ...
func appendCoord(newCoord: CLLocationCoordinate2D, newTime: Double) {
guard coordTimes.count > 1 else {
coordTimes.append((newCoord, newTime))
return
}
let n = coordTimes.count
// So there are at least two already in the array
let c0 = coordTimes[n - 2].coord
let t0 = coordTimes[n - 2].time
let c1 = coordTimes[n - 1].coord
let t1 = coordTimes[n - 1].time
let dt = t1 - t0
let dtNew = newTime - t0
guard (dtNew > 0) && (dt > 0) else {
// decide what to do if zero time intervals. Shouldn't happen
return
}
// Scale the deltas by the time interval...
let dLat = (c1.latitude - c0.latitude) / dt
let dLon = (c1.longitude - c0.longitude) / dt
let dLatNew = (newCoord.latitude - c0.latitude) / dtNew
let dLonNew = (newCoord.longitude - c0.longitude) / dtNew
let tolerance = 0.00001 // arbitrary - choose your own
if (abs(dLat - dLatNew) <= tolerance) && (abs(dLon - dLonNew) <= tolerance) {
// Can be interpolated - replace the last one
coordTimes[n - 1] = (newCoord, newTime)
} else {
// Can't be interpolated, append new point
coordTimes.append((newCoord, newTime))
}
}
The tolerance is important, as you are very unlikely to get exactly matching intervals. Also, for the geodesists amongst you, there is no need to convert into map coordinates or calculate true distances, as the OP simply wants to know if the coordinates can be interpolated.