Making sense of a list of GPS values in an iOS application - ios7

Question

I have a web service that interfaces with the google maps API to generate a polygon on a google map. The service takes the GPS values and stores them for retrieval.
The problem is that when I try and use these values on my iPhone app the MKPolyline is just either a mess or a bunch of zig-zag lines.
Is there a way to make sense of these values so I can reconstruct the polygon?
My current code looks like this
private void GenerateMap()
{
var latCoord = new List<double>();
var longCoord = new List<double>();
var pad = AppDelegate.Self.db.GetPaddockFromCrop(crop);
mapMapView.MapType = MKMapType.Standard;
mapMapView.ZoomEnabled = true;
mapMapView.ScrollEnabled = false;
mapMapView.OverlayRenderer = (m, o) =>
{
if (o.GetType() == typeof(MKPolyline))
{
var p = new MKPolylineRenderer((MKPolyline)o);
p.LineWidth = 2.0f;
p.StrokeColor = UIColor.Green;
return p;
}
else
return null;
};
scMapType.ValueChanged += (s, e) =>
{
switch (scMapType.SelectedSegment)
{
case 0:
mapMapView.MapType = MKMapType.Standard;
break;
case 1:
mapMapView.MapType = MKMapType.Satellite;
break;
case 2:
mapMapView.MapType = MKMapType.Hybrid;
break;
}
};
if (pad.Boundaries != null)
{
var bounds = pad.Boundaries.OrderBy(t => t.latitude).ThenBy(t => t.longitude).ToList();
foreach (var l in bounds)
{
double lat = l.latitude;
double lon = l.longitude;
latCoord.Add(lat);
longCoord.Add(lon);
}
if (latCoord.Count != 0)
{
if (latCoord.Count > 0)
{
var coord = new List<CLLocationCoordinate2D>();
for (int i = 0; i < latCoord.Count; ++i)
{
var c = new CLLocationCoordinate2D();
c.Latitude = latCoord[i];
c.Longitude = longCoord[i];
coord.Add(c);
}
var line = MKPolyline.FromCoordinates(coord.ToArray());
mapMapView.AddOverlay(line);
mapMapView.SetVisibleMapRect(line.BoundingMapRect, true);
}
}
}
}
MKPolygon / MKPolygonRenderer gives the same sort of random line mess. The OrderBy LINQ makes no difference other than to make the random lines a zig-zag going up or down the view.

Answer 1

Since you don't know the order the points were captured in, you can't trace the actual path traveled around the perimeter of the paddock; this is why your polylines are turning into silly-walks all over the map. Lacking that information, you can at best make an educated guess.
Some possible heuristics you might want to try:
Take the average of all the points to get a "somewhere in the middle" point, then order by atan2(l.latitude - middle.latitude, l.longitude - middle.longitude). (Be careful, atan2 is undefined at (0, 0)!)
Take the convex hull of the points captured: for a relatively small number of points you can get away with the simple quadratic time Jarvis's march. This has the approximate effect of wrapping a notional rubber band around the outside of the map push-pins by discarding points that would form concavities, and should also give you the order of the remaining points.