Question

I’m currently developing server side to Application,
in the App I have a lot of interest point in big area (over 1000 points)
and I want to find the nearest points to user device.
I’ve try to used:

.GetDistanceTo(GeoCoordinate);

from the libary:

 System.Device.Location;

example query:

  from point in db.Points
  where ((new GeoCoordinate(point.lat,point.lng)).GetDistanceTo(new GeoCoordinate(coordinates[0],coordinates[1]))<1000)) 
  select point

but in the Linq Query it’s not supported and if I try to use it on a List<> or an Array it takes too long…

How can I do it better and faster?
Thanks

Answer 1

I assume that you are performing this computation often, otherwise iterating over a thousand points should not take a large amount of time – certainly under a second.

Consider caching the points in memory as GeoCoordinates, since I am guessing the bulk of the time may be spent allocating memory and instantiating the objects, rather than computing the distance. From the existing list of GeoCoordinates, you could then do a computation against an existing Geocoordinate that is already instantiated.

Ex:
On the application load, store all points into memory, possibly on a background thread.

List<GeoCoordinate> points = from point in db.Points select new GeoCoordinate(point.lat, point.lng);

Then, take your point you are trying to search and loop over points

var gcSearch = new GeoCoordinate(coordinates[0], coordinates[1]);
var searchDistance = 1000;
var results = from pSearch in points 
              where pSearch.GetDistanceTo(gcSearch) < searchDistance
              select pSearch;

Iif that still isn’t fast enough, consider caching the last searched point and returing a known list if the new search is within the same bounds.

// in class definition
static GeoCoordinate lastSearchedPoint = null;
static List<GeoCoordinate> lastSearchedResults = null
const searchFudgeDistance = 100;

//in search method
var gcSearch = new GeoCoordinate(coordinates[0], coordinates[1]);
if (lastSearchedPoint != null && gcSearch.GetDistanceTo(lastSearchedPoint) < searchFudgeDistance)
    return lastSearchedResults;
lastSearchedPoint = gcSearch;

var searchDistance = 1000;
var results = from pSearch in points 
              where pSearch.GetDistanceTo(gcSearch) < searchDistance
              select pSearch;
//store the results for future searches
lastSearchedResults = results;