Question

i am building an iphone app and i am looking for an idea how to get objects close to my position. the objects have an address listed in my database. now i am interested to show the objects that are close to mine on the map. how do i get that property? at first i thought of floyd-warshall but is there an better solution than coding that myself?

Answer 1

The first thing you’ll need to do is get Lat Long positions in your database instead of Addresses. There is a tool that google provides that does this. There is also a manual approach.

Then you can use the Haversine formula to work out if it is within a certain radius of your location.

Here is a example. You can cycle through your objects(‘s location) on the map and see how far away they are from you.

-(CGFloat) getDistance :(CGFloat)objLatitude :(CGFloat)objLongitude :(CGFloat)currentLocationLatitude :(CGFloat)currentLocationLongitude
{
    CGFloat earthRadius = 6371;

    CGFloat deltaLat = deg2rad(currentLocationLatitude - objLatitude);
    CGFloat deltaLong = deg2rad(objLongitude - currentLocationLongitude);

    CGFloat a = sin(deltaLat/2) * sin(deltaLat/2) +
        cos(deg2rad(objLatitude)) * cos(deg2rad((double)currentLocationLatitude)) * 
        sin(deltaLong/2) * sin(deltaLong/2);

    CGFloat c = 2 * atan2(sqrt(a), sqrt(1 - a));
    CGFloat d = earthRadius * c;

    d = round(d, 4); // round to 4dp
    d = d * 1000; // convert from km to m
    d = d * 0.621371192; // convert from km to miles

    return d; // m
}

You could amend this method to return TRUE if it is within a specific distance and FALSE if it does not:

-(BOOL) doesObjFallWithinMyRadius :(CGFloat)objLatitude :(CGFloat)objLongitude :(CGFloat)currentLocationLatitude :(CGFloat)currentLocationLongitude :(CGFloat)distanceLimit
{
    CGFloat earthRadius = 6371;

    CGFloat deltaLat = deg2rad(currentLocationLatitude - objLatitude);
    CGFloat deltaLong = deg2rad(objLongitude - currentLocationLongitude);

    CGFloat a = sin(deltaLat/2) * sin(deltaLat/2) +
        cos(deg2rad(objLatitude)) * cos(deg2rad((double)currentLocationLatitude)) * 
        sin(deltaLong/2) * sin(deltaLong/2);

    CGFloat c = 2 * atan2(sqrt(a), sqrt(1 - a));
    CGFloat d = earthRadius * c;

    d = round(d, 4); // round to 4dp
    d = d * 1000; // convert from km to m
    d = d * 0.621371192; // convert from km to miles

    if(d < distanceLimit) return TRUE;
    else return FALSE;
}

To find out your current location you will need to use the CLLocationManager which is part of the iOS SDK.