Calculating MAX and MIN Latitude and Longitude wit

问题:

I need to compute MAX and MIN Latitude and Longitude values from a location with certain distance.

I have thousands of locations stored in CoreData, and I want to show only the ones within 5km from users location.

How can I approach this problem?

回答1:

Here's a possible solution:

1. macros to convert Degrees to Radians

   #define deg2rad(degrees) ((degrees) / 180.0 M_PI)
   

2. macros to hold my searching distance

   #define searchDistance 5.00 //float value in KM
   

3. set the minimum and maximum Latitude, Longitude values

   float minLat = userLocation.coordinate.latitude - (searchDistance / 69);
   float maxLat = userLocation.coordinate.latitude + (searchDistance / 69);
   float minLon = userLocation.coordinate.latitude - searchDistance / fabs(cos(deg2rad(userLocation.coordinate.latitude))*69);
   float maxLon = userLocation.coordinate.longitude + searchDistance / fabs(cos(deg2rad(userLocation.coordinate.latitude))*69);
   

4. create predicate as follows

   NSPredicate *predicate = [NSPredicate predicateWithFormat:@"latitude <= %f AND latitude >= %f AND longitude <= %f AND longitude >= %f", maxLat, minLat, maxLon, minLon];
   

This will create a square around userLocation and check if a given location falls into its coordinates.

Update: Swift 2.* implementation

First create a function to compute degrees to radians

func deg2rad(degrees:Double) -> Double{
    return degrees * M_PI / 180
}

Compute and create minimum and maximum Latitude and Longitude values

let searchDistance:Double =  5.00 //float value in KM    

let minLat = userLocation.coordinate.latitude - (searchDistance / 69)
let maxLat = userLocation.coordinate.latitude + (searchDistance / 69)

let minLon = userLocation.coordinate.longitude - searchDistance / fabs(cos(deg2rad(userLocation.coordinate.latitude))*69)
let maxLon = userLocation.coordinate.longitude + searchDistance / fabs(cos(deg2rad(userLocation.coordinate.latitude))*69)

Last create NSPredicate to query CoreData for locations. In my case I am querying for values latitude and longitude but you should change this to match your CoreData object

let predicate = NSPredicate(format: "latitude <= \(maxLat) AND latitude >= \(minLat) AND longitude <= \(maxLon) AND longitude >= \(minLon)")

回答2:

Use the CoreLocation method distanceFromLocation: which returns the distance (in meters) between two points as such:

CLLocation* location = [[CLLocation alloc] initWithLatitude:lat longitude:lon];
if([userLocation distanceFromLocation:location) < searchDistance)
    // do something with close point

A suitable predicate can be constructed as:

NSPredicate* predicate = [NSPredicate predicateWithBlock:(BOOL (^)(NSDictionary* target, NSDictionary *bindings)) {
    CLLocation* location = [[CLLocation alloc] initWithLatitude:[target[@"lat"] doubleValue] longitude:[target[@"lon"] doubleValue]];
    return [userLocation distanceFromLocation:location] < searchDistance;
}];

This has the advantage that it returns the items actually within range, as opposed to the items in a square approximating the range. It's also (probably, we don't know the details) using a more accurate approximation of the range itself. It has the disadvantage that the predicate requires loading every object since it can't be expressed as an sqlite query.