Convert NSString to double - nsstring

Question

I've imported a mysql database into NSStrings using JSON but now I need to convert two of these into doubles to represent latitude and longitude but I'm unsure how to complete my code.
Hazards.h
#import <Foundation/Foundation.h>
#import <MapKit/MapKit.h>
#interface Hazards : NSObject <MKAnnotation>
// #property (nonatomic, readonly) CLLocationCoordinate2D coordinate;
#property (nonatomic, assign) CLLocationDegrees latitude;
#property (nonatomic, assign) CLLocationDegrees longitude;
#property (nonatomic, copy) NSString *title;
#property (nonatomic, copy) NSString *subtitle;
#property (nonatomic, strong) NSString * ID;
#property (nonatomic, strong) NSString * ROUTE;
#property (nonatomic, strong) NSString * ADDRESS;
#property (nonatomic, strong) NSString * LATITUDE;
#property (nonatomic, strong) NSString * LONGITUDE;
#property (nonatomic, strong) NSString * HAZARD;
#property (nonatomic, strong) NSString * RISK;
// Methods
- (id) initWithID: (NSString *) hazardsID andROUTE: (NSString *) hazardsROUTE andADDRESS: (NSString *) hazardsADDRESS andLATITUDE: (NSString *) hazardsLATITUDE andLONGITUDE: (NSString *) hazardsLONGITUDE andHAZARD: (NSString *) hazardsHAZARD andRISK: (NSString *) hazardsRISK;
#end
Hazards.m
#import "Hazards.h"
#implementation Hazards
#synthesize coordinate, title, subtitle, ID, ROUTE, ADDRESS, LATITUDE, LONGITUDE, HAZARD, RISK;
- (id) initWithID: (NSString *) hazardsID andROUTE: (NSString *) hazardsROUTE andADDRESS: (NSString *) hazardsADDRESS andLATITUDE: (NSString *) hazardsLATITUDE andLONGITUDE: (NSString *) hazardsLONGITUDE andHAZARD: (NSString *) hazardsHAZARD andRISK: (NSString *) hazardsRISK {
self = [super init];
if (self)
{
ID = hazardsID;
ROUTE = hazardsROUTE;
ADDRESS = hazardsADDRESS;
LATITUDE = hazardsLATITUDE;
LONGITUDE = hazardsLONGITUDE;
HAZARD = hazardsHAZARD;
RISK = hazardsRISK;
latitude = ; // convert the string hazardsLatitude to a double here
longitude = ; // convert the string hazardsLongitude to a double here
}
return self;
}
/*-(CLLocationCoordinate2D)coordinate
{
return CLLocationCoordinate2DMake(self.LATITUDE, self.LONGITUDE);
}*/
-(NSString *)title
{
return self.HAZARD;
}
-(NSString *)subtitle
{
return self.ADDRESS;
}
#end