Вот как вы можете конвертировать между double -> string и обратно из строки -> double
//Defining Latitude & Longitude
NSNumber *latitude = [NSNumber numberWithDouble:31.5497226715];
NSNumber *longitude = [NSNumber numberWithDouble:74.3436126709];
NSLog(@"Origional > Lat: %.10f, Lng: %.10f", [latitude doubleValue], [longitude doubleValue]);
//Conversion to string
NSString *latitudeStr = [latitude stringValue];
NSString *longitudeStr = [longitude stringValue];
NSLog(@"String > Lat: %@, Lng: %@", latitudeStr, longitudeStr);
//String to double
NSNumberFormatter *numberFormatter = [[NSNumberFormatter alloc] init];
[numberFormatter setNumberStyle:NSNumberFormatterDecimalStyle];
NSNumber *latitudeFromStr = [numberFormatter numberFromString:latitudeStr];
NSNumber *longitudeFromStr = [numberFormatter numberFromString:longitudeStr];
[numberFormatter release];
NSLog(@"Back from string > Lat: %.10f, Lng: %.10f", [latitudeFromStr doubleValue], [longitudeFromStr doubleValue]);