Category to create GPS metadata dictionary from CLLocation and CLHeading for writing to EXIF data in images.

CLLocation+EXIFGPS.h

#import <CoreLocation/CoreLocation.h>
#import <ImageIO/ImageIO.h>

@interface CLLocation (EXIFGPS)

- (NSDictionary*) EXIFMetadataWithHeading:(CLHeading*)heading;

@end

@interface NSDate (EXIFGPS)

- (NSString*) ISODate;
- (NSString*) ISOTime;

@end

CLLocation+EXIFGPS.m

#import "CLLocation+EXIFGPS.h"

NSString * const ISODateFormat = @"yyyy-MM-dd";
NSString * const ISOTimeFormat = @"HH:mm:ss.SSSSSS";

@implementation CLLocation (EXIFGPS)

- (NSDictionary*) EXIFMetadataWithHeading:(CLHeading*)heading
{
  NSMutableDictionary *GPSMetadata = [NSMutableDictionary dictionary];
  
  NSNumber *altitudeRef = [NSNumber numberWithInteger: self.altitude < 0.0 ? 1 : 0];
  NSString *latitudeRef = self.coordinate.latitude < 0.0 ? @"S" : @"N";
  NSString *longitudeRef = self.coordinate.longitude < 0.0 ? @"W" : @"E";
  NSString *headingRef = @"T"; // T: true direction, M: magnetic
  
  // GPS metadata
  
  [GPSMetadata setValue:[NSNumber numberWithDouble:ABS(self.coordinate.latitude)] forKey:(NSString*)kCGImagePropertyGPSLatitude];
  [GPSMetadata setValue:[NSNumber numberWithDouble:ABS(self.coordinate.longitude)] forKey:(NSString*)kCGImagePropertyGPSLongitude];
  
  [GPSMetadata setValue:latitudeRef forKey:(NSString*)kCGImagePropertyGPSLatitudeRef];
  [GPSMetadata setValue:longitudeRef forKey:(NSString*)kCGImagePropertyGPSLongitudeRef];
  
  [GPSMetadata setValue:[NSNumber numberWithDouble:ABS(self.altitude)] forKey:(NSString*)kCGImagePropertyGPSAltitude];
  [GPSMetadata setValue:altitudeRef forKey:(NSString*)kCGImagePropertyGPSAltitudeRef];
  
  [GPSMetadata setValue:[self.timestamp ISOTime] forKey:(NSString*)kCGImagePropertyGPSTimeStamp];
  [GPSMetadata setValue:[self.timestamp ISODate] forKey:(NSString*)kCGImagePropertyGPSDateStamp];
  
  if (heading) {
    [GPSMetadata setValue:[NSNumber numberWithDouble:heading.trueHeading] forKey:(NSString*)kCGImagePropertyGPSImgDirection];
    [GPSMetadata setValue:headingRef forKey:(NSString*)kCGImagePropertyGPSImgDirectionRef];
  }
  
  return [GPSMetadata copy];
}

@end

#pragma mark -

@implementation NSDate (EXIFGPS)

- (NSString*) ISODate
{
  NSDateFormatter *f = [[NSDateFormatter alloc] init];
  [f setTimeZone:[NSTimeZone timeZoneWithAbbreviation:@"UTC"]];
  [f setDateFormat:ISODateFormat];
  return [f stringFromDate:self];
}

- (NSString*) ISOTime
{
  NSDateFormatter *f = [[NSDateFormatter alloc] init];
  [f setTimeZone:[NSTimeZone timeZoneWithAbbreviation:@"UTC"]];
  [f setDateFormat:ISOTimeFormat];
  return [f stringFromDate:self];
}

@end

This was helpful thanks. I have updated the code and ported to Swift 3 to help the next guy.

https://gist.github.com/nitrag/343fe13f01bb0ef3692f2ae2dfe33e86