kCGImagePropertyGPSAltitudeRef The altitude point of reference. Declarationlet kCGImagePropertyGPSAltitudeRef: CFString See AlsoGPS CoordinatekCGImagePropertyGPSLatitudekCGImagePropertyGPSLongitudekCGImagePropertyGPSAltitudekCGImagePropertyGPSLatitudeRefkCGImagePropertyGPSLongitudeRefkCGImagePropertyGPSHPositioningError