ios - trueHeading reference model -

what reference model used when computing trueheading, clheading in ios?

trueheading heading (measured in degrees) relative true north. (read-only)

@property(readonly, nonatomic) cllocationdirection trueheading

https://developer.apple.com/library/ios/documentation/corelocation/reference/clheading_class/reference/reference.html#//apple_ref/occ/instp/clheading/trueheading