ios - How to accurately get Y value from location manager heading -

i'm trying y value location manager heading purpose of animating image on screen angle phone , down. i'm using following cllocationmanager delegate method:

func locationmanager(manager: cllocationmanager, didupdateheading newheading: clheading) {    print("y: \(newheading.y)") } 

when have phone in normal position it's standing upright, i'm seeing y value of -45. however, when angle phone both or down, i'm seeing y values start near 0, regardless of whether i'm pointing phone or down. how can determine y values accurately differentiate between angling device , down?