iphone - For a manual speedometer control, how do I convert needle angle to speed? -

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i'd have manual speedometer control user can drag needle particular speed setting, following:

sample speedometer control

from angle of above needle, how read speed corresponds to? example, above read speed of 100.

i've done calculation myself (but in reverse, given speed, adjust needle angle) on similar app. presumably code knows angle of needle since you're animating it, , prevent needle going below 0 or above 160. i'll assume needle image @ rest pointing straight , down (at 80mph) figure out angle when needle pointing @ 160mph, , pointing @ 0mph putting nslog dumping out angle when animate in response touches , notice values make needle displaying 0 , 160 (you can tweak in code later right). i'm assuming positive angles rotate clockwise , negative angles rotate counter-clockwise.

then speed arbitrary angle is:

double mphperdegree = 160 / (angle160mph - angle0mph); double speed = (currentangleindegrees * mphperdegree) + 80.0;

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