iOS Compass true north correction is wrong.

I´ve compared the iOS Compass app true north direction with a analogic ranger type compass, like the ones of Silva or Suunto campasses brands, and they diverge. I was far of anything metallic that could cause any deviation and proceeded the comparison outdoors.

My conclusion is that the magnetic deviation used to correct the true north at iOS Compass app is reversed.

For exemple, my local mag. dev. is 23 degrees West but Compass app considers 23 degrees East. Then the aplication points 46 degrees to the left, that is completelly wrong.

How can I set my local magnetic deviation in iOS? Is that possible to correct this data manually?