# Core Motion

# Accessing Barometer to get relative altitude


Import the Core Motion library:

import CoreMotion

Next, we need to create a CMAltimeter object, but a common pitfall is to create it in the viewDidLoad(). If done that way, the altimeter won’t be accessible when we need to call a method on it. Nevertheless, go ahead and create your CMAltimeter object just before the viewDidLoad():

let altimeter = CMAltimeter()


  • We need to check if `relativeAltitude` is even available with the following method: `CMAltimeter.isRelativeAltitudeAvailable`.

  • If that returns `true`, you can then begin monitoring altitude change with `startRelativeAltitudeUpdatesToQueue`
  • If there are no errors, you should be able to retrieve data from the `relativeAltitude` and pressure properties.
  • Given below is the definition of a button action to begin monitoring with our barometer.

    @IBAction func start(sender: AnyObject){
    if CMAltimeter.isRelativeAltitudeAvailable() {
        // 2
        altimeter.startRelativeAltitudeUpdatesToQueue(NSOperationQueue.mainQueue(), withHandler: { data, error in
            // 3
            if (error == nil) {
                println("Relative Altitude: \(data.relativeAltitude)")
                println("Pressure: \(data.pressure)")