iPhone accelerometer orientation

After searching for information about the iPhone accelerometer in search engines and the apple documentation, I did not find anything simple as what I am going to post here.

This is a bit of a tutorial, or just an explanation, how to determine the iPhone and iPod Touch (possibly the iPad) and its orientation relative to the flat ground at our feet.

The orientation uses 3-dimensional coordinate concepts, so a person has to be familiar with x, y, z and rotation concepts.  To get the information, you should be developing a native app (I don’t know if this works in Mobile Safari for web apps yet).  The UIAccelerometer class has a class method to provide you the shared accelerometer. The way to do this follows:

[[UIAccelerometer sharedAccelerometer] setDelegate: myObject ];

Additionally, setting the time interval (measured in seconds) to receive updates is a good task:

[[UIAccelerometer sharedAccelerometer] setUpdateInterval:( 1.0f / 30.0f )];

Now your class that myObject is instantiated from must implement the following method (the actual variable names can be customized to whatever you want):

– (void)accelerometer:(UIAccelerometer *)accelerometer didAccelerate:(UIAcceleration *)acceleration

The UIAcceleration object called “acceleration” contains the important 3-dimensional values.

* * *

The iPhone cannot determine how far off the ground it is, but it can tell which side/edge/face of itself is closest to the ground.  That’s the point I am going to work with. Pretend the iphone is a 6-sided box, even though the 4 side edges are very slim, they are still sides of the box.

The device has 6 orientations, when one of the sides is directly facing the ground:

  1. Portrait
  2. Portrait upside-down
  3. Landscape left (rotate the device counter-clockwise 90 degrees from portrait)
  4. Landscape right (rotate the deviceclockwise 90 degrees from portrait)
  5. Face up
  6. Face down

Each time the accelerometer:didAccelerate method responds, you can get information about its orientation from the UIAcceleration object (using the above name) as follows:

float x = acceleration.x;
float y = acceleration.y;
float z = acceleration.z;

And now, comes the final bit of useful information, the pictures.

Basically, the pictures show the values of x, y, z,  for the 6 different orientations.

The way I personally interpret this is by way of the relative positions of one side to its opposite side.  The screen and the back are Z sides, the top with the lock button and bottom with the dock connection are the Y sides, and the two left-right edges are the X sides.

iPhone face-down

iPhone face-down, z approaches 1

iPhone face-up

iPhone face-up, z approaches -1

iPhone portrait

iPhone portrait, y approaches -1

iPhone portrait upside-down

iPhone portrait upside-down, y approaches 1

iPhone landscape right

iPhone landscape right, x approaches 1

iPhone landscape left

iPhone landscape left, x approaches -1

Until I wrote all this info down on paper, exactly as it’s described here, I couldn’t figure out how to interpret weird angles.

I had to do this research because the pre-packaged method for detecting orientation-changes was causing my app to jump back and forth.  Probably has to do with the update frequency too.  But this way now, I can build my own custom orientation code.

Take care everyone.

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2 Responses to iPhone accelerometer orientation

  1. Josh Adams says:

    I was trying to figure out the accelerometer and found your post. Very helpful!

  2. Samir Chakour says:

    Great post, very helpful to me to solve a rotation issue when the device is on face-up / face-down positions, Thank you Tom

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