Get the plane of the device relative to the plane of the earth using an accelerometer

Question

Let's say the iPhone is placed on a flat table. I would like to determine the angle of the plane of the table, where the angle 0 means that the table is exactly perpendicular to the gravity vector. I use the following formula:

    radians = atanf (z / sqrt (x^2 + y^2)

in .h

double      accelerationXAverage;
double      accelerationYAverage;
double      accelerationZAverage;

double      accelerationXSum;
double      accelerationYSum;
double      accelerationZSum;
int         readingsCount;

in .m

#define kThresholdMovementHigh 0.05

- (void)accelerometer:(UIAccelerometer *)accelerometer
        didAccelerate:(UIAcceleration *)acceleration 
{
    // did device move a lot? if so, reset sums
    if (fabsf(acceleration.x - accelerationXAverage) > kThresholdMovementHigh ||
        fabsf(acceleration.y - accelerationYAverage) > kThresholdMovementHigh ||
        fabsf(acceleration.z - accelerationZAverage) > kThresholdMovementHigh   )
    {
        NSLog(@"deviceDidMove a lot");
        accelerationXSum = acceleration.x;
        accelerationYSum = acceleration.y;
        accelerationZSum = acceleration.z;
        readingsCount = 1;
    }

    else 
    {
        // because the device is at rest, we can take an average of readings
        accelerationXSum += acceleration.x;
        accelerationYSum += acceleration.y;
        accelerationZSum += acceleration.z;
        readingsCount ++;        
    }

    accelerationXAverage = accelerationXSum / readingsCount;
    accelerationYAverage = accelerationYSum / readingsCount;
    accelerationZAverage = accelerationZSum / readingsCount;

    float angle = RadiansToDegrees(atanf(accelerationZAverage/sqrtf(pow(accelerationXAverage, 2) + pow(accelerationYAverage, 2)))) + 90;

    labelAngle.text = [NSString stringWithFormat:@"%.2f°", angle];
}

I filter noise by averaging accelerometer readings. The accelerometer update interval is 1/60, and while you are experimenting, I let the device sit for 10 seconds (so it averages 600 samples).

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Answer 1

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