How do I improve the accuracy of this pedometer algorithm?

Question

I've tried several ways of measuring the steps a user makes with an iPhone by reading the accelerometer, but none have been very accurate. The most accurate implementation I've used is the following:

 float xx  = acceleration.x;
 float yy  = acceleration.y;
 float zz = acceleration.z;

 float dot = (mOldAccX * xx) + (mOldAccY * yy) + (mOldAccZ * zz);
 float a = ABS(sqrt(mOldAccX * mOldAccX + mOldAccY * mOldAccY + mOldAccZ * mOldAccZ));

 float b = ABS(sqrt(xx * xx + yy * yy + zz * zz));

 dot /= (a * b);

 if (dot  <= 0.994 && dot > 0.90) // bounce
 {

  if (!isChange)
  {

   isChange = YES;
   mNumberOfSteps += 1;

  } else {
   isChange = NO;
  }
 }

 mOldAccX = xx;
 mOldAccY = yy; 
 mOldAccZ = zz;
}

However, this only catches 80% of the user's steps. How can I improve the accuracy of my pedometer?

Answer 1

Maybe you could combine your formula with location? I think the resolution is around 8m so this could help in your calculations.