I'm tinkering around with trying to build a speedometer using only the accelerometer for use when I go snowboarding. I can read values from the accelerometer but I'm not sure how I would derive the velocity. I was thinking of taking the square root of (X^2 + Y^2 + Z^2) where X, Y, and Z are the values reported by the accelerometer in each axis but that only would only give me the amplitude of acceleration at any given instant. I could multiple that by the sampling frequency to get my instantaneous velocity but this approach fails when I'm moving at a constant speed. Any ideas?