Raw GPS data and measurement of the length of step. One participant freely walked on the level ground. High precision GPS measured 3D positions of the moving participant with a centimeter accuracy at 20 Hz sampling rate (antenna fixed onto the head). The figure presents a small sample (3 m) of a 45 min. test. The top panel shows the sinusoidal variation of the vertical position (Z) as a function of the West-East (X) displacement. The bottom panel shows the South-North (Y) displacement as a function of West-East (X) displacement. The vertical lines indicate the beginning of each step. Dotted circles are raw 20 Hz GPS data. Small dots are 240 Hz interpolated positions.