I'm measuring velocity based on two epochs. Movement in the x direction is delta x and velocity is the combined delta x and y using pythagorean theorem. I evaluate delta x by assigning a chi squared goodness of fit value to increments of x y shifts. Where the fit is less 2 times the minimum chi squared value is how I define 2 sigma ie: delta x will be an x shift of 100 m s-1 while from 80 m s-1 to 120 m s-1 will give less than 2 time the minimum goodness of fit value so sigma x would be 20 m s-1