How do you calculate jump height from a force time graph?

Im looking to write a script that will automatically work out the jump height from force platform data measuring at 1000Hz. The code I have so far is:

      r = find(truedata==0,1,'first');  % find when first 0 occurs
      truedata = (truedata(1:r+1,6)*-1); % cut data

      bw = mean(truedata(1:20,1)); % find body weight
      Jbw = bw*(r*(1/1000)); % work out impulse caused by body weight

      t = [0:1/1000:r/1000]; % change in time
      Jall = trapz(t,truedata); % calculate impulse of whole graph

      impulse = (Jall-Jbw); % calculate jump impulse
      vel = (impulse/(bw/9.81)); % calculate take off velocity
      jumpheight = ((vel^2)/(2*9.81)); % equation of motion

This however is not giving me the correct answers, could anyone see a problem in my calculations?