curvature versus slope
  n = 882056 
  X=0, Y = 78.99 +/- 14.88 
  Weighted least-squares fit:  Y = 86.0 + 0.507 X  + -0.00378 X**2 

slope versus dZ, curvature < 5
  n = 3512 

  Weighted least-squares fit:  Y = 5.3 + 0.175 X  + 0.00020 X**2 
     at X = 100, Y = 24.9 
     effective RMSE xy = 38.1 

slope versus dZ, curvature < 15
  n = 30480 

  Weighted least-squares fit:  Y = 11.2 + 0.185 X  + -0.00009 X**2