def optimal_slope(cn,n,dt,z,i):
    """
    ;+
    ;
    ;   OPTIMAL_SLOPE : Function to compute the slope using a Powell et Leben (2004) <br />
    ;   filter with given characteristics.<br /><br />
    ;   
    ;   Reference : Powell, B. S., et R. R. Leben (2004), An Optimal Filter for <br />
    ;   Geostrophic Mesoscale Currents from Along-Track Satellite Altimetry, <br />
    ;   Journal of Atmospheric and Oceanic Technology, 21(10), 1633-1642.
    ;
    ; @param cn {in}{required}{type:NUMERIC} Filter coefficients
    ; @param n {in}{required}{type:NUMERIC} Filter indices reffered to central point<br />
    ;          (from -p to q - 0 is the center)
    ; @param dt {in}{required}{type:NUMERIC} Mean along-track grid step
    ; @param z {in}{required}{type:NUMERIC}{default:12} Along-track data of height<br />
    ;          anomalies (all dataset for current track)
    ; @param i {in}{required}{type:NUMERIC}{default:12} Index of points to use with<br />
    ;          current computation wihtin track.
    ;          
    ; @returns Optimal slope. Unit depends on distance and height anomalies units.
    ;
    ;
    ;
    ; @author Renaud DUSSURGET, LEGOS/CTOH
    ; @history Created Sep. 2009 from genweights.m (Brian Powell (c) 2004, <br />
    ;   University of Colorado, Boulder)
    ;
    ;
    ; @example dh = optimal_slope(cn,n,dt,z,id) : Compute the Optimal Slope using <br />
    ;   points in z[id], with a dt step, coefficients cn (of indices n).
    ;
    ;-
    """

#Return optimal slope
    #FIX : This version uses a regular grid (uses n * dt)
    #      We should rather use an irregular grid (real array of along-track distances)
  
    #; 1st member EQ 0 if filter centered
    dh = np.nansum((-cn)/(n * dt)) * z[i] \
        + np.nansum(cn / (n * dt) * z[i+n])
    return dh