SPHEREGRAD is the jSphere module of jLab.

 SPHEREGRAD  Gradient of a field on the surface of a sphere.
    [FX,FY]=SPHEREGRAD(LAT,LON,F) computes the gradient of the scalar field
    F on the surface of the sphere.
    FX and FY are the components of the gradient of F in the zonal and
    meridional directions, respectively, with units of F per meter.
    LAT and LON are vectors specifing an evenly-spaced grid, and F is an 
    array of size LENGTH(LAT) x LENGTH(LON) x M, where M is greater than or
    equal to one.  LAT and LON are in degrees.
    The radius of the Earth as specified by RADEARTH is used by default.
    SPHEREDIV(...,R) uses a sphere of radius R, in kilometers, instead.
    Derivatives are computed using the first central difference.
    FZ=SPHEREGRAD(LAT,LON,F) with one output argument returns the 
    complex-valued array FZ=FX+i*FY.
    First and last points
    SPHEREGRAD can use different boundary conditions in the numerical
    computation of derivatives for the first and last points.
    SPHEREGRAD(...,'periodic') uses a periodic derivative with respect to
    longitude.  This is the default behavior.  
    SPHEREGRAD(...,'endpoint') uses the forwards / first backwards 
    difference at the first and last longitude, respectively. 
    For both of these options, the endpoint condition is used for 
    differentiation with respect to latitude.
    SPHEREGRAD(...,'nans')  fills in the first and last values on all four
    sides of the region with NANs. 
    See VDIFF for more information.  
    'spheregrad --t' runs a test.
    Usage: [fx,fy]=spheregrad(lat,lon,f);
    This is part of JLAB --- type 'help jlab' for more information
    (C) 2007--2016 J.M. Lilly --- type 'help jlab_license' for details

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