GRIDDRIFTERS is the jOceans module of jLab.

 GRIDDRIFTERS  Average drifter velocities onto a space/time 3D grid.
    returns a structure containing velocities averaged onto a 3D grid.  
    The grid has bin edges given by LATO and LONO, and has month-long bins
    in time with a semimonthly spacing beginning with year YO and month MO,
    and having N total time steps.  
    The grid size is thus LENGTH(LATO)-1 x LENGTH(LONO)-1 x N.  The first
    time bin corresponds to year YO and month MO, i.e. it is centered on
    the midpoint of this month.  Odd slices correspond to month bins, e.g.
    Jan 1992, while even slices run between midpoints of adjacent months.
    NUM,LAT,LON,U,V,FILLED,SOURCE are all cell arrays of Lagrangian drifter
    or float data, with one instrument per cell, such as those used by the
    FLOATS.MAT and DRIFTERS.MAT dataset.  These are all the same size. 
    NUM is the date in Matlab's DATENUM format, LAT and LON are latitude
    and longitude, U and V are eastward and northward velocity components, 
    FILLED is a flag that is true if the data point is filled, and SOURCE
    contains integers indicating a source dataset.
    FILLED and SOURCE are both optional.  To omit one or both of these,
    replace with the empty array, [], in the input list. 
    If FILLED is included, only non-filled data is used for the averages.
    The output structure STRUCT has the following fields:
        STRUCT.NUM       Date of bin midpoint in DATENUM format
        STRUCT.LAT       Latitudes of bin centers
        STRUCT.LON       Longitudes of bin centers
        STRUCT.U         Bin-averaged eastward velocity
        STRUCT.V         Bin-averaged northward velocity
        STRUCT.EPSUU     Instantaneous eastward local variance in each bin 
        STRUCT.EPSVV     Instantaneous northward local variance in each bin
        STRUCT.EPSUV     Instantaneous local covariance in each bin
        STRUCT.COUNT     Number of data points averaged in each bin
    These are all 3D arrays of the same size.  Array entries in which there
    is no data are filled with NaNs, apart from COUNT which will be zero.
    The "instantaneous local variance" is the variance relative to the 
    local mean velocity computed withing each 3D bin.  See Lilly and Perez-
    Brunius (2021), "A gridded surface current product for the Gulf of
    Mexico from consolidated drifter measurements", for details.
    If SOURCE is input, then STRUCT also includes another field:
        STRUCT.COUNTS    Number of data points from each source in each bin
    which is a 4D array of LENGTH(LATO)-1 x LENGTH(LONO)-1 x N x M, where M
    is the maximum value occurring in SOURCE.
    Usage: struct=griddrifters(num,lat,lon,u,v,[],[],lato,lono,yo,mo,N);
    This is part of JLAB --- type 'help jlab' for more information
    (C) 2020--2021 J.M. Lilly --- type 'help jlab_license' for details

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