def example_roi_func_constant(zg, yg, xg):
    """
    Example RoI function which returns a constant radius.

    Parameters
    ----------
    zg, yg, xg : float
        Distance from the grid center in meters for the x, y and z axes.

    Returns
    -------
    roi : float
        Radius of influence in meters
    """
    # RoI function parameters
    constant = 500.     # constant 500 meter RoI
    return constant