def comp_diff_emerge(result, loc1, loc2, time, obs='sstopo'):
    """Compute differential glacial isostatic adjustment between locs 1 and 2.

    Parameters
    ----------
    result : GiaSimOutput
    loc1, loc2 : tuples of (lon, lat)
    time : the time for comparison
    obs : str
        The type of observation, defaults to 'sstopo', which is emergence. See
        print(result) for all possible fields. Note: fields computed in
        spectral space must be transformed prior to comparison, use
        result.transformObservers().
    """
    obs = getattr(result, obs)
    field = obs.nearest_to(time).T
    interper = result.inputs.grid.create_interper(field)
    xx, yy = result.inputs.grid.basemap((loc1[0], loc2[0]), (loc1[1], loc2[1]))
    diffs = interper.ev(xx, yy)
    return diffs[1] - diffs[0]