def calcAlongSegmentCoords(xy1, xy2, asf):
    """Given the endpoints of a line segment, and an 'along-segment fraction,'
    returns the coordinates of the midpoint by calculating simple x and y
    ranges, each times the value of asf."""
    ydiff = (float(xy2[1]) - float(xy1[1]))
    xdiff = (float(xy2[0]) - float(xy1[0]))
    yMid = float(xy1[1]) + (asf * ydiff)
    xMid = float(xy1[0]) + (asf * xdiff)
    return (xMid, yMid)