def convert_source_location(self, source_reading, reference_reading):
        """
        Converts the source (x, y) location from reading into the coordinate
        system of reference_reading.
        """
        offset_x, offset_y = reference_reading.get_coordinate_offset(source_reading)
        focus = source_reading.x + offset_x, source_reading.y + offset_y
        return focus