It is well known that radial error system, probable ellipse system, error parallelogram system are considered concerning how to express the error boundaries of ship's positions; the crux of the problem is how to measure accuracy and how to combine theory and practice. The writer made careful examination of these methods from the point of view of studying the error boundaries of ship's positions in the following four cases. (1) error boundaries of ship's positions fixed by astronomical position lines. (2) error boundaries of ship's positions by position lines gotten by land objects (objects are fixed). (3) error boundaries of ship's positions by bearings gotten by land objects (observed point is fixed). (4) error boundaries of ship's positions by Loran position lines.
