This paper introduces the elements of projection method for a part of circle, especially for a round, and gives an example to explain the use of the projection method for a part of circle in seeking the especial points on conterminous line.

• 介绍圆弧投射法的原理，特别是圆的圆弧投影，并举例说明了圆弧投射法在求相贯线上特殊点中的应用。