Three-point identifying arc measuring instrument and measuring method

The invention discloses a three-point identifying arc measuring instrument and measuring method. The measuring method includes a main scale and two auxiliary scales, wherein one end of the main scale and one end of each auxiliary scale are connected through a central shaft respectively; and the other ends of the main scale and the two auxiliary scales are measuring heads; a main scale vernier is arranged on the main scale, and auxiliary scale verniers are arranged on the two auxiliary scales; and synchronous locking devices mutually matched are arranged on the main scale and the two auxiliary scales. The invention has the positive effects as follows: according to the theorem of three-point identifying center, an arc is measured by three measuring feet extending from an identical circle center and having the same length, and the three measuring feet having the same length are contacted with three arbitrary and un-coincident points on the same arc; and when the conditions are met, the length value of the measur