Direction detecting apparatus and surveying system

A surveying system comprising a surveying instrument that emits a tracking light and a measurement point indicating device, wherein said measurement point indicating device comprising a supporting pole of which lower end indicates a measurement point, a retro-reflector provided at a known position from said lower end of said supporting pole, and a direction detecting apparatus provided at a known distance from said lower end of said supporting pole wherein said direction detecting apparatus comprises an image pickup device with an optical axis orthogonal to an axis of said supporting pole and parallel to said retro-reflector, an arithmetic processing part and a polarization detecting unit for detecting a polarization direction of said tracking light, wherein said image pickup device acquires an image so as to include said surveying instrument for which a direction is to be detected, wherein said tracking light is a polarized light rotating around an optical axis as the center and arranged so as to have a reference light pulse corresponding with a reference polarization direction of said polarized light, wherein said arithmetic processing part calculates a direction angle of said surveying instrument from a relation between a position of said surveying instrument in said image and an optical axis of said image pickup device and detects a direction of a light source of said tracking light emitted toward said retro-reflector, wherein said polarization detecting unit is arranged so as to detect a polarization direction of said tracking light when detecting said reference light pulse and detects a tilt of said direction detecting apparatus based on the detected polarization direction.