Water conservancy project horizontal displacement monitoring usually adopts ( ). a. rendezvous method b. triangular elevation method c. level observation method

Selection of observation methods: - in general, landslide, high slope stability monitoring using the rendezvous method; horizontal displacement monitoring using the line of sight method (movable surveyor's sign method and the small angle method); vertical displacement observation, it is desirable to use the level observation method, but also can be used to meet the requirements of the accuracy of photoelectric ranging triangular elevation method; foundation rebound is desirable to use the level meter and the suspended steel ruler with the observation method. The foundation rebound should adopt the observation method of level meter and suspended steel ruler. Therefore, this question should be selected D.