In this paper, the relationship between the tilt angle and altitude of a PTZ camera and the 3D-to-2D coordinate transformation is built. A vision-based approach is then proposed to accomplish the estimation of tilt angle and altitude based on the observation of some simple objects lying on a horizontal plane, with known intrinsic parameters of the PTZ camera. The patterns could be a circle, a corner with known angle, more than two corners with unknown but equal angles, or more than two line segments with unknown but equal lengths. Furthermore, if we are given a few line segments with known but not necessarily equal length, the estimation of tilt angle and altitude can also be achieved via an optimization procedure. The impacts of parameter fluctuations are analyzed via computer simulations. Experimental results on real images conform to our analysis and demonstrate the efficiency of this approach.