There is too many 2d position estimation with one camera. Is there any 3d position estimation application or technique with one camera? If there is no application or technique why?
It is possible to use one camera for 3d position estimation but it is significantly more difficult.
Depth/distance data must be obtained to generate the 3d representation and several techniques have been developed to this end such as rapidly moving the camera to a known, fixed secondary position to capture a second image for processing or mounting a fixed mirror in the camera view to provide the offset image.
However the reliability and complexity of these methods haven't led to their widespread uptake. Arguably the IR CMOS sensor in the Kinect is not a 'camera' and this system uses a 'point cloud' projection and interpretation method to obtain it's depth data.
Overall the simplified two camera approach has historically been the most cost effective and the maths behind the processing algorithms came from sound processing and radar signal interpretation, distance and range finding.
There is one really simple solution: take a snapshot of the scene, move to the side a few centimetres and take another snapshot. Alternatively, move forward with the camera pointed to one side. Now you have two images and can process them as though you have two cameras or one stereoscopic pair. The only problem I can see with this method is that any parts of the scene that have moved will cause some issues, but it probably wouldn't be too hard to filter those parts of the scene out.