Accurately measuring relative distance between a set of fiducials (Augmented reality application)
You can use structure from motion type of algorithm to estimate the camera pose from the environment, not from the markers and then fuse this camera pose with the marker poses in order to detect the locations of the markers accurately. Knowing the extrinsic pose of your camera (by SFM), you can triangulate all the 3D positions. 

For pose estimation, 5-point methods are usually more accurate than 8-point algorithm.

Presumably, you should do further bundle adjustment so that the overall accuracy increases.