Navigation velocities in a virtual environment are along six axes: fore-and-aft, lateral, vertical, roll, pitch, and yaw. Using a viewer-centered co-ordinate system, the path of navigating through a virtual environment can be measured by storing the history of the viewpoint co-ordinates of a user. These displacement path histories (along six axes) can then be differentiated to produce the navigation velocity histories. After the differentiation, the root-mean-square velocities along the six axes can then be calculated (adopted from So, Lo and Ho, 2002)