Prediction of new landmarks are commonly expressed as:
Xm = Xr + r*cos(phi + theta_r), Ym = Yr + r*sin(phi + theta_r)
However this is only true for point landmarks. What if I am extracting line feature?
This is true because landmarks are often encapsulated by just their coordinates. It's a toy example. In real life, you'd have to use all the parameters necessary to encapsulate the information about the line feature. For example. Lines are often represented by a point and orientation.