Two plane intersection and angle between 2 planes I am trying to implement my problems in different ways. So, may be though this question has some relation to some other questions, please answer me. 
We know; Intersection of two planes will be given a 3D line. (In case of segments of planes, then we will have a 3D line segment for the sharing edge portion of both planes, and my question is referred with this).
If I need to assign weights for each line, then this can be achieved with respect to the degree of angle between two planes. So, I can say when two planes make 90 degrees, then the produced line is more precise than when the line subtends 150 or close to 180 degrees. So the 90 degree case give one weight and 150 case give another weight. (I think, this weight tells, how accurately model these lines. so, I can consider this also as quality of the lines. maybe, I am wrong.) 
So, I want to know some degree of measure to assign weights in order to discriminate the lines in terms of angle between two planes. So, could you please elaborate how I can categorize different weight classes in terms of that angle as I am really poor in geometry? 
Or maybe there are some other measures like; length of line segment or size or plane segments.
Thank you in advance.
 A: Without further information, I would suggest you might take the sine of the angle between the two planes (or its absolute value) as your indicator of how  they are perpendicular.   
For $90^\circ$ this gives $1$; for $150^\circ$ or $30^\circ$ it gives $0.5$, and for close to $180^\circ$ or $0^\circ$ it gives close to $0$. 
A: Your considerations about the quality of the intersection of
two planes are correct. The intersection will be computed
most accurately if the planes are orthogonal. The interection
is not defined for parallel planes and hence it will be inaccurate
for almost parallel planes. 
This can be quantified by looking at the normal vectors $n_1, n_2$ of
the 2 planes. The angle $\alpha$ between the planes is the angle
between their normal vectors, so:
$$n_1 \cdot n_2 = |n_1| |n_2| \cos \alpha $$
In the following I assume that the normal vectors are normalized (their length is 1). One formula for the intersection 
computes the denominator 
    $$q = 1 - (n_1 \cdot n_2)^2 = 1 - \cos^2 (\alpha) $$
so this $q$ could be a possible measure of the intersection quality: 
if $q < \epsilon$
then the quality is bad. The problem is then ill-conditioned. The best quality is $q=1$.
We get the same measure by considering the direction vector of
the intersection line:
    $$u = n_1 \times n_2 $$
It is 
    $$|u| = |n_1| |n_2| \sin\alpha = \sin \alpha$$
    $$|u|^2 = \sin^2 (\alpha) = 1 - \cos^2 (\alpha) = q$$
If $q$ is small then the length of the direction vector is small and the intersection line is poorly defined. 
