Problems with angles

I'm working on a hypersonic vehicle project and in this project it is required that I know the inclination angle at any point on the curve of the vehicle. I found the inclination angles of the curve but for one aspect of this project I need to delete the angles that are greater than the maximum inclination angle (90 degrees). As seen on the pic, [link=http://www.mediafire.com/i/?thaq7m7gpp4zbfz][/link] , I need to keep the angles that are from the green line to the red line and from the green line to the blue line. I've tried using a find indice command to find the indices that correspond to angles that -90<theta<0 but I then realized that this would delete the angles that are negative between the blue and red lines. The signs of the angles are as follows:

right side of blue line: negative
left side of blue line to right above green line: positive
green line to left of red line: negative
right of red line: positive

So as you can see, finding a condition that works is making this hard for me. I've included an excel file with the angles so that you may see how the signs change.

Close up of top:

Close up of bottom:

• I am working on similar project but i want to find angle at any point on the curve road (highways). Can you please guide me how you have completed this task. Any help will be appreciated.

Regards,
Mrinal
• I've decided to approach this problem using 3D geometry. If I take the dot product of the each of the normals of the facets to the velocity vector, I can see which facets are to be ignored by using these indices as logical indices.
• I've decided to approach this problem using 3D geometry. If I take the dot product of the each of the normals of the facets to the velocity vector, I can see which facets are to be ignored by using these indices as logical indices.