Sight distance - How is the angle of sight determined ?

[cyberflow]

Hi everyone,

I was challenged today for the analysis tool of sight distance in C3D.

We placed a minimum of 70m sight distance for our simulation.

But we weren't able to validate/measure any of the distances to have that minimum of 70m.

I wanted to know, how does civil 3d determine the angle of sight when analysing ?

[jeff_rivers · Accepted solution]

I'm not sure what you mean by 'angle of sight'.  The sight distance tool starts at the starting station range you entered for the alignment you entered, and draws a vertical line up from the given reference surface to the given eye height.  It then moves up-station by the entered minimum sight distance value you entered, and draws another vertical line from the alignment and the reference surface up to the given object height.  Next it attempts to draw a straight line from the top of the 'eye height' line to the top of the 'object height' line.  If that line does not intersect the reference surface, then the sight distance test passes at that station.  

It then steps upstation by the interval you entered, and repeats the process.  Draw eye height line, move upstation & draw object height line, attempt to draw the sight line between. 

If at any of the tests the drawn sight line intersects the surface, then the test fails at that eye height station location.  The only angle is the angular difference between the eye height and the object height, if you are using different heights.  

[jeff_rivers · Accepted solution]

P.s. last time I used it, the sight distance tool still defaults to white+layer zero for all the lines it draws.  If you don't already, set unique colors for the lines representing the different components:  green for visible sight lines (sight lines that pass the test), red for obstructed sight lines, and so on.  If you also set unique layers for each, you can turn each component on or off and see exactly what is going on and where any sight lines may be intersecting your surface.   

[jeff_rivers · Accepted solution]

To add one more post in this thread, in case anyone searches this one for a solution, here's a link to an earlier discussion on SSD: 

https://forums.autodesk.com/t5/civil-3d-forum/check-sight-distance-querry/m-p/7670277/highlight/true...

One point I didn't make in this thread is that for a tangent section, the sightline and the path of travel of the vehicle are the same.  However for a curve, the sightline is straight, but the path of travel follows the curve, and the vehicle follows the curve.  So the stopping sight distance is the distance along the path of travel, not along the sight line.  

[cyberflow]

Hi @jeff_rivers ,

Thank you for your description on how the analisys is made !

These are new features we're using in our team and it's pretty nice to know how they properly work.