What exactly are you doing? It does look really interesting, but I'm not sure exactly what you want. Can you clarify what you mean by "where the circle and TIN intersect? Do you mean the edge of the sphere, or is it a solid sphere? You said you want to draw a "line," but it seems like you could draw an infinite amount of lines from the center of the sphere to points where the sphere and TIN intersect.
I look forward to finding out more!
In 2012 there is the Visibility Check, Zone of VIsual Influence probably no what you want. Not sure what version it came out with the Subscription Advantage Pack.
A better way may be to get the Solid Cut Surface for AutoCAD Civil 3D from the Autodesk Labs webpage and then create a hollow sphere half for above the surface and another for the bottom part of the sphere. Then use the Minimum Distance Between Surfaces to get where the two surfaces from the spheres cross the surface.
The intersection of the sphere and surface would be an arc draped onto the face of each triangle of the TIN. The problem is that none of the arcs would be coplanar, so where they intersect at the TIN lines there would have to be a kink. I tried a few tricks to draw the arcs, or to trim things at the intersection with the sphere but I couldn't figure it out.
Instead of a sphere would a cylinder work? The intersection of a cylinder and a TIN would be a circle, which can be converted to a featureline and then draped onto the surface. This gives you something to draw your line to.
Here's what it looks like when I tried it.
If I can create a feature line from a cylinder and TIN intersection, that would be great. Trying to create and analyze 'NO FLY' zones. I need to be able to measure from the center of the 'NO FLY' zone to a point on the ground 4000' feet away.
Thanks for all the tips so far, I will try these out. How do I create the feature line from the cylinder and Tin intersection?
You don't need the cylinder, just a circle around your point of interest. Actually you need to draw an arc of almost 360 degrees, because a circle cannot be converted to a featureline. Once you have the arc use the Create Featureline from Object command, with the Assign Elevations button checked so that it will be draped onto your surface.
Once you have the featureline you can draw as many lines as you like from the center point out to the edge - then you can select all of them and change their start elevations to match the aircraft location overhead.
Could you actually build a spherically curved surface? Obviously not a whole sphere, but the lower portion of it. Use polylines at an elevation or feature lines? Maybe curved feature lines with 3DROTATE command to model the vertical curve of the sphere?
Once you have the sphere as a Tin surface, you can do a volume surface between that and your ground surface. The zero contour is the point of intersection.
Credit where credit is due! Give kudos or accept as solution whenever you can.
This is a task that would probably best be handled with 3D solid modeling, not Civil 3D. You would need to turn the C3D surface object into a 3D solid or surface and then use the 3D tools in Autocad to find the intersection.