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## Intersection of a 3D curve with a 3D surface in AutoCAD.

If you want to display (create a point, get XYZ coordinates, trim curve) an intersection of a 3D curve or line and a 3D plane or surface, you can use the following procedure.

When you have got a simple 3D line and a 3D face, you can use the ilp function in AutoCAD geometric calculator - see Tip 4201.

For more complex cases, use an auxiliar construction with projected curve on the given plane or surface.

1. If the plane is defined by a 3DFACE, convert it to a region using the REGION command. If the surface is defined by a 3D mesh or similar object, convert it first to a surface with CONVTOSURFACE.
2. Switch to a user coordinate system (UCS) defined by the surface plane - on curved surfaces use an approximate projection near the expected intersection point. See e.g. the command UCS with the option obJect.
3. Use the PROJECTGEOMETRY command to project the 3D line (polyline, curve) onto the selected surface. If needed, select the view/project direction: "Ucs".
4. Now the 3D intersection of the original and the projected curves is the requested intersection point - you can use it to create a point (POINT), get its coordinates (ID), etc. - use the osnap INTerstection to acquire exact coordinates.

NB: in case of a 3D spline, the intersection may not appear - this is a result of different interpolation procedures for 2D (projected) and 3D splines

 See also: Tip 11072: How to make an interference line from 3D Faces? Tip 10926: Self-intersecting polylines and polylines with duplicate vertices. Tip 10499: How to fillet edges between two 3D solids? Tip 10054: Boolean operations on 2D polylines. Tip 8322: How to extract intersection lines or edges from 3D objects?