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Finding the midline of two polygons

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Posted (edited)

Ideas/ workarounds. None perfect:


A. You demonstrate most of the technique in your 2nd screenshot.

     These contours look like they only need 10 or so vertices to get close.

      Not a great workflow if you need lots of these:

           Engage the Polyline Tool>Cubic Spline mode.

           Pick a start point at area of low curvature.

           Hover to acquire a snap point on lower contour.

           Hover again perpendicular (prox) on next contour.

           Hover near midway between the acquired point to acquire their midpoint.

           Click to place a vertex at the mid point.

           Repeat at intervals until returning to start.  Maybe 10 vertices? Just get it close.

         If edits needed, Engage the Reshape Tool>Add vertex mode>Cubic Spline mode.

         Add new vertices in the poly via same technique.


A. Alt

     If the orig contours are polylines with only a few vertices:

           Duplicate a contour.

           Edit with the Reshape tool as above to position the vertices between 2 orig. contours


B.  Go 3d! Create a Terrain, adjust the minor contour interval.

     Note: This requires Landmark, Architect, or Designer package.

                 It's not in Fundamentals, or Spotlight.

      Duplicate the layer with your contours.

      In the new layer, Delete anything that is not a contour, then:

            Select all, Modify>Convert>3d Poly (result is a group of 3d polys all at z=0)


           Create a Terrain (aka DTM, aka Site Model). Several ways, but here is one:

                Select each individual Poly in turn.

                Adjust z value to match your landscape data

                              or at least increase by some regular interval.

                Repeat for each 3d poly.

            Select all 3d polys.

            AEC menu>Terrain>Create Terrain from Source Data.

            In the dialog

                  Adjust the Minor contour interval to match your source data.

                  Adjust the Major multiplier to match your source data.

           Click OK to show your DTM. Set drawing to Top/Plan to display things in 2d.

           If OK, Select the DTM, in OIP click the Site Model Setting button.

           Adjust the Minor Contour Interval to 1/2 the original.

           Adjust Major Contour Multiplier as appropriate.

           Click OK to review the contours added between the originals .

           They may be jaggy, but basically in right place.


Give it a try. Post back, maybe can be refined.








Edited by Benson Shaw
Can't Leave it Alone
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Here's how I might try it -


  1. Duplicate the two contours and group them, then work in the group.
  2. Convert the contours to NURBS curves.
  3. Move the inner contour up a known amount.
  4. Loft between the contours with the Create Solid checkbox enabled.
  5. Use the third mode of the Extract tool (Iso-parametric Curve Mode) to extract the midpoint curve. This will give you a NURBS curve contour.
  6. Alternately, you could slice the solid with the Split Tool and then extract the top of the bottom half as a 2d planar surface.




Intermediate Contour.vwx

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Posted (edited)
1 hour ago, Kevin McAllister said:

Here's how I might try it -

That’s great for a few nested closed contours!


 A bunch of intermediates could be simultaneously developed if a single NURBS surface is formed from the whole nest. In Kevin’s step 6 instead of Split, apply the Contour tool (3d tool set, way down near bottom of the list) in Front view. Set the tool spacing/count same as original interval, with start point at the lowest half interval.  If the source curves are a combo of open and closed polys it gets more difficult. Separate NURBS surfaces are needed for each “hill” and for any adjacent areas of open polys. Although I think the Contour tool will cut a selection of surfaces. More steps needed if the new contours need to be isolated or altered in some way

All of these methods are too cumbersome, I think, especially if many intermediate intervals are desired. Even the interval control in the DTM is rather unsatisfying in current state, and not available in all modules.  The desire/wish is a tool or command to automatically develop those 2d intermediates from a selection of polys. A new tool/command? or new mode to the Offset tool? 2d? 3d? Both? Not an unreasonable feature request. 

Edited by Benson Shaw
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