Why is this surface Data Clip boundary not being honored?

[seattlesurvey]

I have created a surface with a Point Group, a (vast) set of breaklines, and a Data Clip boundary (which is at the top of the surface Definitions list).  The boundary is a 2D polyline which has been run through mapclean and which is closed.  This same 2D polyline works in another surface as an effective Show boundary.

In the attachment:

Thick green line is the 2D polyline added as a Data Clip boundary,

Magenta line is the auto-generated surface boundary,

Blue lines are surface breaklines.

Why is the green polyline not being 'seen' as the Clip boundary?  Does it matter that there are added breaklines underlying portions of it?

One other note is that the surface point group contains only 'spot-shot' points (it does not include points comprising breakline vertices).

Ideally, I'd like to create this surface with a Data Clip boundary because the data set is pretty big and I only want to see a small interior portion of it as a surface right now.  I have achieved this using an Outer boundary, a Hide boundary slightly offset from that, and a Show boundary inside of that, but I was under the impression that a Data Clip boundary would cut down on the surface overhead.

Civil 3D 2011, Version 3.

Many thanks for your thoughts.

[sjg]

Move boundary to bottom of definitions. Definitions list is opposite of what you would think. Bottom takes priority.

[seattlesurvey]

Where Data Clip boundaries are concerned, I have read that they should be at the top of the list.  When I use Outer boundaries, though (and these are typically what I use), I do place them at the bottom.

Thanks.

[sboon · Accepted solution]

A data clip boundary prevents the software from "seeing" the definition data that fall outside of it.  It does NOT prevent the software from building TIN lines outside of the boundary.  A data clip should be at the top of the definiton list, but an outer boundary should be at the bottom.  In this case you can use the same polyline for both purposes.  Add it once for the data clip and then again for the outer boundary.

[seattlesurvey]

Beautiful.  A toast to you, Steve, for a resolution and the best explanation I've read of how a Data Clip boundary works.  The surface looks great now.