Non-Coplanar objects in 2D file

Question

Fishpond.dgn All- 
 Please check out the "O" in the attached DGN. I was hoping to just create a region to with a flood fill between the inner O and the Outer O but I am getting a "objects are not coplanar" error. 
 
 This is a 2D file so this shouldn't be an issue. 
 I'll probably figure out another way to skin this cat but if you have any quick ideas please let me know. 
 
 thanks 
 t.

George A. Martin · Answer

Hi Tom! Sorry this was not a quick reply! 
 As Jon stated, the DGN is a container of 2D and 3D models. When you open M-Sta. and Preview the file, it will state whether the Default model (called "Model" in your file) is 2D or 3D. You actually have 3 - 3D models (Layout1, Layout2 & Model) and one 2D Model (fishpond). Layout1 & Layout2 appear as 2D because they are Sheet models but if switched to Design models they are 3D. 
 
 All that said, I don't think your problem is 2D vs 3D even though you got the co-planar statement but the elements that make-up the "O". Both the outer and inner elements are single b-spline curves. There is something about both of those b-spline curve definitions that is causing the problem. I was able to use the Create Region by Flood on 2 b-spline curves I created, without a problem. re defining the b-splines would be the quickest solution. 
 There are other ways to get the same effect but seems you figured one out. Another option is to use the Break by Drag Line across both b-splines and then join the 2 parts of the b-spline together into a Complex Shape for both Outer and Inner loops then use the Difference in the Create region tool.