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However, the solution was much simpler, thanks Dermot.
I simply overrode the Stan
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(Any specialist styles like cabinets, eave, kerbing, built with a wall style already have their own CP override in the style.)
The effect of this surprised me! With a wall style using the Global Cut Plane, when you raise the wall it will disappear when it is no longer 'cut'. However, override the wall style CP and you can raise or lower a wall as you like and the wall will display perfectly. (It will cut each wall based on the it's cut plane and THAT wall's baseline). I was very surprised and excited that it was so simple. Whilst it obviously overrides the cutplane, it's not obvious that it also overrides the baseline that the CP height is taken from. So why doesn't the OOTB standard style have it's own cutplane. I can't see any advantage to use the global cutplane. If you work with straight single or 2 storey construction this problem won't be obvious to you but as soon as you do a split level it becomes a problem to get a mezzanine to display correctly. With the new cleanup rules introduced in v2009 it's easy to create the 2 floors at their own 0 and still have the walls cleanup.
So now I can have units at varying heights and each one will assume it's own cutplane at the right height according to it's own floor level. This behaviour will occur whether I am using the PN and /or x-ref's or it's in the same file. I've been wanting to work with multi storey in a one file approach for years and this tip was a key to allowing me to make this happen easily - thanks again Dermot.
Thought!
It used to be hard to get a high window to display. You had to override the style display rep to raise the cutplane. Unfortunately overriding a cutplane will require to cut that wall (style) off from it's parent wall style and it will no longer respond to style adjustments made to the parent. In ACA2009+ you can have the baseline at various heights and it will still clean up so maybe instead of overriding the cutplane, it might be an option to raise/lower the baseline and adjust the bottom and top of the wall instead (as long as you have this blog tip in place!).