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Where are my elements?

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Object placement is another flexible concept in #IFC. Most #BIM programs only support one type of placement, which normally is local placement (XYZ coordinates relative to a work plane / level / storey).

Other programs (for designing roads, tunnels etc.) also allow linear placement, relative to the axis of alignment (e.g. middle of the road). So even if the road bends, object keep their relative position.

IFC supports in total three different ways to define a position:

• local placement
• linear placement (since version 4.1)
• grid placement

Local placement is used the most. The convention is that each spatial container (like site, building, storey and spaces) have their own local coordinate system. Object placement is defined as coordinates relative to their spatial container’s coordinate system. Some object like surface features are even related to their host element’s local coordinates.

So, all in all, a bit complex, but highly flexible. When you assign a world coordinate system to your site (“GPS”) you can easily calculate your elements global coordinates.

Linear placement is always relative to a curve (which can be anything from straight lines, arcs, or even clothoids). You specify the location basically at a certain length value of the curve, then to a side / up and down. That placement is rather new to the IFC schema. Look out, road planners!

A fairly unknown – and therefore not well supported – option to place elements is relative to a grid. If you create a grid you can place elements at intersections of grids. So, if you change the grid the objects move with it.

Again, who taught us that IFC was static? I keep repeating myself.

Not only can you place columns directly on the grid intersection, but also any other object, also with defined offsets. You can even define the direction for these elements by selecting another grid intersection (diagonal bracings etc.).

Unfortunately not many viewers can handle all of these placement concepts. But if you all demand this, I am sure that this will get better over time.

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