Archive | August, 2013

Positioning of Linked Models

7 Aug

HOK BIM Solutions.

Every CAD software uses Cartesian Coordinates to locate objects in digital space. You start with a fixed Origin (0,0,0) and three Axes (X,Y & Z). In traditional 2D CAD packages, the Origin and Axes are exposed to the user through visual means, whilst in Revit they are not as “in your face”, but make no mistake: they are still there! This fixed system is essential to position elements in space such as points, lines, planes, surfaces and solids.

Startup LocationTo find the fixed Origin in a Revit project, you can either use the older, harder way (link a CAD file that contains some linework drawn at 0,0,0 with Auto – Origin to Origin) or the easier, newer way: Use an unclipped Project Base Point and pick Move to Startup Location in the right-click context menu.

NOTE: In AutoCad, the fixed Origin and Axes are called the World Coordinate System. In Revit we refer to them as the Project Internal Coordinate System.

Fact #2

When you link with the Auto – Origin to Origin option, the Project Internal Coordinate System of the linked models will be aligned to that of the host model. So if at project startup reference models were linked haphazardly (ex: with Auto – Center to Center or arbitrarily shifted around after linking), you are guaranteed to end up with incorrectly positioned models. To circumvent this problem, we can set up Shared Coordinates in each model, share them through “acquiring” or “publishing” tools, and then link with this option. Note that a successful outcome doesn’t happen auto-magically on its own!

Fact #3

In most CAD software, you will find the concept of a User Coordinate System (UCS), which is another Cartesian Coordinate System similar to the World Coordinate System (WCS). A user can define any number of named UCSs and these are positioned in space relative to the WCS. Now you can probably understand why the WCS is fixed since you must have an anchor point in 3D space for relative positioning to work.

When we talk about Shared Coordinates in Revit, we are talking about their equivalent counterpart in 2D CAD: User Coordinate systems. We can define multiple named systems in the Site tab of the Location Weather and Site dialog. Named Shared Coordinate systems (Sites) are located relative to the Project Internal Coordinate System. In Plan views, Project North is defined by the Y-axis of the Project Internal Coordinate System, while True North is defined by the Y-axis of the Shared Coordinate System (the named “Site” per the dialog below):


When you link with the Auto – By Shared Coordinates option, the Shared Coordinate system of the linked models (for the current named “Site”) will be aligned to the Shared Coordinate system of the current named “Site” in the host model, given that Shared Coordinates have been properly set up between models. If you try to link models that have yet to be synchronized (by publishing or acquiring Shared Coordinates), Revit will not allow you to use this option:

Coordinates not Shared

Revit has always strived to describe functionality using terms that are familiar to the building industry, but I think that the ones used while manipulating coordinate systems do not work well for most common model linking scenarios. Since terminology is so different from that used in generic CAD programs, it makes things harder to visualize and understand sometimes, especially for users that started in 2D CAD. The main problem with named UCSs in Revit being called “Sites” is that if a consultant is linking parts of a building to parts of the same or another building, they probably do not think of “Sites” for this exercise!

The primary reason for the current implementation is to give us the ability to place multiple instances of the same building around a site (ex: the same home floor plan peppered around a housing development). For most typical project workflows, we just need one “site” definition for the purpose of locating links in 3D space and usually don’t even bother changing the stock site name “Internal” to something else.

In Part 2 of this post, we will look in more detail at some of the most common linking scenarios and discuss strategies to set up Shared Coordinates and linking Best Practices.