One of the most common issues I see in the setup of any Revit project is the coordinate system. I can’t remember how many times I have seen this set up incorrectly. It’s surprising, considering it is often one of the most critical parts of a project. If the coordinate system is wrong in the design software, and these coordinates get sent to site and built to, we could end up with a building in the wrong place. In my time in the construction industry I’ve seen buildings out of position by a few meters and even built back to front! This is usually down to the coordinates provided by designers being incorrect. (Sometimes a lack of common sense plays a part too!) This is easy enough to identify in most CAD applications. However, the way Revit deals with coordinates is a little different to what we may be used to, such as AutoCAD. If we treat Revit in the same way we treat our other CAD platforms we can come unstuck.
Having a good understanding of how Revit deals with coordinates and interacts with other pieces of design software is essential to successful project delivery.
COORDINATES IN THEORY
Every CAD system uses what’s called a Cartesian coordinate system to position elements. Every element has an X,Y and Z coordinate value that dictates its position relative to an origin point or base point.
When producing construction documentation we are often required to provide setting out coordinates for certain elements of the building. The coordinates we generally give are relative to the national grid coordinate system. The base point for this coordinate system is located somewhere off the coast of Cornwall. This system allows us to pinpoint any location in the country down to the nearest millimetre.
In many CAD applications this is fine. We can produce our drawings and give the elements the correct X,Y and Z positions based on the national grid base point. Using this method we are essentially drawing or modelling the building in the correct place in the world. However the X,Y and Z positions these produce will run to 9 digits.
COORDINATES IN REVIT
When using Revit this gives us a little challenge. Because of the database system Revit uses (well some technical stuff that goes on under the hood anyhow), Revit will struggle with coordinates this long. In fact, Revit has a size limit of 20km from its coordinate base point. All the data in Revit needs to be inside this limit, otherwise we will start to get some serious performance issues.
So if Revit were to use the national grid system as its base point, all our projects would be constrained to a highly desirable piece of real estate in the Western English Channel. At least all our rooms would have a sea view and there wouldn’t be any trouble from the neighbours!!
Obviously we are not constrained to this area when using Revit – we just have to use a different approach to what we may be used to.
Revit uses 2 coordinate systems at all times:
1. The PROJECT BASE POINT this is a base point Revit uses internally to position all the elements we create. This is the centre of the 20km limit and we should create our model as close to this base point as we can. This does not however help us if we need to report positions relative to the OS national grid system. For this we need the second coordinate system.
2. The SURVEY BASE POINT this is a secondary coordinate system that ‘floats’ around in the background of Revit. The base point of this coordinate system is the only element that can be outside of the 20km limit from the project base point. This coordinate system can be repositioned and rotated into the correct position relative to the building. This will allow us to report accurate coordinates relative to a useful grid system. So rather than modelling our building in the right place in the world we are moving the world to our building.
Needless to say the setup and management of the coordinate systems in Revit is something that needs to be considered and implemented before any Revit project can be successfully shared and coordinated with others.
A project kick-off meeting to discuss and agree the coordinate strategy among other elements pertinent to the delivery of a BIM project is essential. A good plan (and sticking to that plan) is the beginning of a successful BIM project.
Cadassist’s BIM consultants can be on hand for these initial meetings to help guide you in the right direction – a useful resource, particularly if you are tackling your first project.
For more information on a project kick-off workshop and other services for transitioning to Revit, contact your account manager or firstname.lastname@example.org