Revit coordinate system is very confusing. Even after 7 years of Revit experience, I still needed hours of research, investigation and experiments to build this guide.
This post covers a lot of ground. You will learn about the mysterious, super secret Revit Internal Origin. You will learn the difference between a Survey Point and a Project Base Point. You will learn how to properly use the True North and the Project North. Good luck.
1- UNDERSTAND THE DIFFERENCE BETWEEN ALL 3 COORDINATE ORIGIN POINTS
There is 3 different origin points in a Revit project: the Project Base Point, the Survey Point and the secret Internal Origin.
PROJECT BASE POINT: This point is used almost exclusively for internal purpose. It is used to place dimensions relatively to the building. It is represented by a blue circle with a cross in the middle. It can also be used to set the angle difference between the True North and the Project North.
SURVEY POINT: This is used to create a "shared coordinates" system among multiple linked Revit or CAD files. That means it's location is most useful when exporting and importing files. It is usually placed relatively to the Site.
INTERNAL ORIGIN: This is the tricky one. This point is invisible and cannot be moved. Most users don't even know it exists. By default, importing or exporting a CAD or Revit file will be made relatively to this super secret point, therefore confusing many people.
2- LOCATE THE INTERNAL ORIGIN
Now that you are aware of this super secret internal origin, you should locate it in a plan view using reference planes. Actually, you should locate it in your Revit Template so you will be able to track it's location for all new projects.
To locate the point, go to your site plan and make sure the Project Base Point is set to visible in the Visibility/Graphics settings.
The next step is to select the project base point and click on the clip icon. A red dash should appear on the icon. The next step is to right-click on the project base point and select "Move to Startup Location"
The project base point should now be located at the exact same spot as the Internal Origin. Mark this spot in the project by creating two reference planes that intersect at the point. You should also add a text note to indicate the location to other users.
The 3 origin points not only have X/Y coordinates, but also a Z-axis elevation. Therefore, you should open an elevation view and unhide the Project Base Point and Survey Point. Create a third reference plane to indicate the height location of the Internal Origin.
3- BY DEFAULT, SET THE 3 ORIGINS AT THE SAME SPOT
In most project, the Project Base Point, the Survey Point and the Internal Origin can all remain in the same spot without any problem. In your template, make sure they all fit together in a corner of your building, at the intersection of grids A and 1.
While the default stance is to keep these points together, the next tips will teach you in which case they should be moved.
4- TOPOSURFACE IS RELATIVE TO INTERNAL ORIGIN
The 3 origins points each can have a different elevation value. When creating a toposurface, you have to set values refering to Absolute Elevation. This value is relative to the Internal Origin.
5- SET LEVELS RELATIVE TO PROJECT BASE POINT
In most projects, it is a smart move to set the project first level at 100'-0'' or 10 000mm. This is an arbitrary value that has no relation to the sea level. The best practice is to set this value in relation to the Project Base Point. Select a level, click on Edit Type and make sure that the Elevation Base is set to Project Base Point.
6- USE THE INTERNAL ORIGIN AS SEA LEVEL = 0
In a project where you want to spot elements in relation to the sea level, you should use the Internal Origin to represent sea level 0.
In this example, the 10 000mm project level fits the 4 835mm sea level. That means we have to move the Project Base Point so it is 5 165mm below the Internal Origin base elevation.
If you want to spot any element in your project in relation to the sea level, create a new Spot Elevation tag that use Relative as the Elevation Origin.
Placing the Project Base Point relatively to the Internal Origin should be your first move when starting a project. Else, you won't be able to model the site using the sea level elevation values.
In a project where the site and project are already modeled without using the sea elevation, the solution is to use the Survey Point as the sea level origin. If you want to model the toposurface with sea level elevation, your best bet is to use a linked site model.
7- CREATE A SPOT COORDINATE FOR EACH ORIGIN
The Spot Coordinate tool is used to specify coordinate relatively to one of the 3 origin. You can find this tool in the Annotate tab.
In your template, it is a smart move to create a Spot Coordinate for each of the 3 origin type. Edit the type of the spot coordinate and use the duplicate tool. Scroll down the settings options and you will see the Coordinate Origin parameter. Create a different tag for all 3 origins (Survey, Project Base and Internal). Relative refers to the Internal Origin of the project.
Once you are done, you should have 3 different Spot Coordinate types like in this image.
As you can see in the image below, each spot coordinate are used to spot the same element. However, they indicate different data since they each refer to a different origin. These tags can be useful when you are confused about the location of the origin or of a specific element.
8- UNCLIP PROJECT BASE POINT BEFORE MOVING IT
If you want your project base point to be in a different spot than the internal origin, you have to move it. However, if you try to move it, everything in the project will also move except the survey point.
To avoid this issue, unclip the project base point first. Move it to the proper emplacement, then clip it again. As you see, the coordinates will be changed: the N/S and E/W coordinates are always relative to the Survey Point.
9- NEVER UNCLIP THE SURVEY POINT
While unclipping the project base point is the standard procedure before moving it, you should never unclip the Survey Point. If you unclip and move the Survey Point, you will cause a lot of pain, horror and confusion for people working in your model.
The only thing that moving an unclipped survey point will do is to move the icon representation of the survey point, not the survey point itself. There is absolutey no reason to ever make such a move.
If you click a survey point and you see something else than 0,0,0 coordinates, that means someone messed up and decided to move an unclipped survey point. Simply change the values back to 0,0,0.
*Edit: Turns out some people like to move an unclipped survey point. Steve Stafford explains in which case that might be useful in this post on his Revit OpEd blog. For the sake of simplicity, I would still recommend never unclipping the Survey Point, unless you have a really good reason.
10- USE PROJECT BASE POINT TO SET THE TRUE NORTH
Each project contains a Project North and a True North. The Project North is a virtual orientation used to model your project so it is orthogonal to your screen. The True North is a real-world north used to properly locate the orientation of your building. To set a True North value, select the Project Base Point and enter the angle.
In the view properties of each view, you can specify the orientation you want to use. In almost all cases, Project North will be used.
Revit weirdness alert: although the True North value is set in the Project Base Point, the value is actually embedded in the Survey Point. Even weirder: you can have multiple Survey Points in a project. That means you can have many multiple True North value in a single project... Strange, huh?
11- SET THE CAD EXPORT COORDINATE SETTINGS
Most users are confused about the project origin when exporting to CAD. The reason is that the Internal Origin is used by default. You can access the exportation options by going to File/Export/CAD Format and by clicking the 3 small dots next to the Select Export Setup menu. Go to the Units & Coordinates tab.
If you export a Revit view to DWG, the default setting is Project internal. That setting will use the Internal Origin as the 0,0,0 point location in AutoCAD.
The other option is called Shared. This will use the Survey Point as the 0,0,0 point in AutoCAD.
Watch out: If you are using the Shared setting and you entered an angle value for the True North, the project will appear rotated once opened in AutoCAD. To avoid this issue, export the sheet where the view is placed instead of exporting the view.
12- USE LINK CAD TO SET THE SURVEY POINT AS THE ORIGIN
The Link CAD tool has more positioning options available than Import CAD. If you want to use the Survey Point as the origin for the CAD file, you have to use Link CAD and select By Shared Coordinates. Else, the Origin to Origin option will match the Revit file Internal Origin to the DWG 0,0,0 point.
13- USE ORIGIN TO ORIGIN WHEN LINKING REVIT FILE
Revit offers a fancy "Shared Site" and "Shared Coordinates" system to link Revit files... but the truth is that you don't need to use these feature in 90% of projects. The Origin to Origin tool works perfectly fine. This option will match the models Internal Origins.
When linking a model, it is a wise move to PIN it immediately. If it moves around by mistake, right-click the model and use the "Reposition to Internal Origin" tool.
There is a few cases where you might want to use the Shared Coordinates system when linking Revit models... Check out the pamphlet below to learn more.
WANT MORE TIPS? DOWNLOAD THE FREE PAMPHLET.
There is even more weirdness to learn about coordinates. Want to know how to create a shared site? How to create multiple survey points and true norths? Our brand new quarterly free pamphlet publication was just released. It contains this whole blog post with extra tips inside an epic 28 pages PDF.