Coordinates in Revit

Objectives

1. Fully understand coordinates in Revit.
2. Understand the concept of “Shared Coordinates”.
3. Work safely with coordinates.

Prerequisites

1. User has basic skills in Revit Modelling.
2. User has basic skills working in AutoCAD.
3. User has basic notions about general coordinate systems.
4. User can import/link files into Revit.

Workflow description

In Revit the coordinates work in a total different way as they do in AutoCAD. This is why people find it so difficult to deal with them. Let’s try to explain it.

The key to the work with coordinates in Revit is the following:

When in AutoCAD we have to place something in its real coordinates and orientation, we would move the drawing and rotate it with respect to the Universal SCP.

In Revit it is quite the opposite. We will never move the model. We move the SCP to meet our location requirements. We will move the SCP origin so that our model is in the right location.

Coordinates and shared coordinates

In Revit we tend to speak always about Shared coordinates. And this is sometimes the origin of some confusion about coordinates in the Revit Models.

We should talk about:

Coordinates

when we want to refer to the system that we are using in a model, or which coordinates are defined on a point.

Shared coordinates 

Coordinates in a file (revit or cad file) will only be shared, or the files will have shared coordinates after a process of transferring the coordinate system used in a file into another. It does not matter if in two files the location is exactly the same. They will be not sharing coordinates unless the sharing coordinates process has been carried out.

Normally there is one file, and only one, that is the source for sharing coordinates. From this file the location of different models is transferred to them, and after that all files will be sharing coordinates, and be able to be linked with the “Shared Coordinates option”.

Origin points in Revit

First thing to understand in Revit is how many coordinates systems we have in a project, that is also how many coordinates origin points we can find in a model:

There are three, and when we first open a new model, the three overlap. They are:

1. The Survey Point
2. The Project Base Point
3. The Internal Origin

To show the origin points, open a plan view, go to Visibility and Graphics options > Site Category > Survey Point/Project Base Point.

Survey Point (SP - Blue Triangle)

This is the point that stores the universal coordinate system, or a defined global system of the project to which all the project structures will be referred. It shows as a blue triangle.

And:

• It is the origin that Revit will use in case of share coordinates between models.
• The origin point used when inserting linked files with the Shared Coordinates option.
• The origin point used when exporting with Shared Coordinates option.
• The origin point to which spot coordinates and spot elevations are referenced, if the Survey Point is the coordinate origin in the type properties.
• The origin point to which level elevation is referred, if the SP is the Elevation Base in the level type properties.
• When the view shows the True North, it is oriented according to the Survey Point settings.

Project Base Point (PBP - Blue Circle)

The position of this point is unique for each model, and this information is not shared between different models. Project Base point could be placed in the same location as the Survey Point, but it is not usual to work in that way.

In general, Project Base Point serves as a local reference in projects, for each building of a complex for example.

And:

• The origin point to which spot coordinates and spot elevations are referenced, if the Project Base Point is the coordinate origin in the type properties.
• The origin point to which level elevation is referred, if the PBP is the Elevation Base in the level type properties.
• When the view shows the Project North, it is oriented according to the Project Base Point.
• If it is not necessary, it is better not to move this point, so that it always is coincident with the third point: the Internal Origin.
• If we have moved the PBP to a different location, it can be placed back to the initial position by right clicking on it when selected, and choose “Move to Startup Location”

Internal Origin (IO - It is invisible)

Not everyone is aware of the existence of this third origin point, but it is essential to know about it to understand the behaviour of Revit Models in some cases.

This point is invisible. It has no graphical representation in Revit. We can not show or hide it from the visibility options menu, like we would do with the other two points (SP, PBP).

(In the picture below an extra green circle has been added only to represent it, to make the explanation easier)

When starting a project the Internal Origin is coincident with the Survey Point and with the Project Base Point. 

The importance of the internal origin

• Project should be modelled in a restricted area around this origin point. Currently it is said, that model should be inside a 20 miles radius circle around the internal origin, so that Revit can compute accurately.
• This is the origin point that is used when inserting external files using the “Origin to Origin” option.
• This is the origin point that is used when copy/pasting model objects from one file into another using the “aligned” option.
• The origin point to which spot coordinates and spot elevations are referenced, if the Relative is the coordinate origin in the type properties.
• The origin point used when exporting with Internal Coordinates option.
• And Revit API and Dynamo use this point as the coordinates origin point for the internal computational calculations.

Effects of moving the points

Let’s see how to move these origin points and their effect in our model.

Notice that we will NOT move the model elements. If we want to define correctly the project location what will be moved are the origin points, and not the already modeled elements.

And more important, we will not travel to the right location in the canvas to place there the model.

Quite the contrary, we model the building around the Internal Origin, and afterwards we define the right coordinates of the project by moving the Survey Point/Project Base Point.

So, let’s move the points. This is the initial situation:

1. Move the Project Base Point

1.1. Movement performed with the PBP pinned

This is like changing the global coordinates of the whole project. Survey Point remains in the same place.

We would have the same result as moving the Survey Point in the opposite direction.  

• The PBP own coordinates change. They are related to the SP.
• Spot coordinates related to PBP do not change.
• Spot coordinates related to SP change.
• To move the PBP pinned will drag also the Internal Origin. See in the image how the IO (green circle) has moved along with the PBP.

1.2. Movement performed with the PBP unpinned

This means just that we are moving the local reference of the project to a different position.

• The PBP own coordinates change. They are related to the SP.
• Spot coordinates related to PBP change.
• Model does not change its location related to the global position or Survey Point.
• Spot coordinates related to SP do not change.
• Notice that the Project Base Point is no longer overlapping with the Internal Origin.

2. Move the Survey Point

2.1. Movement performed with the SP pinned

This is like changing the global coordinates of the whole project. We move the Survey Point. Project Base point remains in the same place.

• Spot coordinates related to SP change.
• Spot coordinates related to PBP do not change.
• The PBP own coordinates change. They are related to the SP.
• The Internal Origin does not move. It still overlaps with the PBP.

Moving the pinned PBP in one direction or moving the pinned SP in the opposite direction would have exactly the same effect in the resulting global coordinates.

2.2. Movement performed with the SP unpinned 

This is maybe a puzzling option. We will move the Survey Point, but it will not have effect in the positioning of elements. The only thing that changes is the own coordinates of the Survey Point.

We locally change the position of the triangle, but let’s say that we are not actually changing the location of the global coordinate system origin point.

• Spot coordinates related to SP do not change.
• Spot coordinates related to PBP do not change.
• The PBP own coordinates do not change. They are related to the SP.
• The SP own coordinates change.
• It is as if we move the triangle, but an invisible SP remains in place.

Special procedures

1. Set coordinates manually

With this procedure we are going to establish in which location our model is actually placed.

But remember that we are not going to move the modelled elements.

We have started modelling close to the Internal Origin of the file. That is the only care that we had when deciding where to start.

Now we have already some elements, and we are informed that point A has the following coordinates: (N -8250, E 5210, Elevation 1000).

We have no other file with the correct spatial reference (no DWG, no RVT).

We are going to set the coordinates manually:

1. Go to Manage tab > Project location Panel > Coordinates menu > Specify coordinates at a point

2. Select the point. In this case point A.
3. Specify the coordinates.

4. See that the Survey Point changes its position. The model has not moved, but the global (SP) coordinates of point A change. PBP has not moved. Internal Origin has not moved.

2. Share coordinates with a DWG

With this procedure we are going to establish in which location our model is actually placed with help of a linked DWG.

But remember that we still are not going to move the modelled elements.

We have started modelling close to the Internal Origin of the file. That is the only care that we had when deciding where to start.

Now we have already some elements, and we are informed that point A is the same point A that we can find in a DWG file, that we know that is in the correct location.

Now is when really we are going to share coordinates between the two files: DWG file and our RVT model. Share means that we are going to transfer the information in one file to the other. And this connection will remain.

Follow these steps:

1. Set the view properly

Go to a site view and ensure you are using “Project North”.

2. Insert the DWG.

Select the correct import units and CENTER to CENTER Positioning option.

3. Move and rotate the DWG Link

So that Points A overlap, and lines of the DWG are aligned with the walls.

Remember, we don´t move the Revit modelled elements. DWG Link and Revit file are still not sharing coordinates. They just overlap in the view.

4. Share the coordinates.

Select the DWG Link, and go the the Shared Site menu in the properties  palette. We will see two options.

• Publish. To transfer the coordinates in the revit project to the DWG link. NEVER publish when we are using a DWG Link.
• Acquire. To transfer the coordinates in the DWG link to the Revit project.
• In this case we don´t want to change the DWG link, we want to modify the revit model, so we choose the Acquire option > Reconcile.

5. The Survey Point has moved.

Coordinates related  to SP in model are now coincident with those in the DWG link. We have not moved model elements.

6. Remove the link

In most cases after this sharing coordinates process we can remove the DWG link or unload it. This will prevent us from moving it unintentionally, what would result in us starting to experience issues with the coordinates.

7. Or stop sharing the site with the DWG

If we do not want to remove the link we could also stop sharing the site between DWG link and host project. This would prevent also some common issues:

If in the same project we have several models (which happens almost always) we can use the DWG link as the unique source of coordinates for all of them.

Once we have shared coordinates across all models, we will be able to work with them knowing that their global positioning is correct, that they shared the same system, and that if we link one into another their location will match perfectly.

3. Share coordinates with Revit files

With this procedure we are going to establish in which location our model is actually placed with help of other linked Revit Model. In this second Revit Modelled the coordinates have been configured following any of the two previous procedures.  

But remember that we still are not going to move the modelled elements in host model.

Procedure is the same as with a DWG Link:

1. Insert RVT link

Center to Center (They still don´t share coordinates).

2. Move the link and rotate it

until it is in the right position relative to the host revit model.

When working with revit linked files we must not forget the Elevation. Linked file must be also moved in elevation to the correct position.

3. Select the link and go to the Shared Site Menu in the properties palette.

• Publish. To transfer the coordinates in the revit host project to the RVT link.
• Acquire. To transfer the coordinates in the RVT link to the Revit HOST project.

In this case we don´t want to change the RVT link, we want to modify the revit HOST model, so we choose the Acquire option > Reconcile.

We advised to never publish when we are sharing coordinates with a DWG linked file. When we are using revit links we could do it both ways, publishing or acquiring, it only depends on which file contains the correct location of the project.

4. Reset coordinates

It could happen that we are on the need of having the coordinates reseted, so that we have a file as it were a new one, with the survey point in the initial position.

This could be for a number of reasons:

• Simply we want to start over.
• We have acquire coordinates from a DWG/RVT Link and we realise that we have done it wrong.
• We have shared coordinates with a linked file, but at some point the coordinates of the project change.

In that case it is better not to try to move the linked files, but reset the coordinates and start the procedure right from the beginning.

For resetting the coordinates, we need a clean model. Have a clean DWG just with a circle with center in the Universal SCP 0,0,0. Or have a clean REVIT model with a model reference based on the Internal origin of the model.

1. Save the DWG and keep it for this and future uses.

2. Insert the DWG into the model using ORIGIN to ORIGIN positioning
3. Select it and acquire coordinates from this clean file.
4. Save the project.
5. Remove the link.
6. Save the project again.
7. If necessary start the sharing coordinates process again from the beginning.

5. Insert generic DWG files

This is another case. We have already set up the coordinates of our Revit models, following any of the possible procedures.

At this point  we need to insert some DWG links to complete the information in the project.

• They have to be in place.
• We know that the coordinates systems are coincident although the files are not sharing coordinates.
• This DWG links are not necessary the one that we have used to acquire coordinates.

What then:

1. Insert the DWG link. Go to Insert Tab > Link CAD > find the file
2. The view is in Project North
3. Select the correct import units.
4. Select the option “Shared Coordinates” although files are not sharing coordinates. We will get this warning:

Do not  panic. Just read and notice that this is just what we wanted, to align the World’s coordinates in DWG link with the coordinate system of the Revit model.

5. The DWG link lands in place.
6. When first we save the model after this process, we will get some additional options for saving positions of the DWG links. One menu for each DWG that we have linked:

Save 

We will be sending the information back to the DWG. It is not recommended as changing the DWG file is not a good practice. This change consists on creating an additional SCP different to the World coordinate system that would be the one used by Revit to insert the file.

Do not Save 

nothing happens, we will be asked again later.

Disable shared positioning 

DWG link will stay in the place that was inserted, but the connection between link and host is disabled, so that the DWG link is not changed at all. This is the recommended option like explained also in Procedure 1.

Same applies for RVT links.

Tips&Tricks

• Always link and reload links in the same view mode: Project North or True North. It is easier to remember to use the Project North option since it is more usual in views.
• Do not ever try to move a linked file (nor DWG nor RVT) if what you want to achieve is to change host file coordinates. Remove it, reset the coordinates and start again from the beginning.

• When sharing coordinates with a file by acquiring them from it, unload every link in the model so that they are not affected by this change. If not, the active file will publish automatically coordinates to the links, and this can have undesired results if we are not very confident with this processes.

• If you find that when inserting a DWG using the “Shared Coordinates” option it is not landing in the right position according to the World Coordinate system and with reference to the model, check in the DWG file the existence of an additional SCP. If that is the case, remove it.

Bottom-line

Coordinates in Revit are a very troubling subject for many people. They do not understand how it works because it does not work as it does in a CAD environment, which they feel that is the intuitive way of getting it.

In any case coordinates are very simple if we understand that we have to model the project close to the Internal Origin of the file, and after that we will indicate which are the correct coordinates for that area, but without moving the model element. Instead we will move the Coordinate system origin.

Something important is also that:

• A file can have been configured with the right coordinates, but not share coordinates with any other file. Then they will not be ready for spatial coordination even though they can be located with the same coordinates.
• Sharing coordinates is a process to establish a connection between the coordinates information of two (or more) files, where the source should be unique for the models involved in the same project.

Write by Roberto Molinos