Shared Coordinates and Civil State Plane Coordinates
Understanding shared coordinates can feel like trying to find and unlock the “Dark Web.” You might be better if you just didn’t try. Unfortunately, some of us are forced to deal with things we don’t understand. In this article, I will break it down for you as simply as I can. We’ll also go over the use of shared coordinates with a CAD civil file with State Plane Coordinates—another “black magic” practice—although the two don’t always work well together.
Let’s first start to understand what shared coordinates do for Autodesk® Revit® users and their projects. Shared coordinates are a coordinate system you establish specific to your project and can be communicated by acquiring or publishing to or from a linked model. Sounds simple enough, right?
Okay, so what are State Plane Coordinates? The U.S. Geological Survey defines the State Plane Coordinate System (SPC or SPCS) as “a plane coordinate system (N-S and E-W lines are perpendicular) in which each individual state has from one to six zones, depending on the state's size and shape.”[1] In other words, State Plane Coordinates are defined points on one of more than 100 Cartesian grids that each overlay all or part of a U.S. state. When you get a SPC civil CAD file (such as a site plan) from the surveyor or civil engineer, open the file and perform a “Zoom Extents” command. You will normally find that there is some minuscule point or marker way off in the distance that makes your site really small. This point is most likely the geo-reference point, and it will be in a different location in relation to your site on each project. The point will normally be located at the origin point or 0,0,0 of the drawing.
Our site is located in the square on the top right of the diagram. The site is the entire airport property of approximately 3,300 acres.
The origin point of the drawing is located in the circle at the bottom left of the drawing, also known as 0,0,0.
We know that Revit does not like site files or models larger than 20 square miles. You will more than likely encounter this error, and we will discuss this workflow later in this article.
Now, we get to the nitty gritty: Revit’s shared coordinates. Due to the size and complexity of the work on my last project, using shared coordinates made the most sense. This was a large-scale aviation project in Tampa, Florida, with 18 different consultant firms and 135 Revit models (10GB of data) exchanged weekly among the team. I first created an architectural site model and linked in a site plan provided by the airport. This file was not geo-located, so it was a perfect start for locating our project using shared coordinates. In this case, we located some of the airport control monuments and designated one of them our shared coordinate point. We also located a second monument and used it as a secondary control point to make sure we had everything dialed in. The control monuments had northing and easting values, which we used to actually locate our project in Revit.
The control monuments can be found on the Airport Control Survey. We located this data in the CAD survey and linked it into our site model.
This is the site model with the CAD site linked into it showing our control monument. We will place our survey point on top of the monument.
To clarify, we are setting up our shared coordinates in our ArchSite.rvt model and “sharing” them with the other models. Our project has seven different buildings or structures on one large shared site, so we used the same shared coordinate point for every building, although we could have used a different point with each building. We felt that one common coordinate point would be much easier than seven. Now, back to our process. We have linked in our site plan and found a reference point (airport control monument). Next, we will place the survey point on our control monument, assign the northing and easting information, and save the location. This will establish our shared coordinates. It is also important to save the location or position of the site. We are actually saving the position of the building even though we have not yet placed our building model on our site.
You will have to unclip the Survey Point in order to record the coordinates.
The survey point has been placed on top of our control monument, and we have assigned the northing and easting values and elevation to it.
At this time you will also want to place the project base point. The project base point sets the origin of the model and preserves the position of the building on the site, whereas the survey point locates the project within the physical world. I recommend that you designate a common reference point for the project base point as well.
The next two diagrams show the location of the site on the map and the saved location or position.
Let the Sharing Begin
Let’s assume you already have a building model developed and you are ready to place it on the site. Link your building model into your ArchSite.rvt model and locate it in the desired location. Be sure to also locate it in the Z axis as well. I use Manual Center to place my building and then rotate or position as needed. Once this has been completed, you can now share your coordinates. Select your building model from any view and look at the Properties dialog.
You will see a button in the properties dialog next to Shared Site that says . This is one of the places Revit displays the shared position of the project. To share the coordinates to the linked model, you will select the Coordinates pull-down, then Publish Coordinates, which can be found on the Manage tab.
The Location Weather and Site dialog will pop up. It will display the current position of the linked project, which will be Internal. I like to rename the internal position to identify that I have designated shared position or coordinates for the linked model. Once you synchronize your project, a dialog will pop up and ask you to do one of three things. In our case, we will select the first option, Save. As the option text states, this saves the position back to the linked file. To confirm the coordinates saved back to my linked file, I normally select the linked file and look at the Properties dialog next to Shared Site to make sure it displays the name of my shared position. We can also open the linked model, go to the Manage tab, and select the Location button and then the Site tab, which will display the project position.
These steps should be performed for each linked model.
When I create my BIM project execution plan, I typically include some instructions and require that my consulting engineers use my Revit site model to set up their shared coordinates. Before taking this step, double check that your shared coordinates are correct and everything is set up exactly the way you want.
State Plane Coordinates
If you only need to do shared coordinates and have no need to work with State Plane Coordinates, your work here is done. If you do need State Plane Coordinates or are just sick and want to continue reading, then by all means let’s continue.
As I mentioned earlier, my firm recently used State Plane Coordinates to coordinate with our civil team when we exported our Revit models for a project that contains a 1.4 mile-long elevated train structure with four train stations. Making sure it all fit and was in the correct place was important, to say the least.
This is the MasterSite.dwg file that is linked into Revit. It is within Revit’s 20 mile limit. The origin point marked with the X.Y is located at 0,0,0.
In your Revit models, locate the Survey Point and mark it with an annotation symbol that will export with the model and show up in the CAD file. This will act as a reference point to help you position everything in the correct place. Next, turn on all the layers in the CAD file that you exported from Revit and move everything until the exported survey point is placed at 0,0,0.
You will need a similar workflow for the other civil files that will be included in your Site Model. In your civil file, draw a circle and enter for the x and y coordinates your northing and easting values that correspond to the shared coordinate survey point. Now, turn on all the layers and move all the CAD data so that your circle or reference point is placed at 0,0,0.
The circle on the far left is our reference point. It is located at 0,0,0 in our CAD file and corresponds to our shared coordinate point or survey point.
Once all these files are correct, link them into your master site and place the circle or reference point on your located CAD survey point. You still need to make your MasterSite.dwg file geographically correct, so link it into your geo-located survey and share it with your civil team.
The diagram below shows the workflow and file associations for Revit and CAD files that pertain to the shared coordinates and State Plane Coordinates workflows.
Using shared coordinates and State Plane Coordinates will help your team keep things consistent with common reference points. Don’t forget to check these reference points from time to time to make sure no one has moved them. Autodesk has given us these tools to help establish and locate our projects within the physical world. If that doesn’t work you can always try black magic or consult the Dark Web.
[1] http://www.usgs.gov/faq/categories/9794/3025
