Wednesday, March 03, 2010

Coordinates, Base Points, Surveys, Linking and Other Confusing Things - Part 1

If you browse through the forums, blogs, how-to videos and books you will find a wealth of information on the above titled subject. And it's all conflicting. It's just about the most confusing subject you will encounter in Revit. I will now add my voice to the many others. The method I'm proposing can be boiled down to a common point which is graphically represented in all files. You will then line everything else up based upon that.

The architect must set this up BEFORE anything is done in Revit. No modelling, importing, linking, not anything.
Step 1. Set up the AutoCAD
AutoCAD survey drawings usually have 3D contours and 2D drafted information. The 2D stuff is at 0 Z elevation, the 3D is at it's actual height off of Sea Level. The 0,0,0 or Origin can be anywhere and sometimes it relates to something but sometimes not. We will be changing this.
Save as.
Give new name.
Move everything so the 0,0 is at the bottom left (don't adjust the Z). This could be at the property line intersection.
Make sure there is nothing outside of a 10 mile radius sphere from the 0,0,0. That means you will need to take a look in the Z.
Draw a circle on an individual layer at 0,0,0.
Rotate these to Project North. Make sure there is a North Arrow visible.
Break this file into two files, one with 2D plan info and the other with contours. You can use the wblock command or saveas.
Of course, you should audit and purge the AutoCAD file as well.

Step 2. Not in Revit yet...

Get the longitude/latitude information relating to the AutoCAD site file.
Open Google Earth and go to the address. Find the longitude/latitude of the corner of the site that corresponds with the 0,0, write it down.
The Google Plugin also works great as well. It will import the location to Revit's 0,0,0 as well as draw some Reference Planes and bring in a flat image. It will also give you the true north.

Step 3. Revit Architecture

Revit has a 10 mile diameter of where you can draw or even import. You cannot have anything outside of that.

Manage Place and Locations will assign the Longitude/Latitude numbers to your Revit 0,0 (Origin)
Revit 0,0,0 is found by default at the centre of the elevation markers. It's also the default location of the Project Base Point and Survey Base Point. The Revit origin can not be moved.

Make a view called AutoCAD Site Plan.
Import the 2D AutoCAD. Origin to Origin. Current View Only turned on. You cannot change Revit's origin. The origin can be found at the nexus of the elevation markers in plan. This point is dead centre on your screen when you start from a standard Revit Template.
Then Import the 3D AutoCAD Contours, Origin to Origin. Current View Only turned off. You won't see this file yet, it's above your view.
Go to elevation, move the contours down to level 1.
Go to the site plan view.
Confirm that your Survey Base Point(triangle with an x) is at the centre of the circle you drew in AutoCAD.
Click on the Project Base Point (a blue circle with an x). Click in the 'Angle to True North' and type the difference between your project north and the true north. You may have to type a negative in front of the degrees to get the expected results.
The Project Base Point and Survey Base Point will be in the same place. That's fine.
You may also want to draw two Reference Planes based on the centre of the AutoCAD circle which is also the origin. Name them something logical.
Adjust your elevation markers and datums to focus on where your building will be.
Start drawing grids and modelling etc.
Step 4. Other Linked Revit Files...
Start your new Revit Structure or MEP file.Very first thing you do is link the Revit Architecture file (which we just setup) Origin to Origin.
Check that the Z has come in correctly.
Adjust your elevation markers and datums to focus on where your building will be.
Draw Grids and Levels over top of the linked Revit Architecture as needed.
Start drawing grids and modelling etc.
About Exporting back to CAD...
When exporting back to AutoCAD the origins of both AutoCAD and Revit will line up as long as you have used the default location of the Project Base Point.

5 comments:

David C. Banks said...

Does "do this first" mean that I can't get the topo survey into my Revit project that was 90% complete before we received the site topo file?
Also, which version of AutoCadd were you working in? I have not upgraded my version because I have been working almost exclusively in Revit!

Jay Polding said...

'Do this first' means that it's a best practice. While it is possible to bring in a topo after the fact it will mean more work and pain. Plus, how will you get the proper site levels, sun studies etc if you don't have the accurate site data loaded in?

Jay Polding said...

There was a 'Nic' who wanted to leave a message. Nic, would you please expand upon your comment and then I can publish it? Thanks!

Me! said...

Hi,in my case I have done almost 90% of my project using Autocad Origin to Origin base point, until it going for submission which requires correct coordinates. How to set this Shared Coordinates from the Survey Plan...Help me :( Thanks

Torsdag said...

Hi! Do I have a question for you:
What about inputting HAND DRAWN surveys? How do I set up my project in this instance? What level should I input the survey in, and what else should I be mindful of?
Thx!