Monday, July 08, 2013

Shared Coordinates

Shared coordinates seems to be one of those subject areas which causes a mental block. Not actually sure why, but it does. I know others including Steve Stafford having documented & shared this with people time & time again. I even posted something a few years back which defined the process. Anyway, this is my quick step guide to setting up shared coordinates by “specifying a coordinate” at a point. I am 100% confident with this process, a bold statement I know, but I have helped setup some mega projects using these steps & it works every time for me.

Start by opening the survey drawing in AutoCAD; as good practise check the unit of the DWG, this is achieved by select or typing “DDUNITS”. It’s also worth Auditing the DWG for any errors using the “AUDIT” command in AutoCAD. Next using the “ID” command, ID a point, this may be a benchmark or a survey pin on the survey plan. In the example below, it is an Easting & Northing of 100000,100000, with the units of the DWG set to millimetres.


In Revit, start a new project. Next go to the Site view & notice the Survey Base Point & the Project Base Point. Do not move them right now.


Next import the DWG survey, but it is essential to import “centre to centre”. Do this by going to the Insert tab > link CAD this will open up the Import CAD dialogue. Locate the DWG & ensure the units are set & that you are importing centre to centre.image

Once you have everything setup, click the Open button & the DWG will be imported. If you zoom all, you should see the DWG imported & the elevation symbols, very small. Resolve this by changing the scale of the view. If for any reason the DWG is not imported centre to centre, you need to go back into AutoCAD & check the DWG file as there may be an ordinate dimensions setup which can mess with the result. Remove the ordinate dimensions from the CAD file & go back a step to import again.


Next we need to align the coordinate system of Revit with the imported DWG file. Start by upclipping both the Survey Base Point & the Project Base Point.


Next we will move them to a location of 100000,100000 which is on the bottom left hand corner of the DWG. You may want to put the view into thin line mode to help with the line weights, as this can be distracting. Ignore the Northings & Eastings, we will resolve this in a moment. Place the Survey Base Point as below. Do NOT clip the Survey Base Point yet.


Next go to the Manage tab & the Coordinates. Choose the Specify Coordinates at Point command.


Pick the intersection. If this is problematic, draw a model line from the intersection & pick the end of the line where it intersects with the DWG.


In the Specify Shared Coordinates, type in the Northing & Easting to match the DWG file. You may also want to address the elevation level if you know this.


Once you have done this, click OK, to set the shared coordinates. Select the Survey Base Point & now click the paper clip, this will lock the Survey Base Point in place.


With the Project Base Point still unclipped you can then move this either to the Survey Point or to another reference location on the linked CAD file, if you know another survey coordinate or even a grid intersection, place it there. Once you have chosen a suitable location, clip the Project Base Point.


So as a double check, its good practice to save the file & then export the survey view as a DWG by shared coordinates back to CAD to see if everything has been setup correctly. To do this, go to Application Menu, big R>Export>CAD Formats> DWG files


This will open the export DWG dialogue; next go to button with 3 dots which is the Modify Export Setup.


Go to Units & Coordinates & ensure the Coordinate system basis is set to Shared. Select OK.


This will return you to the export dialogue, choose Next.


Name the export ; then Export the DWG to a suitable location. Ensure Export views & sheet links as external references is NOT ticked.


Next open the resulting DWG in AutoCAD. Use the ID command to double check the coordinates have been passed back correctly.


In this example we have zoomed into the location of the Survey base point & used ID in AutoCAD to check the coordinates. We can see from the example that they are spot on in the resulting DWG export.


As an extra check, you can always Xref in the original Survey CAD plan. If they drop in correctly on top of one another, you can be confident that the shared coordinates between AutoCAD & anything you export from Revit are correct.


Aaron Maller said...

Whats also very confusing for a lot of people (and programmers) is that things are worded in the UI much different than they are in the API. Here is where that gets crappy: There is the Shared Coordinate Origin, the Project Base Point, and the Relative Origin... Which is really just the original *for real* origin of the file, that you can never ever move.

That gets even more crappy ever since we got the functionality to either Specify coordinates at a point (which i love, and which i teach) and the option to simply move the freaking PBP or SP, which acts differently in a clipped or unclipped state. That changes how things move (moving the vase, or moving the tablecloth), and a lot of folks get lost with that. So i tell them ALWAYS use specify at a point, and dont ever move the SP and PBP. As far as that goes, my expectation is the PBP will always stay rigth on top of the Relative origin (and its set that way in our template), but why? Because some exporting methods can ONLY go to the relative origin. You go to export to some file formats, and it SAYS Project Internal, but thats not Project Base. Its the relative, which can mess people up.

So now there are three origins, 2 that are moveable, in two different ways. LOL. Factor in to account rotating True North (always do this BEFORE you specify coords), and people have a lot to get confused over. Ive seen people Rotate TN without realizing they need to pick a base point for rotation, lest they end up confused about why all of their coordinates no longer reconcile.

Once you get it, it makes sense... but IMVHO even though the visibility of the PBP and SP are great things, i think the ability to physically move them manually in the project is a travesty. Ive never seen anything good cme out of it, but ive seen a lot of bad.

Jon Frost said...

Hi David,

Great post, I've seen coordinate systems messed up countless times in Revit...
I'd be interested to know why you haven't used the acquire coordinates tool to align the survey coordinate system with WCS in AutoCAD. I've used this method successfully many times (Provided the AutoCAD file is set up correctly)but i'm sure there must be some pitfalls i'm missing

DanD said...

Hi David
I will have to agree with Jon about using acquired coordinates from landinspectors / surveyours 3D ACAD files. I have set up many projects through the years and have also had my fair share of unexpected out comes and Revit coordinate system nightmares. I have also used the method you have prescribed with success.

But if you need to deal with 3D land survey data. By far the best way I have found up to now is to:
1. Clean up the ACAD file for irrelevant data.
2. In a view set to level 0 & project north. Link the ACAD file Centre to Centre, usually meters (to avoid the 10 mile truncation issue).
3. Move the ACAD file from a known point (a building corner or relative datum or seting out point)to your Revit SP/PBP (which are both set at 0,0,0 in your template of course). Basically the point in the ACAD that you want to be your Revit 0,0,0 bottom left grid.
4. Rotate the ACAD around your 0,0,0 SP/PBP so that it aligns with your bottom left grid / desired project north
5. Unclip both SP & PBP
6. Now here comes the magic. Go to - Acquire coordinates. Click on the ACAD file.
7. If you now go to a "True north view" the ACAD will have rotated it´s self according to the real world coordinates. Eliminating the need to use rotate project north altogether (unless you need to adjust later due to the design). It will also have fed your PBP/SP with the correct land surveyor ACAD / WCS coordinates. Clip your SP/PBP. I usually pin the ACAD at this point as well.
This method leaves your SP/PBP static with your Revit relative internal 0,0,0. You can of course check this with a spot coordinate tag set to show relative (should show 0,0) and one set to show survey point or survey base point coordinate origin this should read the real coordinates.
I tend to do this in a separate terrain model file then link origin to origin with my building model. This of course enables the shared coordinate system to read the coordinates from the linked landscape file, so that we are able to work with and switch between relative or real coordinates and levels. Any ACAD files exported from Revit should also export correctly.
Hope this simple method helps someone who has been scratching their heads about this initial project start up procedure for some time, with not many definitive answers to be found out there on the web.

BIM Coordinator at Erik Møller Arkitekter Denmark

Darryl Store said...

To follow on from Aaron's comment: I don't ever bother rotating True North. I always teach users to have a separate site and building file. True North and Project North stay the same in the site file(unless you really need them to be different), link in the building file once your SC's are setup in the site model, move and rotate the building into position and publish coords to the linked building. Much better than having users mess up the SC's by rotating TN.

nlh said...

Thanks for the post! Clear and simple!
Do you use the same workflow when working with consultants (interior architects, for example) who use Archicad, when you're using IFC for file exchange?
There seems to be some issue with big coordinates and Archicad.

HMR said...

Thanks for these great tutorial. it was very useful. plain and simple.thanks.