Trying to understand "Acquire Coordinates"

[dbaldacchino]

I'm trying to understand the full depth of the Shared Coordinates subject. The following has me baffled....

When you link a dwg file into Revit and position it somewhere underneath your building, we typically then use the "Acquire coordinates" command. This moves the shared coordinates origin and aligns the orientation to match the World Coordinate System of the dwg that was picked (True North matches the direction of the Y axis). So far so good.

When you use the "Relocate Project" tool (or "Rotate True North") BEFORE using the Acquire tool, both your model and the dwg "move & rotate around". Basically the relative distance between the two stays the same. When you use these tools AFTER acquiring coordinates, the dwg stays put and the distance between the building and the dwg changes depending on your input. This makes sense as now Revit sort of "pins" the dwg to the shared coordinates and moves the building around instead. Even if you move the dwg around and you relocate the project, the relative distance between the two changes. Again, I understand this.

But now let's say I get a new site dwg and I want to acquire coordinates from it. When I do that, Revit tells me that the shared coordinates have already been synchronized! How can that be if I just linked it in? Shouldn't the command just align the shared coordinate system to the WCS of the new dwg, just like it did in the first place? I would expect that even if I move the original dwg around after aquiring coordinates, that it would let me use the tool again, as now the WCS of that dwg would no longer be matching the origin & orientation of the Shared Coordinate System. Any thoughts?

[tomnewsom]

All this horrible 'slippiness' that comes with wrestling with the Shared Coordinates function has lead me to use a workflow that just keeps a constant 0,0 point in all my xrefs, and Project North oriented to the WCS of those XREFs. It does mean a bit of re-centering of AutoCAD drawings, but importing files Origin-to-Origin is far more reliable, in my experience.

[patricks]

Yeah but what if you get a DWG survey which has the origin more than a mile away from the actual survey drawing?

[tomnewsom]

Yeah but what if you get a DWG survey which has the origin more than a mile away from the actual survey drawing?
I re-centre the Autocad objects in the autocad file to match my 0,0

Yes, this has to be done on a per-original-file basis, but I find it saves time and hassle, overall.

[jyodak]

No one has asked me, but my opinion is that the whole thing is confusing, backwards and needs revisiting. Why is Project North fixed while True North is rotatable? Last time I checked, North (True or Magnetic) was pretty much fixed, generally in a northerly direction based on a very wide set of shared assumptions, maps going back thousands of years, and a host of devices from astrolabs to compasses of all ilks, to our current standard of GPS and satellite navigation systems. Project North, on the other hand, has long been something that has rotated to every point on the compass, from North to South, but has always had the top of the sheet as the reference point.

Perhaps this is just my misunderstanding of the principles involved, but rotating True North in Revit seems to change the model, something that should NEVER HAPPEN HERE! This makes North NOT North, IMHO.

Jan S Yoder
"Always Question Assumptions"

[Andre Carvalho]

I re-centre the Autocad objects in the autocad file to match my 0,0

Yes, this has to be done on a per-original-file basis, but I find it saves time and hassle, overall.

Or if you want to keep your original CAD file where it is, create a new ACAD file and X-Ref the original file that is located far away from the origin, into the new file, locating it at your 0,0. You can use that one to import into Revit. Changes on your original ACAD file will be reflected on the one you just created, located at 0,0 and in Revit too.

Andre Carvalho

[patricks]

No one has asked me, but my opinion is that the whole thing is confusing, backwards and needs revisiting. Why is Project North fixed while True North is rotatable? Last time I checked, North (True or Magnetic) was pretty much fixed, generally in a northerly direction based on a very wide set of shared assumptions, maps going back thousands of years, and a host of devices from astrolabs to compasses of all ilks, to our current standard of GPS and satellite navigation systems. Project North, on the other hand, has long been something that has rotated to every point on the compass, from North to South, but has always had the top of the sheet as the reference point.

Perhaps this is just my misunderstanding of the principles involved, but rotating True North in Revit seems to change the model, something that should NEVER HAPPEN HERE! This makes North NOT North, IMHO.

Jan S Yoder
"Always Question Assumptions"

It may seem to be backwards in the whole view of things, but it actually matches the typical workflow in most architecture offices pretty well.

I'm referring to the fact that many times a building is started in Revit BEFORE you have actual site survey data. You may have a general idea of where the building will be on a site, and how it will be oriented, but most of the time you want it oriented orthogonally to your sheets.

So you draw the model with north straight up, oriented orthogonal to the sheets. Then when you get your site data, you rotate true north away from project north (or you acquire coordinates from the survey DWG) to get the building oriented correctly on the site, and also to get your site plan oriented with true north as up on the site plan sheets.

I suppose the same tool could have been called Rotate Project North, but the fact is, most all plan sheets in a set are going to have Project North orthogonal to the sheet, and True North will only be orthogonal on a few sheets (the site/civil sheets). So if the tool were called Rotate Project North, that might be confusing that it would modify all the views where Project North is straight up on the sheet, and that's what you DON'T want.

I dunno, it all seems perfectly logical to me. Well except for that "already synchronized" error mentioned in the original post. I don't know what's up with that, and I've seen it myself as well.

*edit*
Actually, we're about to start a new project where we already have a current DWG site survey. But the process is still the same. You want to draw the building orthogonal (believe me, it will be MUCH easier), link in the DWG, and then acquire coordinates or rotate True North to make the site drawings match the survey, which has True North as up.

[benmay]

I believe I have a answer to this question:


But now let's say I get a new site dwg and I want to acquire coordinates from it. When I do that, Revit tells me that the shared coordinates have already been synchronized! How can that be if I just linked it in? Shouldn't the command just align the shared coordinate system to the WCS of the new dwg, just like it did in the first place? I would expect that even if I move the original dwg around after aquiring coordinates, that it would let me use the tool again, as now the WCS of that dwg would no longer be matching the origin & orientation of the Shared Coordinate System. Any thoughts?

There are few things you need to be careful of here,

The first is a big one; when you link the new site drawing are you linking centre to centre? If you link by shared co-ordinates this linked drawing is recorded as being already aligned to the shared co-ordinates

Next I always make sure if I am updating my site drawing that it is a fresh drawing, I am not 100% sure what Revit is doing to these drawings when I acquire co-ordinates, I have noticed that Revit wont let me save the project if I have the drawing open in AutoCAD, this makes me think Revit is saving information to the DWG file. This could possibly be causing problems if you re-use this drawing further down the track.

Finally if the new site drawing has no change in co-ordinates you will receive the message saying it is already synchronized because there is no change.

[patricks]

I believe you should always first link in the site DWG center-to-center, move and/or rotate the DWG as needed under the building to get it in the right place, and then use Acquire Coordinates.

By this method, you should not have to use Rotate True North, and if you change your site plan views to True North, you will find it has automatically picked up True North from the DWG and rotated the view so that True North is straight up.

[dbaldacchino]

Thanks all. I wasn't questioning the rationale of the way Plan North and True North work. I think they make perfect sense. We always try to make our Plan North as close as possible to True North. A view set to True North has North in a vertical direction (Y-Axis), which makes sense. Rotating True North isn't really changing the direction of true north....we're just defining how far off the two are from each other.

I think I found out what the problem in my case is. I was testing things out (working on some demonstration materials for a class) and created a site DWG. Then I opened the same DWG, moved some things around and did a Save As to a different file. When you link in file #1 and acquire coordinates, Revit seems to "tag" that file as already being acquired from. It seems to record this information in relation to some internal link ID/handle and will remember this as long as the link is not removed (whether a CAD file or another Revit linked file). Now when I link file #2 (which was made from #1), the identifier that Revit seems to be using to know whether the file has been already acquired from is the same, and thus the reason for the warning. Revit thinks they're the same file, even though they're really not, but the internal ID that Revit seems to be looking at is the same. You can try this for yourself and it is easily reproduceable.

If on the other hand you create file #2 from scratch and link it in, Revit will acquire coordinates without issues. To me, this is proof of my thinking above. You can keep going by creating file #3, #4, etc and you can acquire from each of these files. BUT, if you try re-acquiring a second time from each previously acquired links, it'll tell you that coordinates are already synchronized.

Now this behavior is still not right in my opinion. When you acquire coordinates, you're really matching the Shared Coordinate system in Revit to the WCS of the linked DWG. I tried rotating and moving around each unique DWG (to make sure my WCS's were all in unique locations in my Revit project) and acquired from each file. I confirmed where the origins were by adding a ref plane at the point where the origin of the WCS in my DWG was and used the Specify Coordinates at a point tool to confirm (did not change the value; just used it to read the info). So let's say I have 5 uniquely positioned DWG files in my project and #5 was the last I acquired from. Clearly, the Shared Coordinate system is syncronized with this file, but not the others. Yet if I try to re-acquire, Revit says that they're synchronized. So the question is....what does "Synchronized" really mean? To me, it should mean that the WCS of the DWG match the origin and orientation of my project's Shared Coordinate System. If a project is relocated with the Relocate tool and True North is rotated with the Rotate tool, then the coordinates shouldn't be assumed as being "synchronized" anymore with any of the links, unless for some bizarre reason they match up.

To prove this further, I removed #1 and then re-linked (doesn't matter if you do Origin to Origin or Center to Center). I moved the link around and rotated by a certain amount. So in the meantime, nothing has changed...I removed the link completely and re-linked file #1, so the Shared Coordinate system should still be the same (synched with #5). But now when I try to acquire coordinates from #1, it lets me do that just fine, which clearly demonstrates (to me at least) that Revit is just tagging a file as already having been acquired from, rather than really checking that the WCS of that file matches the Shared Coordinate system in my Revit project. For any one location, there can only be one Shared Cooridnate origin and orientation. Of course through the Manage Place and Locations dialog, you can have several in your project (equivalent to multiple UCS's in a DWG). So to me, Revit should be checking the actual Shared Coordinate system of my current location with the link I choose and if they match, issue that dialog, rather than looking at if the DWG was already "tagged" as having been acquired from and assume coordinates are "synchronized".

I hope this makes sense, but I'm trying to fully understand how Revit thinks and works :)

[benmay]

Exactly what I thought, there is something saved in the DWG file

Nice to know I am not just being overly paranoid.

[dbaldacchino]

Well, the UCS in the dwg is saved from Revit when you publish coordinates to the link and you save locations in the DWG from Revit. Unless you explicitly save those locations, nothing is written to the dwg. The "tagging" that I'm alluding to happens within Revit only. In fact when you remove a link and re-link, Revit doesn't remember that you acquired coordinates from this link anymore.

[Gadget Man]

That thread is actually very good! I've learned quite a lot from it :) Thank you all!

[Revitator]

I haven't understood the full picture yet, but I've done some testing:

When you acquire coordinates from a DWG and then Save, Revit creates a new UCS in the DWG (to keep track of the shared coordinates?). You can see the new UCS in AutoCAD: They seem to be called 'REVIT_60_...' and the name of the Location (usually 'DefaultLocation').

Using Acquire Coordinates changes the origin and orientation of the shared coordinate system: If you Acquire coordinates, nothing moves but all the coordinates change. When you next Save, Revit will want to update the saved locations in all the linked files (RVT and DWG). In contrast, Specify Coordinates at a Point just moves the current Revit file relative to the existing shared coordinate system.

[dbaldacchino]

Actually a UCS named "REVIT60-something" will be recognized as a Named Location in Revit called "something".

When you acquire coordinates from a DWG with no UCS that Revit can recognize, the WCS is used to locate the Shared Coordinate System in the host Revit project. If a recognized UCS exists, then Revit will use that instead. If no recognized UCS exists and you acquire coordinates, Revit would want to save a recognized UCS back to the DWG, but you don't have to since it's really the WCS. Yet when you go to Manage Links, you'll see that the checkbox for "Locations not Saved" is checked. If you save locations, by default the UCS in the DWg file will be named "REVIT60-DefaultLocation". You can change this name through Revit by selecting the link and go to the properties, click the button at the bottom (which typically would read "DefaultLocation") and go to the Named Place and Locations dialog to rename the location (which is just a UCS that Revit recognizes).

EDIT: Specify Coordinates at a point has the potential of moving the Shared Coordinate origin and Project North to True North orientation for the Current Named Place and Location. Any Shared links will appear to stay where they were, while the building appears to "move". Those links will be tagged as having Locations not Saved, as what really happened is that the Shared Coordinate System in the host file has moved. Unless you save the locations back to the links, next time you open the Revit projects, those links will ALSO move. When saving locations (publishing coordinates) back to the link, you're modifying the UCS in those links; you're modifying the relative distance of the geometry to the UCS. If you don't save that new UCS, the link will move as it maintains the original relationship to the Shared Coordinate Origin and Orientation, which could move if you specify coordinates at a point (if you change the values that the dialog pulls up).

[tomnewsom]

Now, I'm reading all this, and the words are making sense, but I just can't get my head round it all. Everything seems so relative! At least I know where 0,0,0 in the WCS is in any Autocad file I care to name. I like that sort of certainty :)

[justin.black]

I agree with Tom. The words all make sense to me but I just can't wrap my head around it. At least when I put it at 0,0 then I know where it is.

[dbaldacchino]

I'm working on a tutorial to give some visual cues to this "abstract" subject. It's not as abstract as we think by the way. It's that the terminology we're used to is different ,and we don't "see" the familiar coordinate system, origin, orientation, etc, but it's all there nonetheless!

The founders of Revit did a great job at thinking it through and I can tell they went beyond what the average person would think of. However, we're bound by a cultural constraint if you will, and we get confused by a different way of doing things because we're familiar with seeing the coordinates, etc. I think the interface/display could solve this issue but nothing has been done since this solution's inception to make that better. We've been given "rules" that all users should follow, such as those you'll find around these forums by Leonid Raiz and David Conant. They are great rules, but some of us just want to learn the whole inner workings ;) I think I finally have a really good grasp on the subject, but now I need to find a way to visually illustrate it, because that's really what's missing.

[tomnewsom]

That sounds great! I have to understand something in order to use it and hate following 'blind' instructions, so a visual guide would be very useful :)

[mruehr]

Now, I'm reading all this, and the words are making sense, but I just can't get my head round it all. Everything seems so relative! At least I know where 0,0,0 in the WCS is in any Autocad file I care to name. I like that sort of certainty :)

Not really if you change the base point your WCS only apears to be 0,0,0 in acad

[rrijswijk104343]

I'm working on a tutorial to give some visual cues to this "abstract" subject. It's not as abstract as we think by the way. It's that the terminology we're used to is different ,and we don't "see" the familiar coordinate system, origin, orientation, etc, but it's all there nonetheless!

Did you publish this tutorial somewhere?

Thanks anyway, Renzo

[dbaldacchino]

Not yet, but you can bet I'll post it here when I do get to finishing it up :)

[jkgolden64]

Is there any way to make insert "origin to origin" the default instead of "center to center". I believe this is a root cause of lots of problems. When you create the first model it is drawn "wherever". As soon as you start a second or subsequent model and insert the previous model, someone not familiar with the problems that can be caused will insert it "center to center" because it is the default.