Revit Coordinate System and project site setup

[Anonymous]

Hello Everyone...

i think the subject speaks for itself and I think that many will come across this question and move on either because they are as confused as everyone else when it comes to this topic or because they know something about it but hey are in doubt (not 100% sure)

I have read ALL of Revit help on survey point, project base point and shared coordinates. I have read about the subject on more than 5 blogs some with several trials and attempts on the same topic back from 2012 till today. 

I have eave also read several posts on this forum which surprisingly although it is AUTODESK Revit forum yet this topic is the least discussed here and mostly no conclusion or a clear solution which applies to all is found.

i have watched on YouTube more than 20 tutorial videos...and all of them either say

1. don't do this and do only that
2. or... Forget about this feature because it has a bug and do this instead
3. or... Move and rotate and relocate points and model without clear clarification why is this point moved clipped and why that is moved not clipped. And funnily enough not two single videos do the same. One tells you rotate the model with base clipped and the other not clipped and in the comments they argue if survey point should have been un clipped first or not

Not to mention those with 100 theories on the 20 miles and some say 12 and others 15 and one claims he has been testing and he depicts the findings of another blogger and says it is more like 17 miles!! Why 17 and why 12 or why 20?! It can't be trial and error there has to be a math behind all that or is it not the case?

So here it is what I think:

1. it cannot be possible that there is so much contradiction and confusion on ONE topic which used to work so fine and easy in Autocad
2. it cannot be that there is no clear guideline or a demo which clarifies the whole process or different process/cases
3. it cannot be that no one really gets the differences and similarities if any between clipping and not clipping and what measure should be taken to troubleshoot the project if those points got knocked out by mistake! 
4. It cannot be that with a software as big as Revit someone jumps and says you have to do this in AutoCAD C3D then bring to Revit because Revit cannot do
5. it cannot be that there is someone who still says today that the elevation topopoints for the toposurface which are placed in Revit are not relevant to the survey point! I read this in a reply to a post here yesterday! I am not sure how AUTODESK admins monitoring this forum read all those contradicting replies and leave them to confuse others! I don't think users are seeking an answer which says use this tool but more of an answer which clarifies why and what this tool does

My question:

with all due respect to all bloggers and youtubers who has 10 times more experience than me; it is not personal...my aim is to understand which is right so please if anyone here knows how this works in Revit and how it is really used on a real project please explain

Sorry about the long narrative above, I am not complaining, I have only reached a dead end after checking wi several classmates and colleagues who turned out not far off than I am. 

I know now most of you experts have work and enough of everything else but I believe many of us out here would be so thankful to whoever can contribute to this post, on his/her free time or over the weekend.

thanks

[Anonymous]

"... I think that many will come across this question and move on either because they are as confused as everyone else when it comes to this topic..."

You're right, I almost moved on because your OP is very confusing (at least for me). Can you please clarify what it is you're looking for?

It sounds like you're trying to understand what the Survey Point and Project Base Point are, what they do, and how they work together. Is that correct?

[Anonymous]

Hello @Anonymous

Thank you for your answer and interest 

I did mention the nature of my question in the subject and at the end of my introduction I titled my query with the word  "Question"

i cannot not expect anyone to give a comprehensive reply if they don't understand where the problem is and where the question is coming from. If that would have been the case then the question would have been half a sentence and the replies maybe even shorter and more confusing

but if you insist and since you were such a gentlemen not to ignore my post although it was long; if you have any idea what the 20miles is, where from it is measured and how and with respect to what/which point...that would be great for a start 

I have been reading on this for the past 3 hours trying to find a clue... The only reliable statement I found so far is that AUTODESK says before it was 5mile now it 20miles not sure what's with the 8, 12, 17 figures all those blogs talk about!

[loboarch]

---

@Anonymous wrote:

but if you insist and since you were such a gentlemen not to ignore my post although it was long; if you have any idea what the 20miles is, where from it is measured and how and with respect to what/which point...that would be great for a start 

I have been reading on this for the past 3 hours trying to find a clue... The only reliable statement I found so far is that AUTODESK says before it was 5mile now it 20miles not sure what's with the 8, 12, 17 figures all those blogs talk about!

---

I am not the person who responded but I can take a shot at the 20 mile limit restriction and where it is measured from.

Revit accuracy and graphic display will begin to break down when geometry is drawn more than 20 miles from the "origin". Origin in this case is kind of a loaded term. The "origin" this is calculated from is the internal Revit origin point. This is not really user visible. The project base point in a from scratch file starts at this internal "origin", but if the project base point is moved "unclipped" it will move away from this origin. If  the project base point is moved clipped it will "drag" the internal origin with it. So basically you don't want to move the project base point un-clipped more than 20 miles from where it starts, or import a DWG with geometry more than 20 miles from the internal origin. If you want to make sure the project base point is located at the "origin" you can un-clip it, and right click to select "Move to Startup Location". That is documented here in the help.

http://help.autodesk.com/view/RVT/2017/ENU/?guid=GUID-AFCA59C6-9E00-4576-BCA0-63EB3342B68C 

This limitation used to be 5 miles but was changed to 20 miles a few releases ago. I am not sure if this number is "arbitrary" but it is related to the way Revit converts from "real" numbers to binary equivalents and back again. The 20 mile limit change was related to the way this is done internally.

[RDAOU]

@Anonymous

If it is not urgent I will try put something together over the weekend... discussion already started here http://forums.autodesk.com/t5/revit-architecture/site-massing-coodinate-system/td-p/6344270  so no harm in developing something more complete out of it

[RDAOU · Accepted solution]

@Marie.Joe

On the 20miles - As mentioned; it is in principle the limit after which graphical representation of elements & objects tends towards becoming less reliable/accurate. You might also want to note that the behavior of the graphical representation varies relative to how such elements are introduced into the Revit file…from personal experience

• It is less dramatic when the whole model is generated in the same Revit document,
• It is more dramatic when the geometries exist in a linked or imported model (example a CAD file…etc) and even more dramatic when the linked file contains a set of geometries/element with extents > than 20 miles (the latter depends more on the size of the project and/or how it was drafted in the other software)

Why this behavior; it is because the world is round and Revit is flat like a platform and the extents of this platform end at 20 miles. It might be easier to understand this if one sees the world of Revit from the perspective people saw the world (flat) before they discovered it was round.

The Coordinates Systems and how that relates to the 20 miles limitation:

In a Revit model there are two coordinate systems ie: two origins.

•  The first system is FIXED and is referred to as the World Coordinates System. Its origin is the 0,0,0. The Site Survey Point is a Benchmark/Reference Point on this coordinates System and is the origin of the shared coordinates.
  1. When the Site Survey Point is clipped, it is attached to the WCS. When it is moved, it moved the position of the whole WCS along with it.
  2. When it is not clipped, it is detached from the WCS and it move solely.

• The second system is variable and it can be moved/relocated and rotated within the space of the WCS (similar to a named UCS in AutoCAD). This coordinate system is always orthogonal to the Revit Plate and its Y-axis relates to the Project North. Its origin is the always fixed at the center of the 20 miles radius Revit Plate and this origin is referred to as the Startup Origin (SEE SCREENCAST HERE). This whole coordinate system is variable => Coordinate System + Plate move together. The Project Base Point is a floating Ref. Point on this system.
  1. When the PBP is clipped, it is attached to this coordinate system. ie: when moved it moves this whole coordinate system including the plate & stratup origin along with it
  2. When PBP is not clipped, it is detached from this coordinates system and moves independently.

In a NEW  project started from a Revite default template, both origins mentioned above (including Site Survey Point & Project Base Point) are located at the same exact point with coordinates 0,0,0. That is the Origin of the WCS. The project can be located more than 300 miles away from WCS origin but it should not be located/relocated (all together) more than 20 miles away from the center of the plate. Therefore, when when first setting out the project (in particularly when it is done manually – not using the Relocate Project Tool), one should the move/relocate Project Base Point clipped.

• When the Relocate Project tool is used, Revit ignores the status of the clip and moves the plate along with the Project Base Point anyway. Regardless how the project is relocated, one should always make sure that the Project Base Point is clipped/re-clipped when not working with it.

The Math behind the above:

1. Accuracy is at its best when the orthogonal projection of a geometry from the spherical Globe (real world) onto the Revit Plate (virtual drafting canvas) is carried out closest to the center of the plate (where it touches the sphere). The principle is similar to that when reading the world’s map unfolded on a sheet of paper; the continents closer to the edges are not drawn to their actual dimension relative to thosee continents that fall at the center of the map. (those at the edge always appear smaller although in reallity they are bigger)
2. The inaccuracies and variations are relative to the way/method which the different platforms/software use in order to compute the projections from the Sphere onto the Platform and such computational methods are dependent on the way each software/platform handles Integers (ie: number representations - signed vs unsigned) and the type of float (single vs double or long double precision)…The same apply to the computation Revit performs when it prompts the user when a line is not 100% straight (ex: 0.0001 degrees off axis) or because it is too short. The same line in another software might have not really made a difference... The size of float is typically 32 bits, and that of double is 64 bits. For greater precision, most scientific and engineering computation should be performed using the double data type; however, not all do. (Revit does)

Of course there are mathematical equation one can go through if one is interested to figure it all out down to the smallest detail but the easy interpretation of the above is as such:

Consider Revit’s 20 miles Radius Plate as a circular stretched Canvas and the project as the painting. When one paints a portrait on a canvas, one may not always the start at the exact center; HOWEVER, when one doesn’t, one always needs make sure that the whole painting composition ends up on the Canvas....Same applies to Modeling in Revit.

1. The Site (a painting) should all fit on the Revit Plate … an extreme example: if one is working on a Master Plan for a site 10 x 10miles, this 10 x 10 miles site should fit on the Revit Plate.
2. Subsequently, the Project’s Base Point should be located within the boundaries of that site (ex: if the project will be built at an Address X in London it doesn't make sense to have any benchmark/reference point set at a Street Y in Beijing...)
3. To maintain optimum precision, both points 1&2 above must be satisfied. However; taking into consideration the scale/extents of the project (ex: 10x10miles), if for any reason the user decides or needs to UNCLIP AND RELOCATE the Project Base Point away from the center of the Revit Plate (which is normal to happen at a certain point during the the project), ONE DOESN'T really need to go for the extreme and re-place/locate the PBP 18miles away from the center of the Revit Plate (Startup Origin) because then, part of the above mentioned 10x10miles plot example would fall off/outside the 20 miles precision limits.

In 90% of the cases, one doesn’t need to move the PBP unclipped more than +/- 1 mile away from the Startup Origin (on large scale projects) and a few 100 feet (on regular projects)

For clipping and unclipping; I usually keep in mind that moving the project base point unclipped too often as well as excessively clipping and unclipping it with every move; can get the project to diverge away from the Revit plate/Start Up origin as well as makes one lose track of the Startup origin’s location with respect to the WCS (unless the Startup point has been marked and one is keeping track of its original location). Hence; I usually stick to the following rule

1. not move the Project Base Point Unclipped (too often and never when large coordinates are involved)
2. not move the Survey Point Clipped (at all if possible)

Once Project Base Point and Survey Point are set…they should remain clipped and intact. If further points need to be defined, Spot Coordinates are used. And there are 3 types of Spot Coordinates each reading to a different point/origin

1. To Survey Point (default): It reads coordinates with respect to the WCS (not to the Site Survey Point)
2. Relative: It reads coordinates with respect to the Startup Origin (center of the Revit Plate) – can be used to track the Startup Origin
3. To Project Base Points: It reads coordinates with respect to the Project Base Point (What the builders use on site)

Another important thing to note is that when working with large scale projects and sites with multiple buildings, Shared Coordinates come into play. The site would then be modeled in the Master File and the buildings each in a separate file which then would be linked into the site model where it is set-out/positioned and oriented. Once set, the coordinates can then be published from the master file to the links and the Shared Coordinates would be set and saved for each building site. Each of these buildings should have it’s own Project Base Point in its own Revit file. And that is rarely moved further than the envelope of the building. What gets updated is the location of the Site Survey Point. That is how each of the linked buildings identifies its location on the project site (it’s PBP with respect to the Site Survey Point).

...to be continued

[RDAOU · Accepted solution]

The following graphic rep. of the workflow I use for setting up the site’s master file (In Revit) …probably if I find more time I will do the same for shared coordinates and acquiring site coordinates from AutoCAD or AutoCAD Civil 3D… not that much different, same principle basically but with 3-4 steps more on preparing the file in CAD/DWG before importing, acquiring coordinates and publishing/republishing to all links in order to have all in line with the Master Revit File.

[Anonymous]

Hi Marie, first off, the all important link... http://au.autodesk.com/au-online/classes-on-demand/class-catalog/2013/revit-for-architects/ab1412#ch...

This link will take you to a video by David Baldacchino from a presentation he did at Autodesk University.  It is appropriately named  "Navigating through the storm, using coordinate systems in Revit". I have blogged about this video before and in my opinion, it is a must see video for anyone trying to understand the Revit coordinate system. I give it to all my students. 

There are a few misconceptions with the coordinate system, some of which have already been highlighted by previous posts. I personally find these misconceptions have a lot to do with poor naming convention in Revit. The software is riddled with examples. (I have complained about this before). For example, many users think that a survey point level, is the distance from the "survey point" in Revit. It actually isn't. The height comes from the shared coordinate system origin (aka Internal Origin, aka Startup location).  I would have a guess that this is what the user was talking about regarding topography in the post you read. (That is another example, Autodesk have a right click menu that moves the project base point back to the "startup location". Then they have new repositioning options referring to it as the "internal origin" and linking refers to it as just "Origin").

The image above highlights some of what I am referring too. It is all explained in the presentation by David and there is a much better and more comprehensive image in his handouts showing the relationship of levels to points. At the link, you will also find a handout PDF as well as a ZIP file of DWG's and RVT files for you to play with, including the families he uses in the presentation.  

I think you may be finding so many different solutions and answers because there isn't necessarily ONE right answer. Revit isn't so black & white. 

There are many processes and use cases as you have seen and they are not necessarily wrong. With the addition of the new ability in Revit 2016 & 2017 to "reposition" links to project base points & internal  coordinates, these options are likely to only increase as every project is different in size,needs and complexity. The various options are great if you know what you are doing to solve various problems, but for those learning, it can be extremely confusing.

[Anonymous]

Another link. You have likely come across Steve Stafford's blog Revit OpEd in your research, but if your looking for a consolidated list of all his posts discussing shared coordinates he has this summary list which was recently updated a few weeks ago... http://revitoped.blogspot.com.au/2012/05/shared-coordinate-post-summary.html 

hope it helps. 

[Anonymous]

@loboarch Thank you for the reply and the link. I did have it bofore because I usually search Autodesk help before anything 🙂 then I go searching blogs if the help files did not clear out the confusion 100% ... But it was good reading it again together with the other replies I got from @RDAOU and @Anonymous

the arbitrary part is what was confusing me and was mostly what I was asking about in my original question so that I can understand when to do something and when not to do it based on a true understanding how it works and not following the contradicting recommendations of "do this and don't do that" 🙂 it's more clear now but I still have to read through the replies here again.

[Anonymous]

@RDAOU Thank you for the lengthy and detailed explanation. I surely appreciate the time you spared to prepare this reply.

i love the representation of sphere vs plate! That made lots of sense. And many thanks for the clarification of the 20 miles restriction and how it is determined by Revit. I have to admit it's the first time I read this although I have read so many posts on blogs, tubes and AU and none made reference to how the projection from sphere to plate, float, precision and computation...the flattened world map also added some sense to it. So far I have only read on other blogs that there are two Cartesian Systems or grids.

lots of thing makes more sense now especially with the canvas and painting example and the clipping and not clipping survey point and base point tips. I have to go through what you wrote again and try it out again in Revit before I can say that I can now close this chapter and move on to the next.

And thanks for the images too 🙂 first I was finding it hard to keep up till I saw them ... Thank you

[Anonymous]

@Anonymous Many thanks for your reply. I have seen that AU session before more than once. He makes sense at certain points but at others not really. I am 100% sure that he is a master of his trade but I was lost more than a handful of times..

Example: 

1. he once refer to the shared coordinates system as fixed then he says that the project base point is fixed and the shared coordinates system moves and rotates...I see that only the 0,0,0 is fixed and the WCS! If one switch to true north view it works exactly opposite to what he was explaining in his presentation! The grid on the shared coordinates system would be orthogonal and the grid attached to the project base point would move and rotate...so on that point he made no sense at all
2. on the clipping and not clipping ... He kept clipping and un-clipping till the base point was far from the start up and he had to un-clip the base point and restore it to start up. That is what I want to avoid. I don't know! And many times he takes back what he says to the extent that I lost track of what he is saying and what the conclusion was!
3. also when I get to statement like that one he made on plan view vs section and that we do one thing and Revit does the opposite! Hehehe I am not sure what to say on that! 
4. And when it came to the most interesting part I was waiting for on the reasoning behind the 20 miles limitation! His clarification was "for some Odd reason Revit behaves like that"... Well that helped a lot! 🙂 thanks to @loboarch and @RDAOU clarification above I don't have to settle for an "Odd" clarification...

I have read lots of Steve Stafford's blog posts (better than the above AU session for sure) and he has lots of super helpful things and interesting post. But on this particular topic I think he should archive all the old ones (I read them all btw) and just make one new up to date post which sums up all his findings. Following the progress of the experimentation is slightly confusing especially with the changing things in every new version of Revit.

Again many thanks for your reply I really really appreciate it...and believe me it did help me see things I have missed out on in my previous readings

[Anonymous]

Hi Marie, glad you found it helpful. The presentation is long and there is a lot to absorb, so I don’t think you should try and learn it all from the presentation video. If you haven’t already, you should read the handout instead, which is obviously going to be clearer than someone trying to deliver this material to a lecture theatre full of people. David’s presentation is more about helping people to see the relationship of these points to each other, which can be misunderstood.  

So with that in mind, I won’t make comment about any minor fumbling’s he has during the presentation, but I will clarify item 4, where you have misquoted him and the context of it. He doesn’t attempt to explain in his video why there is the limit or where it has come from, only that it exists.

What he is actually demonstrating is that when you create a circle around the origin with a radius of 52800 ft. (10miles) Revit throws a warning saying “Please enter a value less than 30000ft”. BUT, if you enter 5280 ft. and scale the circle by a factor of 10x, (resulting in a radius of 52800ft) Revit lets you do it. Go figure.

It is in this context prior to demonstrating the above he says “For some odd reason, Revit does not allow us to type in any dimension larger than 30000 feet”.

He then goes on to demonstrate unclipping the project base point and demonstrating this limit, but most importantly, you are not able to nudge the unclipped Project base point over this limit, which is expected behaviour, BUT if you use the arrows that show up when you click the project base point, it lets you move it over the limit! Not expected behaviour! Something Autodesk may need to address as I tested 2016 and it still does this.

For the metric readers (including myself) the converted limits of the above paragraph are 32.18km diameter or as per Revit warnings; 16km radius. In a metric template, it won’t let you type in the millimetre equivalent of 16090000mm. It gives an error saying it must be less than 9144000mm. (Which is equivalent to 30,000ft, so same error). So if you want the actual limit, enter 1609000mm and then scale the circle up 10x.Then you can try the above of moving the base point.

In regards to item no.3, see if the handout helps and maybe do some testing. I thought David’s video demonstrates it well when he has the plan and section on screen, but his handout may be clearer for you.

What he is demonstrating is (quoted from his handout):
"Moving a CLIPPED survey point = moving the shared coordinate system by the same amount in the same direction.
Moving a CLIPPED project base point = moving the shared coordinate system by the same amount in the OPPOSITE direction". People mistakenly think they are moving a clipped base point when they actually aren’t.

So it is similar regarding the behaviour you mentioned about True north, (from David's handout):  "Changing the angle to true north to a positive number clockwise results in the shared coordinates system rotating around the project base point in an anti-clockwise direction. This is how Revit allows us to align our building in a convenient way, where Project North is always assumed as being at the top of the drawing area under the ribbon UI (Project Internal Coordinate System), and True North can be at any angle relative to Project North (Shared Coordinate System)"

This hopefully explains a little, why you are seeing the opposite as you mentioned in item 1. 

[RDAOU · Accepted solution]

She is somewhat right…

There are some essential misinterpretation and misconceptions of scientific terms, conventions and terminologies in both the Presentation and its attachments. He starts off ok on the introduction and general idea (namely page 4) but then whatever he states on page 3 and then on 5 in my opinion is either not properly worded or whoever wrote it or copied it from has no knowledge of neither International Conventions of coordinate systems nor Physics, Geometry and Kinematics.

If you or who ever visits this posts have watched and read what came therein and is are at ease with how all this functions in Revit, you/he/she do not need to read on further. One can stick to what one know because the rest from this point on is just for the General Knowledge of those who were confused by the contradicting information here and there.

@Anonymous if you are a Student - EU, private message me your name and university and I will send you a recommendation to access the University's Electronic Library. Guaranteed you will find reliable sources and published researchs on this as well as as other AEC topics.

Remark: The rest is neither arguable nor a material for discussion unless the reply is paired with scientific references from an accredited source.

However; first things first though… It is NOT TRUE that Revit DOES NOT PROMPT the user with the 20 miles LIMITATION ERROR when one drags the project point unclipped outside the 20 miles boundary manually (ie: moving the geometry away from the Startup Origin). IT DOES… Moreover; as accurate as Revit is, it proves consistency with the above behavior when using the “Relocate Project tool which ignores the status of the Project Base Point and forces the relocation of both Project Base Point + Startup Origin together EVEN IF THE PROJECT BASE POINT IS NOT CLIPPED…only to preserve the Startup Point in the same position with respect to the Project Base Point (ie: within the 20miles circle)

On the referenced AU Presentation.

The first mistake is on page 3…One thing one thing there makes sense and is where it mentions that any point in space is defined by 3 Cartesian Coordinates X,Y,Z on fixed axes that meet at an origin. The wrong and inaccurate parts of the statement is the reference made to what he calls Revit’s Euclidean Space and the 0,0,0 origin.

• The accurate statement would be the “any point in the Euclidean Space encompassing the 3D Space of any Modeled Geometry is defined by 3 coordinates X,Y & Z”. In Revit Environment there can be more than One Euclidean Space namely when one starts talking about Shared Sites linked into a Master File. Each of these sites is an independently defined Euclidean Space encompassing the modeled geometry of that instance and each has its own Origin (NOT necessarily at 0,0,0). I would assume that the most probably presenter meant to say something else but the words didn’t serve him well.

More mistakes on the part related to the Terminologies…If one is to criticize Autodesk on some naming conventions and if a spot coordinate origin should be to Survey Point or something else; one shouldn’t throw terminologies with no bases to build upon. (Misconceptions)

1. The comparative equivalence between SYSTEMS and Coordinates (whether internal, user defined or shared) is wrong! Coordinates are defined points in a System. A System doesn't really require a definition its a 3D Space and it's name speaks for itself.
2. There is no such thing as Internal Coordinates System that may be referred to as an equivalent to the WCS which is a global reference as coordinate system. There is only the World Coordinate System and it is the same in both AutoCAD and Revit and every single other AEC platform.
3. Autodesk did not reinvent the wheel in Revit and Revit does not have a coordinate system of its own to be called Internal Coordinate System. Revit is a 3D environment and in it there is the Global System relevant to the World and the Secondary System(s) relevant to the Geometry/Model(s). EXACTLY the same principle as in AutoCAD and other software.
4. Revit only introduced two visible reference points which did not exist in AutoCAD; the Site Survey point (by default attached/clipped to the Origin of the WCS) and the Project Base Point which by default clipped/attached to the Origin (also known as the Startup Origin) of the Secondary Coordinate System relevant to the model(s). In the Default Revit Template both systems are at point 0,0,0 of the Primary Coordinate System (the WCS)
5. UCS and Named UCS in Row 3 of the table above are the same, “User defined secondary Coordinate Systems”. There is no unknown category for named UCS’s which he place in equivalence with Sites. Sites are defined Locations/Positions in the 3D space of the WCS which may be associated with Secondary Coordinate Systems.
6. Revit does not greet the user with a blank canvas. The X axes and Y axes are there on all view. User can access them on the Site View where they are visible or on any other view by switching them on in the Visibility Graphics. Selecting either of those reference point will reveal the orientation of the Axes X/Y/Z.
7. The statement “The Center of the Relocated Project Base Point will mark the Origin (equivalent to the WCS Origin)” is a wrong statement…The Startup Origin is not equivalent to the WCS Origin. They only coincide at point 0,0,0 in a new/blank Revit template folder until the user defines the project location (Use Relocate Project) then the startup Origin is move to a new point with X1,Y1,Z1 with respect to the 0,0,0 Origin of the WCS.
8. The statement “The Center of the Relocated Survey Point will mark the Origin (equivalent to the UCS Origin)” is a wrong statement. The WCS is the common System which defines the shared Coordinates/Positioning and not the UCS!!! He defined them the opposite way round.
9. All AEC platform follow the same concept. A Primary Fixed CS which the Global Coordinate System and secondary User defined Coordinate System(s) relevant/aligned to/with the Geometry/Model. If any system may be called SHARED it is the common System between all platforms as well as between all Models – The Global Coordinate System (WCS).

In every science which is based on mathematical computations (physics, geometry, kinematics as well as in programming of related AEC software), coordinate systems are used to describe the (linear) positioning of points as well as to describe the angular position of axes, planes, and solid geometries. In the case of geometries and 3D projections (in Revit as well as other platforms), the orientation of a SECOND coordinate system (ie: the Coordinate system relevant to the Model in Revit or ACAD’s User Coordinate System or UCS) at a defined node/origin (the Startup Origin in Revit), IS DEFINED based on and with reference to the Global Coordinate System which is widely/commonly known as the "World" Coordinate System. ie: it’s positioning with respect to the ALL TIME FIXED WCS.

The positioning & orientation of a Geometry/Model (ie: a Project North in Revit) is represented by an orientation matrix, which includes the Cartesian coordinates of three points defined with respect to the WCS on an ordered triplet of lines (axes) that are perpendicular to each other pair-wise and intersect at a defined position. These points are used to define the orientation of the axes of the local/secondary system in each and every model (the X1, Y1 & Z1) and they represent the tips of the 3 unit vectors aligned with those axes. <--- This system is the Variable System and not the SITE which rotates below the Project (as the presenter called it)

• It (the Secondary/Local System) is what Rotates (not the SITE as emphasized throughout the presentation using the term “Rotate the Site under the Project”). The site, in essence is a set of fixed geographical location ON EARTH which does not change or Rotate and it is defined with respect to the World Coordinates System. Using the term Rotate the Site under the Project is wrong. The only thing which can get the Site to rotate under the project is the Earth Rotation and then the Project Rotates with it because they are and should be clipped together. Hence; the bases of the argument in the presentation is wrong. Many do that maybe but wrong is wrong even if everyone does it. The other concept of the drafting paper which was mentioned would have been a better concept to build upon considering the site is fixed and the model is on the drafting paper which may be rotated/oriented and positioned on the site.

• Reiteration: It (the Internal/Local System) is not the Shared Coordinate System because scientifically speaking there is no such thing/term as “Shared Coordinate System”. There is a common Primary Coordinate System and then there are Shared Coordinates/Positioning which determines the positioning and orientation of the Secondary/Local Coordinate Systems (or UCS’s) of the various models with respect to the common Primary Coordinate System, and this common system is the Global Coordinate System (WCS) which is the only system in common (or what one may call SHARED) by all relevant models within the Revit Environment as well as all other Architecture, Engineering and Construction platforms (namely when having multiple buildings/geometries on one site or when importing from one platform to the other – for that same reason, prior any impost of site surveys from CAD to Revit all named UCS’s are to be deleted and only the WCS is to be preserved)

The part related to the ERROR AND INACCURACIES occurs when Revit translates the coordinates from all those Cartesian Systems to the Real World Coordinate System – also known as the Geographical Coordinate System. The Geographic Coordinate System (the system no one mentions) is the coordinate system that enables every location on the Earth to be specified by a set of numbers, letters and/or symbols, usually the latitude, longitude and elevation. The transformation/translation (done by Revit, other software and/or manually) is based on map/mapped projections which is basically the projection of the positioning coordinates (latitude/longitude) from the sphere onto horizontal plane and the Elevations (in case of 3D onto a Vertical Plane) and viseversa…That is where the used float type and numbers representation, based on which the code/program translates Real Numbers into binary, play the major role or are the reason behind the odd or unexpected behavior WHEN LARGE COORDINATES are used. Those inaccuracies start becoming significant once the coordinates approach the 20miles limit from the origin of the Secondary Coordinate System (Not the WCS 0,0,0 )

While there are different methods and types of mapped projections; Revit relies particularly on Orthographic Projections onto a Horizontal plane (ie: Axes X1,Y1 – That of the Secondary Coordinate System / UCS) where accuracy is at its best when the origin of the Horizontal Plane of the Model (or Euclidean Plane of that Model) is at the Zenith (the imaginary point directly "above" a particular location, on the imaginary global sphere (Geographical Coordinate System); where the Z1 axis of that Plane is in the opposite direction of the gravitational force pointing to the center of the sphere. That is the 100% accuracy point/origin (at the Startup origin in Revit). The further the model moves from that origin the more the projection gets distorted ie: the higher the inaccuracy. The limit for acceptable distortion using double float is 20 miles from that Startup origin (the Zenith) … If Autodesk would develop Revit to use “Double Long” float (ie: highest known) instead of Double float; probably that limit would increase by some 20, 30 or 40 miles but that wouldn’t eliminate the inaccuracies but just set a new limit. Would be probably useful for town and city planning but not 90% of the projects.

Now watching that presentation as well as several others on AU...the question is; does Autodesk Audit the material on Autodesk University!!! or does it still go by popularity voting?!!

[Anonymous]

Wow, great write up RDAOU. I am shocked its not a perfect document. You the man. 

I am a designer, not a scientist, so beating a presentation to an inch of its life with science and mathmatics is great, I will just take your word for it because I don't have any "scientific references from an accredited source". 

I think I will stick with the tracing paper idea though. My simple mind.  

p.s. screencast below in Revit 2017, still no warning message. You shouldn't be able to do this in my opinion, even if it is unlikely to be moved this way. 

[RDAOU · Accepted solution]

@damo3 / @Anonymous (too)

I am not using 2017 so I cannot really tell if they removed/missed out on (I DOUBT that it was and I believe the warning/prompt is still there) ... This has been there since the very early versions of Revit although the limit was recently increased to 20 miles with double float (Previously - in the old versions (pre-2013) it was 5.6 miles if I am not mistaken).

What you are observing/demonstrating on your screen is actually the result of all those misconceptions in videos/presentations/blogs found online. And it is not related to the 20 miles limitation.

What is happening in your screencast is the following:

• The 20 miles/32km diameter relates to the limits of the vertical projection on the Model's horizontal plane... Meaning; ideally if the center of the 20mile/32km is at the Startup Origin (100% accuracy case), any geometry or element can be anywhere within 10miles away from that origin as long as the EXTENTS of such geometry or Element are completely circumscribed by the 20miles radius. (this includes the Project Base Point which is deemed as an element too)

• This limit relates to the extents of the Model's Horizontal Plane (Vertical Projection) where any given geometry which is to be translated accurately onto the Geographical Coordinate System. (AND NOT RELATED TO THE DIMENSIONS OF THOSE GEOMETRIES) caps used for emphasis

The limit for Dimensions/Measurements Input (reference to the circle you placed with 30,000 ft / 9144m max allowable radius/distance input)

This max. input is governed/based on International Standards/Conventions for systems of measurement and Units conversion. It is ABSOLUTELY NOT related to the 20 miles limitation …This standard is based on multiples of 3 ft (US Imperial units) / 0.9144 (metric units) / 1 yd (British Imperial) and that has really nothing to do with the 20 miles diameter limitation which relates to the accuracy of equating/computing projections from the spherical surface onto the projection plane and the precision of converting Real Numbers used in those projections/computations into binary.

• Hence, the prompt only appears when
  1. the Project Base Point is moved/dragged more than 10miles away from the Origin (outside the 20 miles dia. circle) and NOT outside the 30,000 ft / 9144 m Radius’ed Circle which you have drawn. (ie: When you unclipped and moved Project Base Point, it was still within the 20 miles.ie: still within the limit)

When it comes to dimensioning/measurements, the limit in Revit is set to 30,000 ft (5.68 miles) / 9144 m / 10,000 yd (input) depending on which template which one uses (US Imp, Metric, British Imperial). Revit limits the dimension only when it is entered into the annotation because in such case user is asking Revit to compute it. That is where Revit prompts the user saying it will not carry out the computation for values that exceed the set Standard. (A Perfect EXPECTED BEHAVIOR when a software is coded according to International Standards) => prompt stating that input should not exceed 30,000 ft or 9144 m.

• HOWEVER, when it comes to doing it manually by click start point then click end point, Revit allows the user to do that (ie: Revit allows it respecting the User’s wish not to abide by the standard)

• One Standard which I can remember and which relate to the above are
  1. the ASTM Metric Practice Guide, US Dept of Commerce, National Bureau of Standards, Handbook 102.
  2. USCS - United States Customary System (which also has its equivalent standard world wide - ie: EN, BS, DIN which are equivalent to the ASTm in other parts of the world)

What is worth noting, is that this convention is not only applied in and/or not only a characteristic of Revit. It is used worldwide and in all fields of Science (Physics, Mathematics, Kinematics…etc), Fabrication, Sports, Aviation, Space ...etc  even Beauty Contest lol  examples:

• Human Scale and Proportion (reference to Da Vinci's Vitruvian Man is based on the correlations of ideal human proportions with geometry described by the ancient Roman architect Vitruvius - Distance is equivalent to 3ft to 2x3 ft Human arms's reach, stride, height) <- same increment! is it a Coincidence?
• 30 ft Agility Tests (point to point distance)
• Archery (Darts shooting) - Distance Archer to Target 30 ft / 9.144m
• Aviation (length tip to tip of most Airplanes is a multiple of 3 ft / 0.9144m)
• Aviation (Ideal cruising altitude for long dist. flights is 30,000 ft / 9144 m)
• Beauty Pagents perfect measure Bust 91.44, Waist 66.04, Hip 91.44 lol don't ask me why - people think thats the as hot as a woman can be
• Most interesting example of all is Edge of Space (Theoretically – although it is still arguable – is at roughly plus or minus 300,000ft / 91,440 m above Sea Level

     =>Can you imagine if Revit increase the measurement limit? logically speaking it would be an increment of x10

     =>from 9144m/30,000ft to 91,440m/300,000ft

     =>Projects would literally be in Outer Space

　

The reason behind this confusion or misconception is (I think) based on the fact that a Circle (which is a very particular case) was used for the demonstration of what is described by many as an ODD/UNEXPECTED behavior and when a circle or arc is used one unconsciously associate it with Diameter or Radius; therefore, the confusion with the 20mile diameter limit measured from the origin (startup origin) of the horizontal plate in Revit (the Modeling Platform with Axes X1/Y1) … If one would use a LINE to demonstrate the dimension input limitation; it would be easier to realize that the two limits 20 miles dia. vs the 5.68 miles (30,000 / 9144m) point to point distance input are not related.

　

With all due respect to everyone but it is sad that Autodesk host speakers who calls all the above ODD and UNEXPECTED...and it is not Autodesk's fault that such info is not in Revit's help documents because the above is not really Revit related...Engineers should either use as is or be more informed before making assumptions and before questioning Revit's Integrity.

[Anonymous]

I'm not sure how to express my gratitude! I didn't  think anyone can break it down to the tiniest detail and actually get it to work just as described!

Thank you!

Where and how does one get all that info from!

[Keith_Wilkinson]

Blimey, that's quite a set of answers...  did you get up to much else this weekend?  

Set up properly though the whole thing can be really straightforward but it helps if you can get the project team to sit down and agree how things are going to be done at the outset rather than letting everyone run off and do their own thing.

[Anonymous]

The problem is not sitting with the team and talking the problem is that not two people give the same explanation specially when one asks why? It's been always do this then this then that and not otherwise but why did that happen? The answer is either "a bug" or "for some odd reason" not just in the team but also on all those blogs and surprisingly also on AU!

I am pretty sure there are lots of people out there who are looking for someone to explain the 1+1 and the ABC of it so they don't be just robots...and maybe when something goes wrong and one needs to backtrack then one can do it unhesitatingly.

thanks to this forum though there are many nice people who are willing to share their valuable experience 🙂 as you can see from the above it turned out there are no odd reasons or bugs! and Revit as it is on this subject, all perfect and good

i actually did print those replies and gave also to some classmates and colleagues who were surprised and thankful too ... I hope RDAOU doesn't mind

[Keith_Wilkinson]

My point is that if you set your models up correctly you generally don't need to worry about the 20 mile restriction or anything else.

The way we do it is thus : 

We have a model for the building - this is just built driectly in the template file roughly centred between the elevation markers.

This model is issued to structures and services who link it into their template origin to origin and build their models accordingly.  

All these models link correctly when loaded origin to origin.

Next we have a 'site' model.  Into this we link the site survey (usually CAD) using centre to centre.  We then specify the known world coordinates at a station point.  typically these surveys will be orientated to true North.

Now we link our building model into the site model, locate it correctly on the site and save it's position.  

the site model is issued to structures and services.  They open this an use 'reload from' to load in their model replacing our own - they save the location of their model.

Now all the models have the same origin and aslo the same shared global location.  

Job done.

Agree this approach at the outset or something similar and your life will be much easier.  No pissing about with autocad drawings miles from the project origin and unless you are buidling the Extra Large Hardon Collider the 20 mile raduis limit will never be an issue.