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Thread: Revit Export to CAD produce wrong Coordinates then what Revit has - Workaround???

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    Junior Member phxarch's Avatar
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    Revit Export to CAD produce wrong Coordinates then what Revit has - Workaround???

    Dear friends, I am having an issue with exporting Revit to CAD (per client instructions), where the CAD exports produce a different Coordinate system then what Revit holds. For our project, it is imperative that both of these coordinates match, and we can't afford to move the coordinates in the CAD exports manually, due to a high volume of exports for every issuance (100+ sheets). Has anybody written a CAD script that allows for a quick modification of CAD exports' coordinates to correct Revit Coordinates? Thank you and I appreciate any advice you might have,

    Rafael (phxarch)

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    Forum Co-Founder Twiceroadsfool's Avatar
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    You can export to CAD using either Project Internal (Relative Origin, not Project Base) or Shared Coordinates.

    When you say its *different,* you need to provide more information about whats *wrong.*

    1. What coordinates are you expecting to see in CAD?
    2. In Revit, are those coordinates the Shared Coordinates, Project Coordinates, or Relative Coordinates?
    3. Are you trying to use State Plane Coordinates for Surveying?

    I ask question 3, because Revit will typically to CAD in Inches, and the export needs to be in feet. But exporting to feet trips up the coordinates (for me), so i export mine to Inches and then convert the CAD file to feet and allow it to scale the mspace objects. Which is something that has to be done in each CAD file, but it can be automated i imagine.

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    Junior Member phxarch's Avatar
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    Revit Export to CAD produce wrong Coordinates then what Revit has - Workaround???-revit_n-e-coordinates.jpgRevit Export to CAD produce wrong Coordinates then what Revit has - Workaround???-cad_n-e-coordinates.jpg

    Aaron,

    I have attached two images, one for the Revit file, and the other from the Revit-to-Cad export, which bring up different coordinates. As per your questions, I am referring to the 'North'ing & East'ing' coordinates, which need to match. The only Revit models that are giving us (our firm) the headache, are the ones that are linked through 'Shared Coordinates'. The models that are linked through 'Origin-to-Origin', export with correct coordinates. Also, we're working in Metric, primarily in meters.

    Rafael

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    Forum Co-Founder Twiceroadsfool's Avatar
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    Im sorry, those images and that paragraph dont provide any more insight.

    1. You are exporting each model individually to CAD, and some models are correct and some models are not? Is that correct?

    2. Please screen capture the CAD Export Dialogue from the models that are correct. And from the models that are not. Specifically, the Units and Coordinates tab, for both models: Those that work correct, and those that dont.

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    Member HansLammerts's Avatar
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    You cannot export sheets with shared coordinats. The options should accually be grayed for sheets, but they forgot to program it that 'clever'. Only single views and 3d models can be exported aligned to the shared coordinats you defined in Revit.

    My workaround:
    -Export and use as much as possible DWG as 3D model, objects better, coordinats are better.
    -Sheets : try to avoid them, prefer pdf.
    - if DWG is required to deliver, always make sure at least 2 point are marked with xyz
    Reciever should do the aligning of and cleaning up of DWG.
    I just noticed some dutch developer is working on a DWG manager for Revit.. That would be the day..

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    Mr. Revit OpEd
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    When I've chased this situation down in the past it has always come down to the accuracy of how the coordinates were defined to begin with. Specifically how the coordinates were establish in Revit. Since the error you show begins at the first decimal place that's pretty close and makes me suspicious about how the point was defined to begin with. I have observed over the years:

    When Acquire Coordinates (AC) was used, and an error was observed, the coordinate distance from origin was huge and I've come to the conclusion that AC has some threshold for acquiring and converting accurately. When that happens there is usually a much larger discrepancy so I'd be inclined to doubt it in your case.

    If Specify Coordinates at Point (SCaP) was used then the discrepancy was the result of picking a reference in a CAD file or picking the endpoint of a line segment sketched after snapping to CAD data. In either situation I've found the accuracy of that point to be in error.

    In some cases we determined that the element in the DWG was actually at the value Revit reported but the units in the DWG were disguising it. In other cases there were multiple segments/elements in the same location or very nearly which made it possible to snap and select a slightly different location than was intended. For example in one project, a block (datum symbol) in the DWG was referenced for this origin. It was a triangle and it was assumed the triangle was equilateral so the point for Survey Point was taken from the center of this triangle. Closer examination proved the block wasn't exactly equilateral...really really close...but not. The error was established at the outset but not observed until much later when the team was told the site situation changed a little and they observed a discrepancy, prompting them to look closely at it again.

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    Junior Member phxarch's Avatar
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    All,

    Thank you for your insight. I want to provide some clarification:

    Our client has asked that all Revit models (ours and consultants) use a specific North'ing/East'ing coordinates, which specifies the center of the project. This point in Revit was created through 'Specify Coordinates at Point', in our main Central file. From this file, we're exporting Revit sheets to Cad. From our office (architect of record), we're working with 12 or so Revit models. Some of these models are linked through 'Origin-to-Origin', while others are linked through 'Shared Coordinates' system. Per the client's request and BIM Execution Plan, we owe our client Cad sheet exports of ALL Revit sheets that we issue, at every milestone. The Cad sheets exports that we produce are supposed to have the same North'ing/East'ing coordinates, as the Revit models. While all the Revit models retain the correct coordinates, as we export sheets to Cad, the cad sheets exports from the Revit models that are linked through "Shared Coordinates" system, do not retain the same North'ing/East'ing coordinates, so we have to manually re-work those coordinates, which is a nightmare due to the high volume of sheets produced. My question is whether or not there is a work-around or a CAD script that would allow us to speed up our process. (Aaron, I hope that clarifies our issue.)

    Hans & Steve, thank you for your inputs, much appreciated.

    Rafael

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    Forum Co-Founder Twiceroadsfool's Avatar
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    Im not misunderstanding what you are trying to do, at all. Either your coordinates are screwed up, or your export process is screwed up. Plane and simple.

    Typing Northing and Easting in bold many times isnt changing anything, either. But if you dont want to post the screenshots i asked about earlier, good luck i guess?

    But what Hans said is also true: If you need to export SHEETS, you CANT use Shared Coordinates. Sheets Export with the views at Project Internal (when its a single plan on a sheet). Nothing you can do about that, aside from a TON of moving stuff in AutoCAD, and then having to move the viewport as well.

    if they just want the BACKGROUNDS in Shared Coordinates, thats a different story.

    I dont know why anyone in your office would have agreed to that in the BEP at all, since its never been possible to do without a crapload of rework.

  9. #9
    Mr. Revit OpEd
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    That the BIM execution plan requires this means it is fanciful and even impossible, even if you were using AutoCAD too. Unless only one model space viewport is allowed per viewport in paper space (sheet). The logic is only remotely relevant to plan or RCP oriented views too. Elevations, sections or details can't possibly be oriented to the same origin in a meaningful way. To claim otherwise demonstrates a lack of understanding, and especially of what data exported from Revit to DWG format is for and what is a reasonable expectation.

    Exporting to DWG from Revit is fundamentally intended to provide background files for other trades to use as an external reference for their own work. Exporting to DWG for sheets and their views does not provide a viable result for ongoing work or use. It is a direct response to "I need your information in DWG format". The reasons for this request often, "We need to print later" or "We (the owner) has AutoCAD and we want a record of the building design in a digital format we can take advantage of during our use of the building". For the printing response PDF wins hands down. Regardless, it is only ever a translation from one to another. There are always translation issues no matter the software or language.

    To reiterate what Hans wrote, exporting to DWG from sheets isn't viable and even impossible if a sheet contains more than one viewport. Revit can't put the model space elements (each exported view) in the same place relative to this singular origin and then show each unique collection of view elements in different viewports in paper space. Consider that the only way that would be possible working in AutoCAD, forget Revit, is to use completely different layer settings for each viewport's information and create saved layer states to manage when/if they are visible.

    Revit could do that, meaning it isn't beyond the ken of the developers to work out a strategy for it. Personally, I doubt it will ever come to pass. Also ignored by Revit is that AutoCAD users usually create base files that are externally referenced extensively and viewed differently via paper space. Revit does not do that at all. Thus the expectation that Revit models exported to DWG from Sheet views creates an illusion of real usefulness, which is dashed pretty quickly once someone actually digs into those files.

    Exporting via Shared Coordinates is for model to model relationships (3D or 2D regardless) and that includes model space elements to other models but not paper space arrangements of model space revealed elements.
    Last edited by Steve_Stafford; September 28th, 2017 at 11:22 PM.
    Dave Jones and Nurlan like this.

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    Member HansLammerts's Avatar
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    In my field of work it is usual that pricipicals (governamental) ask for DWG that need to be in coordinates. Nothing new, been the there as long as DWG format itself. Off course not every section or detail, but the 'civil layout' definitely needs to be. This will not change and is a big hurdle for Revit. Principals are often not aware that BIM software can't deliver what they where used to. The problem comes up way too late in many cases, in the AS BUILD stage where you need to deliver over 100+ dwg, .. Expected in coordinates.. Contractors have been battled this more than once. Most are aware now, so they are keen on this BEP rule to get paid for the extra AutoCAD services provided.

    Edit : Btw I know other software that can be it. Just the irony os that better DWG does not come from Autodesk, sorry to say
    Last edited by HansLammerts; September 29th, 2017 at 04:54 AM.

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