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WorldCoordinateSystem, IfcMapConversion and IfcSite

I have trouble interpreting how values in WorldCoordinateSystem will move&rotate objects in IFC when combined with IfcMapConversion.
To visualise, I made a diagram showing the relations.
Assuming my interpretation of WorldCoordinateSystem is correct, should RefLatitude/RefLongitude point to (A) or © in the attached picture?? See the yellow comment.
.

https://thinkmoult.com/ifc-coordinate-reference-systems-and-revit.html

@Moult made a good explanation. There is some redundancy between having a mapping and also storing the Lat/Long mostly for backward compatibility.

in addition to the great explanations by @Moult, and to come back to your actual questions - the answer should be “C”. The RefLatitude and RefLongitute shall be used (as documented for IfcSite)

If asserted, the Longitude , Latitude and Elevation establish the point in WGS84 where the point 0.,0.,0. of the LocalPlacement of IfcSite is situated.

This is however redundant to applying the local placement of the site to the geographic reference point as established by IfcMapConversion. The attributes RefLatitude, RefLongitute and RefElevation are mainly there due to backward compatibility with earlier versions of the IFC Schema. When using IfcMapConversion, those values should not be used for geo referencing.

Also take a look at this post:

Yes, and this is the reason for my post.
“0.,0.,0. of the LocalPlacement of IfcSite”: Does this mean point B) or C) in the diagram where IfcSite (C ) is located at [196.0, 387.0, 0.0]?

#8=IFCSITE('<guid>',#2,'Site',$,$,#34,$,$,.ELEMENT.,(58,44,40,04562),(5,38,57,37631),31.,'',$);
#34=IFCLOCALPLACEMENT($,#48)
#48=IFCAXIS2PLACEMENT3D(#55,#56,#57);
#55=IFCCARTESIANPOINT((196.0,387.0,0.));
#56=IFCDIRECTION((0.,0.,1.));
#57=IFCDIRECTION((1.,0.,0.));

(@Moult has reviewed my diagram earlier this year, but did not comment on this possible error)

good point, the sentence of “0.,0.,0. of the LocalPlacement of IfcSite” can indeed be misinterpreted. But what is meant is:

0., 0. ,0. (the origin) within the object coordination system of IfcSite, that is established by IfcLocalPlacement with an offest/rotation relative to the project coordinate system.

That would still be C

The spec should better read

If asserted, the Longitude , Latitude and Elevation establish the point in WGS84 where the origin of the object coordinate system of IfcSite is situated.

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Dear gentlemen,
The industry needs a proper guideline for this puzzle so everyone knows that the answer is “C”. The best would be to see this as an ISO Standard, but until this happens an official bsI guideline would do just fine. The reason is that we will see more and more issues concerning the coordination of models. So the guideline will solve many open issues that we usually have at many kick off meetings in BIM projects.

Isn’t @Moult working with someone in this direction?

Thanks for the explanation @TLiebich!

I absolutely agree that it is confusing. I am sorry to say that I am not working with anyone on this currently.

However, it is on my to-do list to build a little utility which will do things like:

  1. Report coordinates
  2. Replace coordinates
  3. Convert coordinates

It will be a free and open-source tool, which hopefully we can all use to verify and correct model coordinate behaviours once we have all tested it and double and triple checked the behaviour :slight_smile:

I believe a little tool, plus perhaps a bit of rewording in the IFC spec docs, can help resolve this over time. It’s good to see IFC mature!

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Dion, there is one initiative from the dutch academic university that could be of interest.

In civil design we have many challenges (on projects) for aligning ifc files to national grid or even project grids. Tools that make things easier have high demands!

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Thanks Hans,
It sounds interesting, especially when I see “Cesium”

Dion, When i use Blender i get this error messages. I tried 2.80 but it said “needs updating” This message i get when 2.78 is installed. I want to test your tools with some of my CAD models/geometry from Inventor and DWG. / step. Geometry made to IFC in various ways with various aspects like CRS / Color and ‘Block definitions’ Could you help me with this?

Ehsan, i really think our (… hate the word…) [ AEC design industry ] is getting way to much ‘layers of software’ to use. If its not yearly versions of your favorite ‘BIM authoring tools’ its all the addons and support that will maken to much dependancies for IFC and other open formats.

Get it to work and without a doubt …you will know next year it will be different and probably will not work anymore.

Hey @Hans_Lammerts - it looks as though you have got Blender 2.7* installed. It requires Blender 2.8*.

It would be good if you could join the IRC chat - I will be able to help you live: https://blenderbim.org/community.html

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Thanks! Sorry to post it here as sidenote. We will be in touch!