Required Data for IFC - data-conversion

Question

I'm working on a project where I need to generate an IFC file, and am given not much more information than geometry (I have access to the density and heat-conductivity of materials, and basic labeling for Objects).
So far I could only find what IFC can store, never what IFC needs to store.
What do I need to include in an IFC file so it is properly functional?
What does an IFC file need besides basic geometry?

Answer 1

Disclaimer: I have not read (or bought) the standard. My knowledge primarily stems from working with IFC files and trying different things. And reading the buildingSMART documentation. So I can't give you a hard guarantee, but I am rather confident my information is correct/usable.
As an alternative to buying the official standards file, you could look into the official documentation by buildingsmart. (Also have a look here for more general information and availability of other/more modern releases).
Now assuming you are familiar with the basic STEP file layout (header and data segment), let's jump to what an IFC file absolutely has to include to be considered correct (as far as I understand the documentation; there might be parsers/loaders which can load incorrect/incomplete files, but we aren't aiming for them). Also note I am building this example for IFC 4.0. This should be correct for the current IFC 4.1 standard, but probably not for the older IFC2X3 standard (there have been some relaxations in IFC4 from IFC2X3). Also I am skipping on names and descriptions - you can set those fields for testing to recognize your structures in a viewer (it's easier than comparing GUIDs).
IfcProject
The root of all elements is the IfcProject. It also contains most basic properties and definitions for all other elements. The attributes required per documentation on this entity are only the unique id. But for a working example you usually also need a minimal unit assignment and representation context.
#20= IFCPROJECT('344O7vICcwH8qAEnwJDjSU',$,$,$,$,$,$,(#19),#13);
In the unit assignment you define required units, starting from geometric units to monetary, thermal, etc. The minimum is length, area and angle to meaningfully define geometric items. So for our example we include only those: metre as length, square meter as area and radians as angle. If you need foot or inch or degree you can define those as derived units.
#10= IFCSIUNIT(*,.LENGTHUNIT.,$,.METRE.);
#11= IFCSIUNIT(*,.AREAUNIT.,$,.SQUARE_METRE.);
#12= IFCSIUNIT(*,.PLANEANGLEUNIT.,$,.RADIAN.);
#13= IFCUNITASSIGNMENT((#10,#11,#12));
The representation context defines for a given class of representations (=geometric/parametric descriptions) the basic coordinate system. So the simple case would be a 3-dimensional right handed system at point zero. IFC is working with the z-axis pointing up - this might be important if your are working with models/files originating from 3D/OpenGl applications which usually assume the y-axis pointing upwards. You also need a precision value - I am using 1.0e-5 here, but you might want to test out if you can go with less or need more. The precision is usually applied when comparing points/edges when combining geometry (during constructive solid geometry steps). If you have errors, try a different precision value.
The second attribute of the representation context is the context type. This is a string identifying on which representations this context should be applied. The documentation states that values are based on "implementers agreement" - which means AFAIK "look what the others are using". From my experience using "Model" works for 3D geometry. Using "Plan" for 2D plans and sketches should work, too.
#14= IFCDIRECTION((1.,0.,0.));
#15= IFCDIRECTION((0.,0.,1.));
#16= IFCCARTESIANPOINT((0.,0.,0.));
#17= IFCAXIS2PLACEMENT3D(#16,#15,#14);
#18= IFCDIRECTION((0.,1.));
#19= IFCGEOMETRICREPRESENTATIONCONTEXT($,'Model',3,1.0E-5,#17,#18);
Spatial container for elements
Elements can't be added to the IfcProject directly - they need to be placed into a spatial element which is contained in the project. There are three possible choices: IfcSite, IfcBuilding and IfcSpatialZone (see section Spatial Decomposition on the IfcProject page). The IfcSpatialZone is defined as non-hierarchical spatial element - its usage is slightly different from the other two (elements are added using a different relation).
A single site is sufficient as spatial container. Adding all elements to it might be sematically vague (mostly fences are directly added to it, other elements are usually inside a building) but not incorrect. (IFC does not care if you have electrical appliances in your garden). As nearly all attributes of IfcSite are optional we can skip on those. But beware: if you give your site a representation (=some geometric shape) you will need to include a placement for it. The site will be aggregated into the project to be related to it.
#30= IFCSITE('20FpTZCqJy2vhVJYtjuIce',$,$,$,$,$,$,$,.ELEMENT.,$,$,$,$,$);
#31= IFCRELAGGREGATES('0Du7$nzQXCktKlPUTLFSAT',$,$,$,#20,(#30));
Elements
Actually that is all that is needed as absolute minimum structure. Now you can add your elements - entities of some type derived from IfcProduct. As all those elements have some sort of meaning attached to it you either need to select those closely matching the objects you have, or you might want to use IfcBuildingElementProxy which is the most "meaningless" (or better: no specialized semantic meaning) object type. The following code places one proxy without geometry. The placement references the same coordinate system definition that is used to create the coordinate system out of convenience as it doesn't transform or move anything. Your geometry would be added through a product definition shape which has shape aspects and finally some geometry items. The building smart documentation has a few examples with assigned geometry.
#40= IFCLOCALPLACEMENT($,#17);
#41= IFCBUILDINGELEMENTPROXY('3W29Drc$H6CxK3FGIxjJNl',$,$,$,$,#40,$,$,.NOTDEFINED.);
#42= IFCRELCONTAINEDINSPATIALSTRUCTURE('04ldtj6cp2dME6CiP80Bzh',#12,$,$,(#41),#30);
Conclusion
So there isn't much needed as bare minimum to add elements:
a project
basic unit definitions
one spatial container
The complete example file would be:
ISO-10303-21;
HEADER;FILE_DESCRIPTION(('IFC4'),'2;1');
FILE_NAME('example.ifc','2018-08-8',(''),(''),'','','');
FILE_SCHEMA(('IFC4'));
ENDSEC;
DATA;
#10= IFCSIUNIT(*,.LENGTHUNIT.,$,.METRE.);
#11= IFCSIUNIT(*,.AREAUNIT.,$,.SQUARE_METRE.);
#12= IFCSIUNIT(*,.PLANEANGLEUNIT.,$,.RADIAN.);
#13= IFCUNITASSIGNMENT((#10,#11,#12));
#14= IFCDIRECTION((1.,0.,0.));
#15= IFCDIRECTION((0.,0.,1.));
#16= IFCCARTESIANPOINT((0.,0.,0.));
#17= IFCAXIS2PLACEMENT3D(#16,#15,#14);
#18= IFCDIRECTION((0.,1.));
#19= IFCGEOMETRICREPRESENTATIONCONTEXT($,'Model',3,1.0E-5,#17,#18);
#20= IFCPROJECT('344O7vICcwH8qAEnwJDjSU',$,$,$,$,$,$,(#19),#13);
#30= IFCSITE('20FpTZCqJy2vhVJYtjuIce',$,$,$,$,$,$,$,.ELEMENT.,$,$,$,$,$);
#31= IFCRELAGGREGATES('0Du7$nzQXCktKlPUTLFSAT',$,$,$,#20,(#30));
#40= IFCLOCALPLACEMENT($,#17);
#41= IFCBUILDINGELEMENTPROXY('3W29Drc$H6CxK3FGIxjJNl',$,$,$,$,#40,$,$,.NOTDEFINED.);
#42= IFCRELCONTAINEDINSPATIALSTRUCTURE('04ldtj6cp2dME6CiP80Bzh',$,$,$,(#41),#30);
ENDSEC;
END-ISO-10303-21;
Note that loading this one up doesn't show anything, because it doesn't contain any geometry. Also please note that I have not yet verified if it is error free - I currently don't have my IFC tools at hand (if you would like to verify your files have a look at stepcode which can check if your files are syntactically correct - it won't check semantic meaning or enforcement of the mentioned concepts in the building smart documentation.)
Also good to know is that the order of references/ids (like #20) can be freely arranged - you can reference elements that you add later in the file and the references only need to be unique to this one file. This means the lines of the example file can be shuffled and it is still a valid file - parsers usually use a two-step apporach to create an in-memory representation (1. parse into IFC classes, 2. resolve references).