IFC coordinate system

IFC coordinate system and georeferencing of its elements in IFC format.  Practical examples with Edificius, Revit, Archicad, Tekla, EdiLus

The proper georeferencing of BIM models plays a critical role whenever you need to identify a set of coordinates or the coordinate reference system of an IFC file, part of a typical BIM collaboration workflow. So if you need to be sure that you are defining the parameters of a project correctly in order to export them in the IFC standard format, then you’ve found the right page!

Today’s topic is the IFC ‘coordinate reference system’. In fact, I’ll show some practical examples of how to export these values from the most common BIM authoring architectural (Revit, Archicad, Edificius) and structural (Tekla and EdiLus) software or even read them with the aid of aBIM viewer online.

Then I’ll also explain what IFC coordinate reference system means, and how the hierarchy for spatial placement of entities within an IFC file is structured.

Identifying the coordinate reference system with usBIM

How to identify the project coordinates in an IFC file

If you receive an IFC file, then you probably need to find the coordinates of the reference system. In this case, you’re going to need a IFC viewer that enables you to view and read the IFC properties, including those related to geographic coordinates (IFC coordinate reference system).

Believe it or not but it’s so simple. You just need to follow these 4 steps:

• log in to usBIM with your ACCA account;
• upload the IFC file to the usBIM cloud;
• open the file by clicking on the right-hand side menu;
• select the terrain entity (IfcSite);
• read the geographic coordinate values (latitude, longitude, elevation) directly in the properties toolbox.

Also remember that usBIM.viewer+ is certified by buildingSMART International and is already used by thousands of users around the world.

IFC coordinate reference systems – Edificius

At this point, we can now see how to export the IFC project and define the IFC coordinate reference system with Edificius, the BIM authoring software for architectural design integrated with a MEP modeller.

First of all, you can identify the project origin, in its local reference coordinate sytem (0;0;0). Now this is quite a simple procedure. Just set a reference node at (0;0;0): this now represents your project’s ZERO origin point in space.

To set the relative coordinates of an object in relation to the local system of coordinates, we can proceed as follows:

• select the object;
• edit the XYZ values in the properties toolbox.

This positions the object in space according to the values assigned to the cartesian coordinates.

To set the project site’s geographical coordinates (this means we want to define theIfcSitecoordinates):

• access General Data in the toolbox on the left;
• set the geographical coordinate values (latitude, longitude, altitude), inserting them manually or selecting them from the locations archive that you can see available in the dialog box.

At this point, all you need to do is export the project in the IFC format. The file will maintain all of the information and data assigned to the objects, including the coordinates that characterize the site position.

To export the file in the IFC format:

• click on the File icon on the toolbar at the top left hand side of the interface;
• click on the Exports option;
  
• select the IFC Format;
• define the IFC Export Settings;
  
• click OK to confirm and save in the desired folder path.

Edificius interface | IFC coordinate system settings

IFC coordinate reference systems – Revit

To set the user reference system in revit, follow these next few steps:

• select Manage from the menu
• click Coordinates on the ribbonbar
• now select Specify coordinates at point
• insert the coordinates.

To specify the IfcSite absolute coordinates:

• select Manage from the menu
• click on Location
• insert the geographical coordinates in the pop-up dialog or select the location from the archive.

Revit interface| Dialog Location

IFC coordinate reference systems – Archicad

To set the user reference system in Archicad:

• select the Move user origin command present on the toolbar
• click on the desired point.

To specify the IfcSite absolute coordinates:

• click Option from the menu toolbar
• select Project preference
• click on Project location
• insert the coordinates in the pop-up dialog.

Archicad interface| Dialog of the Project location

IFC coordinate reference systems – EdiLus

To set the local reference system coordinates for a point in EdiLus:

• select the object;
• edit the XYZ values in the properties toolbox.

Confirming, position the object in space in relation to the assigned cartesian coordinate values based on the EdiLus reference system.

To set the geographical coordinates and therefore georeference the project site:

• access General Data in the toolbox on the left;
• set the geographical coordinate values (latitude, longitude, altitude), that you can see available in the dialog box.

At this point all you need to do is export the project in IFC format. The file will maintain all objects related information, including the site coordinates and position.

IFC coordinate reference systems – Tekla

To set the base point coordinates in Tekla, proceed as described below:

• click on File
• select the Project properties
• click on Base Point
• insert the coordinated values in the dialog.

Tekla interface| Base point dialog

Verifying the IFC coordinate system

To verify the correctness of the project positioning information within the IFC (exported with a BIM authoring software – such as Edificius, Revit, Tekla, EdiLus, etc…) you can use an online BIM viewer.

Reference systems in IFC format

After having dealt with the practical part, I also wanted to explain some of the fundamental concepts that may be useful.

To manage the reference systems in the IFC format you can use:

• absolute coordinates, when referring to the global reference system;
• relative coordinates, which refer to the positioning of another element;
• constrained coordinates, if you need to refer to the axes of a defined grid.

Each element will be correctly positioned in space through appropriate hierarchies.

Conceptually, the geolocation of a project starts from theIfcSite, an entity used to build the spatial structure of a building.

TheIfcSite is defined through a system of absolute coordinates or a system of coordinates relating to another entity. Let’s assume we’re using the absolute coordinate system.

First, let’s determine the position of theIfcBuilding (building’s elevation) through a coordinate reference system with respect to the IfcSite.

Next, we can define the IfcBuildingStorey, that represents the levels of a building.

Each entity (IfcWall, IfcSlab, IfcBeam, etc.) is associated to the IfcBuildingStorey through specific relationships (IfcRelContained, InSpatialStructure).

Basically, given an entity, for example a column, we can in the first instance assign it to a certain level of the spatial structure, specifically the storey, that is in turn assigned to the site terrain. Lastly, the terrain is georeferenced through the coordinates of a point (that usually corresponds to the project origin point).

The illustration below summarises the concept of entities hierarchical placement.

Infographics that illustrates the hierarchy for spatial positioning of entities

IFC4 and reference systems

IfcCoordinateReferenceSystem is a new entity provided by IFC4 that serves to define a coordinate reference system only through qualified identifiers (see the specific section of the buildingSMART International website).

A coordinate reference system allows you to define the geographical position of a point on the earth’s surface and is related to the real world by a datum (that represents the way in which a geoid is approximated – i.e. the real earth’s surface – through the ellipsoid – a more or less regular solid). The datum is the combination of the ellipsoid and the orientation with respect to the geoid.

There are global and local datum: the so called WGS84 (provided by Google Maps and widely used in GPS systems) is based on global datum: the ellipsoid is positioned at the earth’s centre of gravity and can be used in all continents. Instead, the local datum, make the ellipsoid coincide with the real earth surface in a particular point, called “base point”.

Datum and reference systems: terrestrial, geoid and ellipsoid surfaces and their perpendiculars

Anybody interested in cartography and digital map making, who needs to know exact latitude and longitude values in different spatial reference systems, could refer to the EPSG (European Petroleum Survey Group), a structured dataset of coordinate reference systems with a wordlwide geographic coverage of data and coordinate transformations. EPSG has reached even greater importance after the growing use of GIS software.

EPSG online service interface

With regard to the IFC4, the reference system adopted for the geolocalization is stored in the Name attribute of the IfcCoordinateReferenceSystem.