South Africa’s definition of its coordinate system makes our lives difficult when it comes to setting up BIM projects. The reason for this is because the coordinate system is left-hand rectilinear instead of right hand rectilinear.
Anybody with a rudimentary understanding of applied mathematics will understand how the algorithms within the authoring platforms are unable to interpret a left-hand rectilinear coordinate system. The purpose of this video is not to confuse the audience with the intricacies of our workarounds within South Africa but rather just give step-by-step instructions on setting up a coordinate system for a project at the very beginning.
In my experience, not setting up the coordinate system in a logical way at the beginning of the project is what necessitates a lot of re-work later.
While I do have a site I can use in Cape Town for illustrative purposes, it is very close to sea level, and I want to show you how the coordinate system works when we are above sea level. For that reason, I’m going to randomly choose some other coordinates up in the Highveld. I do not have much information available to me except what is within Infraworks and so that is the data that we will use even though the topography is very inaccurate. We choose the South African coordinate system for Infraworks and generate a project.
Next, we will set up the project in civil 3D with the same coordinate system and then transform the coordinate system through 180° and rotate the model through 180° and live with the fact that South now points north and North Point South. This transformation and rotation allow us to interact with Revit according to a commonly accepted protocol where we work our coordinates in the negative direction both in the Easting and the Northing. All that is required of us is simply to link the civil 3D model in the center to center and then to acquire the coordinates from the Civil 3D drawing.
After we bring the Infraworks model into our civil 3D model (without the transformation) we rotate the Infraworks model elements by 180°.
The false Northing of South Africa’s coordinates that places the origin on the Equator about 4000 km away from where we are imposes a significant constraint on Revit interpreting points files. Unless you want to normalize points around some other coordinate closer to the site in Revit, I strongly recommend that you use civil 3D to export the topography or to generate the topography from CAD links. Unfortunately, when you export the topography, you will need to un-transform the coordinate system first (we do not need to rotate the model back again). We can then link this topography into Revit by shared coordinates.
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