What coordinate reference system is used?

I’m looking at some existing point cloud 3D Tile assets like Melbourne and Montreal and I’m just trying to figure out how to tell what coordinate reference system they use? The Ion docs say if I import LAS that is georeferenced it will be converted to EPSG:4326 so I’d expect lat/lon type values. But for example with the Montreal point cloud I see:

  • the root has a boundingValue that is a box that has:
    - center: -4.19877, 5.7869, 385.0733
    - x axis: 20535.3929, 0, 0
    - y axis: 0, 27448.6577, 0
    - z axis: 0, 0, 409.5056
  • root also has a transform that is:
    - column 0: .9599, .2801, 0, 0
    - column 1: -.2000, .6852, .7002, 0
    - column 2: .1960, -.6722, .7138, 0
    - column 3: 1253502.8130, -4294682.7306, 4530092.1469, 1

So with those large numbers on the bounding box axis and column 3 of the transform being the transposition part, I’m seeing very large values for X/Y/Z points that doesn’t seem like lat/lon. So what coordinate system is it and how am I supposed to know what CRS a given 3D Tiles file is using?

I think I’ve managed to answer this myself. Looks like the coordinates are using WGS84 but in ECEF XYZ rather than lat/lon/height.

Hi @bpepers, thanks for coming back and reporting your solution—that’s always greatly appreciated by the community. The 3D Tiles specification is a great resource that I would recommend you check out. The section relevant to your question is here: 3d-tiles/README.md at main · CesiumGS/3d-tiles · GitHub

Feel free to add any follow-up questions or create new threads for new topics. Welcome to the forum!

Thanks for the welcome! I have read that section of the document, and the whole document, at least a dozen times so I’m very familiar with how points work but I don’t think anything in there actually tells you what CRS those points use. This link says that uploaded LAS will be re-projected to EPSG:4326 so I know it’ll be WGS84 but the part I was missing is that it is also using ECEF coordinates rather than latitude, longitude, and elevation.