The Cesium 3D engine seems to operate with Earth Centred Earth Fixed coordinate system (ECEF - related to the the WGS84 ellipsoid), as the 3d coordinate system that is used for rendering.
https://cesium.com/learn/cesiumjs/ref-doc/GeographicProjection.html?classFilter=projection
Thus Cesium seems to require that 3D tiles must be placed using WGS84.
But what if you wanted the rendering to happen in a coordinate system where for the 3D Tiles were placed on a plane and not around an ellipsoid like in ECEF, thus allowing a vertical axis
to represent height above or below sea-level There are many 3D datasets that are geo-referenced in for instance UTM. And there are certain features that would be easier to implement if you can assume “flat earth” projections
I do belive that the OGC 3D Tiles specification allows for coordinate refernce systems such as UTM but I don´t know of any 3D Tiles rendering engine that has implemented support for it? If someone else knows about one I would love to learn about it!
I would as you might imagine love for Cesium to add support for rendering in a UTM reference frame. Would that be something Cesium would consider implementing?
Best regards,
Jan-Erik Vinje