DME to 3dtile file

hello everyone,
I have some DME terrain file,I want to change these file to 3dtiles.Follow GitHub - CesiumGS/3d-tiles: Specification for streaming massive heterogeneous 3D geospatial datasets ,but couldn’t found any structure description related to the terrain(for example, coordinate system, latitude、lontitude and height storage arrangement).Where can I find relevant information?

3D Tiles geometry (vertex positions) is stored in a Cartesian XYZ coordinate system, not longitude/latitude/height. The coordinate system of each tile may be whatever is convenient for the tool producing the tileset (and for precision, the origin should be near the center of the tile). However, after any transforms are applied, including transforms on the glTF nodes and transforms on the tiles in the tileset.json, the coordinates should almost always be expressed in Earth-Centered, Earth-Fixed. See this section of the spec:

Follow this spec,I seem to understand.
3D Tiles is also a collection of files.Take quantized-mesh as a comparison,the overall structure is the similar,has is an entrance and many terrain files.The difference is that each file of b3dm can use its own coordinate system and set the location of each b3dm terrain in tileset.json.Quantized-mesh is to set the display range of each level. UE will find the corresponding terrain file according to the range and load it.Is that right?

Yes, that’s right. The terminology we use is that quantized-mesh terrain uses implicit tiles, whereas in 3D Tiles the tiles are defined explicitly in the tileset.json. However, it’s also possible to use implicit tiling with 3D Tiles using this extension:

Implicit tiling is also supported in 3D Tiles v1.1 without an extension, but this is not supported by Cesium for Unreal just yet: