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lidalps3d.fr

/!\ PROJECT STILL IN ALPHA PHASE /!\

Project goal

An open source reuse of IGN data to get a detailed 3D web visualization of the French Alps.

Contact me if you'd like to reuse my work, help out, or report a bug.

Glossary

  • WebMercatorQuad — standardized OGC tiling grid, the Web Mercator tiling scheme used both by the imagery tiles and by the terrain cells here.
  • WMTS — Web Map Tile Service, OGC standard for serving images split into tiles (this is how IGN imagery is served).
  • 3D Tiles — OGC standard for streaming large 3D scenes as tiles, with levels of detail.
  • glTF / .glb — standard 3D mesh format; .glb is its single binary file variant.
  • LiDAR HD — IGN's high-density aerial LiDAR survey program (the source point cloud).
  • RGE ALTI — IGN digital terrain model (regular elevation grid), here at 5 m resolution.
  • iTowns — 3D web rendering engine (based on three.js) used by the webapp.

Third parties

  • IGN — LiDAR HD, RGE ALTI, WMTS (orthophotos, IGN map)
  • Camptocamp — points of interest, topo guide, search
  • PoissonRecon — surface reconstruction from the point cloud
  • Inspiration for terrain generation / normals computation + base architecture of the C++ builder: OscarPilote/LidarTerrainMesh
  • OpenTOpoMap Additional map layer

Full dependency details: NOTICE.md.

Note

Claude is used for the implementation.

TODO

  • Better CI and tests
  • Update install scripts (project.toml / environment.yml are out of date.)
  • Enrich the database (LiDAR HD coverage)

How to build tiles?

Terrain is built with a small GUI:

python alpineview_builder/gui/main.py
  1. Draw a rectangle on the map ("Select rect" button) to choose the zone to build.
  2. Check the paths (builder/coarse executables, RGE ALTI folder, output folder) and the options (processes, force rebuild).
  3. Click "Build". The GUI chains: fine reconstruction (LiDAR HD), coarse reconstruction (RGE ALTI), then tileset assembly (ogc3d_tiler).

--> terrainPack.json gets updated along with the .glb files.

Build workflow

   LiDAR HD (.laz)              RGE ALTI 5 m (.asc)
         |                            |
         v                            v
   alpineview_builder          alpineview_coarse
   (Poisson Recon + cleanup / simplification / cropping)
         \                            /
          \                          /
           v                        v
              .glb tiles (position only)
                        |
                        v
              ogc3d_tiler/build_tileset.py
                        |
                        v
              a single file: tileset + subtrees
              (webapp/src/terrainPack.json)

Coordinate system. The whole pipeline works in a single frame: a Mercator projection centered on the Alps (metric, no distortion over the covered area), the same tiling scheme as the WebMercatorQuad grid used by the IGN imagery tiles.

Altitude. Tile Z stays in NGF69 (the raw altitude from the source files) end to end.

Tile naming. The terrain is first split into tiles of about 190km2 (Zoom 11 Pseudo-Mercator).

Inside, one subfolder per level of detail, then one file per tile:

The level of details are relative to the Pseudo-Mercator level 11.

public/pm/
└── 1024.700/            <- cell (x.y at CELL_LEVEL)
    ├── 0/0.0.glb
    └── 1/0.0.glb  1.0.glb  0.1.glb  1.1.glb

Poisson Recon and post-processing. For the LiDAR HD zone: point cloud → implicit surface reconstruction (PoissonRecon) → keep the main connected component → simplification ("Quadratic Error Metric simplification") → crop to the tile's exact boundaries.

RGE ALTI 5 m vs point cloud. Beyond a certain level of detail (Zoom 15), using the point cloud's precision is pointless, it's faster to use the RGE ALTI 5m data.

The 3D Tiles tileset. ogc3d_tiler/build_tileset.py

I more or less follow the standard: https://github.com/CesiumGS/3d-tiles/blob/main/specification/ImplicitTiling/README.adoc

With the difference that everything is written into a single .json file, committed directly to the repo.

Webapp workflow

  terrainPack.json
         |
         v
  3D Tiles tiles (.glb) loaded on the fly by iTowns based on camera placement
         |
         v
  Fetches the WMTS tile matching the zoom level
         |
         v
  UVs computed for each vertex
         |
         v
  Normals computed and a "skirt" added to avoid holes in the mesh.

Coordinate system. Pseudo Mercator, metric

Note UVs and normals are recomputed dynamically to minimize the size of requests to cloud storage.