WMS Float32 DEM Protocol

Most of this app's terrain sources are tile-native (XYZ Terrarium/Terrain-RGB, or a COG read directly in-browser — see Bring Your Own Data). Some elevation providers only expose a traditional WMS GetMap service, with no tiled/COG access at all — IGN France's LidarHD is the motivating example. float32dem:// (lib/float32dem-protocol.ts) is a custom MapLibre protocol that bridges that gap: it issues a WMS request per tile, decodes a real float32 elevation grid out of the response, and re-packs it as standard Terrarium raster-dem, so every other part of the app (hillshade, terrain-analysis derivatives, 3D terrain) can treat it exactly like any other elevation source.

A standalone demo of this protocol, decoupled from the rest of the app, is deployed at terrain-viewer.iconem.com/maplibre-raster-dem-wms-float32-generic.html (source: public/maplibre-raster-dem-wms-float32-generic.html) — useful for testing a new WMS endpoint in isolation before wiring it into the main app.

1. The WMS request

A GetMap request asking for image/geotiff — a real georeferenced float32 band, not an 8-bit styled/quantized image:

https://data.geopf.fr/wms-r?SERVICE=WMS&VERSION=1.3.0&REQUEST=GetMap
  &LAYERS=IGNF_LIDAR-HD_MNS_ELEVATION.ELEVATIONGRIDCOVERAGE.LAMB93
  &STYLES=&FORMAT=image%2Fgeotiff&CRS=EPSG:3857
  &BBOX={bbox-epsg-3857}&WIDTH=514&HEIGHT=514

{bbox-epsg-3857} is MapLibre's own tile-source placeholder — the same mechanism an ordinary type: "raster" WMS source already uses — substituted per-tile with that tile's bbox in EPSG:3857.

2. Decode → re-encode

The response is a real GeoTIFF, parsed client-side with geotiff.js. Band 0 is read as raw float32 meters, then re-packed into standard Terrarium RGB (the same encoding described in Terrain Analysis Rendering Pipeline) and rasterized to a PNG — so MapLibre's native raster-dem consumer needs no awareness this data ever passed through WMS or GeoTIFF at all:

const tiff = await GeoTIFF.fromArrayBuffer(arrayBuffer)
const image = await tiff.getImage()
const elevationData = (await image.readRasters())[0]
for (let i = 0; i < elevationData.length; i++) {
  const v = elevationData[i] + 32768
  const intPart = Math.floor(v)
  rgbaData[i*4 + 0] = Math.floor(intPart / 256) & 0xFF
  rgbaData[i*4 + 1] = intPart & 0xFF
  rgbaData[i*4 + 2] = Math.floor((v - intPart) * 256) & 0xFF
  rgbaData[i*4 + 3] = 255
}

The RGBA buffer is drawn to an OffscreenCanvas and converted to a PNG blob — what's actually returned to MapLibre.

3. Protocol registration

Registered as maplibregl.addProtocol('float32dem', withTileResultCache(float32demProtocol)) in components/TerrainViewer.tsx. Tile URLs are the scheme plus the WMS URL with its https:// stripped (re-prepended by the handler before fetching): float32dem://data.geopf.fr/wms-r?...&BBOX={bbox-epsg-3857}....

4. Relation to the app's COG terrain path

float32dem:// exists specifically for the "no COG, no server-side GDAL WMS bridge" case in the pure-browser rendering path. In lib/source-builder.ts, a "wms-raw" source kind branches on rendering mode:

• Client-side (@geomatico/maplibre-cog-protocol, which needs an actual COG file) always falls back to float32dem://, since there's no COG for it to read.
• TiTiler-backed mode instead passes WMS:<url> through GDAL's own WMS minidriver, letting the server treat the live WMS endpoint as an addressable raster dataset and handle reprojection/tiling itself — no float32dem:// involved.

float32dem:// is also reused as a building block by two other parts of the app: LRM (its ancestor-tile fetch rewrites the WMS request size for a WMS-raw upstream, since such sources have no real overview pyramid to exploit) and lib/cog-contour-protocol.ts/ContoursLayer.tsx (contour generation).

5. Exposure in the app UI

components/TerrainControlPanel/wms-picker-panel.tsx is a generic "bring your own WMS" picker — fetches GetCapabilities, lists available layers, builds a {bbox-epsg-3857}-templated GetMap URL — shared between the basemap and terrain "Add Source" flows. A format prop selects image/png for ordinary basemap layers or image/geotiff for elevation/terrain sources; the resulting URL is routed through source-builder.ts's "wms-raw" case described above. It sits alongside COG uploads as one more entry in Bring Your Own Data, not a separate standalone feature.