Meaning
Terrain relief combines rendered imagery with a separate numerical inspection plane. The RGB colours do not encode numerical elevation. GEBCO provides shaded land and seafloor relief globally through its public WMS. USGS 3DEP provides elevation-tinted hillshade from a regional, multi-resolution DEM service. Their imagery implements the same versioned Data Product contract; their Sources and Channels keep separate identities and attribution.
Composition
A map may place the global GEBCO Layer below the USGS detail Layer. The example uses a user-configurable minimum zoom for the detail Layer. Provider alpha is authoritative: transparent USGS pixels reveal GEBCO, and USGS request failures leave the global Layer visible. The Layers retain independent visibility, opacity, and zoom controls. This uses ordinary Mapbox raster surfaces through the globe-to-Mercator morph.
The example's Map Connections explicitly declare one raster replacement group. USGS has higher priority within its saved zoom interval; GEBCO is the baseline. Rendering retains both native raster surfaces while numerical inspection chooses one actual contribution. Provider controls and attribution remain separate.
Delivery
Both providers use on-demand Edge Functions with fixed upstream endpoints, bounded PNG payloads, timeouts, and a bounded one-hour in-memory cache. The provider services may change their mosaics; these tiles are not described as immutable releases. Authorization checks the exact map Layer, Source, and Channel before every cache hit or provider request. The browser never selects an arbitrary upstream URL. HTTP caching is private and varies by Authorization; shared HTTP caches cannot bypass map access checks. No acquisition worker or local GDAL job is required.
Channels retain pixel_delivery@1 with kind=imagery and sampling.mode=none. They separately declare point_inspection.tile_query_value=elevation-tile with one numeric elevation field in metres. The same cached raster-inspection-v1 plane feeds hover and Details; moving inside a tile does not issue coordinate requests. Fields controls use the existing provider-neutral declaration.
GEBCO_2026 numerical subsets come from the official CEDA OPeNDAP dataset, not from relief colours. Requests address a bounded native-grid subset, at most 512 by 512 samples. The server validates DAP2 dimensions and coordinates and resamples nearest-neighbour into north-up Web Mercator tiles. The native grid has 15 arc-second spacing. Its signed elevation is relative to mean sea level: negative values describe seafloor depth. At low zoom, inspection is limited by the delivered tile resolution. The numeric release is explicitly GEBCO_2026; when updating the WMS to a new annual grid, update the numerical release too.
USGS numerical tiles request raw F32 BIP with the None raster function and nearest-neighbour resampling. Its independent validity mask is authoritative: a NoData sample may contain zero and must not become sea-level elevation. USGS retains the native DEM's vertical reference, which varies by source; values are not transformed into a unified survey datum or averaged with GEBCO. In an explicit replacement group, the inspector shows the regional value when valid and only the global value otherwise. An ordinary map with independent Layers still lists their independent values; matching quantity alone does not imply replacement.