---
type: Rendering Constraint
title: Terrain relief imagery and elevation
description: Global shaded relief with tile-local signed elevation and regional detail.
tags: [rendering, raster, terrain, coverage]
visibility: public
---

# 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](/rendering/raster-replacement.md). 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.

# Sources

- [GEBCO WMS](https://www.gebco.net/data-products/gebco-web-services/web-map-service)
- [GEBCO grid and terms](https://www.gebco.net/data-products/gridded-bathymetry-data)
- [GEBCO_2026 numerical dataset](https://data.ceda.ac.uk/bodc/gebco/global/gebco_2026/ice_surface_elevation/netcdf)
- [USGS 3DEP service](https://elevation.nationalmap.gov/arcgis/rest/services/3DEPElevation/ImageServer)
- [USGS products and coverage](https://www.usgs.gov/3d-elevation-program/about-3dep-products-services)