Module mercator 

Reproject web-mercator (XYZ) DEM tiles onto a geodetic (EPSG:4326) grid.

Elevation tiles are almost always served in the web-mercator XYZ tiling (Terrarium, Mapbox Terrain-RGB, …), while Cesium quantized-mesh terrain is served in the geodetic TMS scheme (EPSG:4326). Those are different projections, not just different tilings — web-mercator’s latitude axis is non-linear — so producing a geodetic terrain tile means resampling (warping) the mercator DEM, not merely cropping and stitching it.

MercatorDem holds a contiguous block of decoded web-mercator DEM tiles stitched into one grid and lets you sample it by longitude / latitude (bilinear). From there:

• MercatorDem::geodetic_grid produces the 2^n+1 elevation grid that crate::terrain::encode_terrain expects, and
• MercatorDem::buffered_geodetic produces the halo-extended BufferedElevations that crate::terrain::NormalMode::BufferedGradient expects for seam-free normals.

Fetching the source tiles is left to the caller (HTTP, disk, cache, …) so this module stays free of any IO/async assumptions — supply an already decoded tile via MercatorDem::new / MercatorDem::from_tiles.

Example

use terrain_codec::quantized_mesh::TileBounds;
use terrain_codec::mercator::MercatorDem;
use terrain_codec::terrain::{encode_terrain, TerrainOptions};
use terrain_codec::tile_coords::geodetic_tms;

// Target geodetic TMS tile we want to emit.
let (w, s, e, n) = geodetic_tms::tile_to_bounds(12, 7252, 2852);
let bounds = TileBounds::new(w, s, e, n);
let grid_size = 257; // 2^8 + 1

// Source web-mercator DEM: decide a source zoom, find the covering XYZ
// tiles, and assemble them (the closure does your fetch + heightmap decode).
let src_zoom = 13;
let tile_size = 512;
let (x0, y0, tx, ty) = MercatorDem::tiles_covering(src_zoom, w, s, e, n);
let dem = MercatorDem::from_tiles(src_zoom, x0, y0, tx, ty, tile_size, |z, x, y| {
    // fetch z/x/y.png, decode to elevations (tile_size², row-major N→S)
});

let grid = dem.geodetic_grid(&bounds, grid_size);
let terrain = encode_terrain(&grid, grid_size, &bounds, &TerrainOptions::default());

Structs

MercatorDem

A contiguous rectangular block of decoded web-mercator (XYZ) DEM tiles, stitched into a single elevation grid and sampleable by longitude / latitude.