Terrain and elevation#

One terrain raster serves every elevation function: the contours on the map, the climb on a route, the ascent a router weighs, the altimeter, and Peak View. The source is Copernicus DEM GLO-30, and obc-dem converts it to the OBCT format.

Data flow#

Terrain cells are baked and revised separately from map cells, and the assembler copies the selected cells into the terrain region of the map. A map schema change therefore does not force a new terrain bake.

One sampling truth#

The packer and the device use the same no_std sampler, obc-elevation. This matters more than it sounds: the packer bakes the ascent the router weighs, and the device measures the climb the rider sees. If the two sampled differently, a route would promise one climb and report another.

OBCT section 5 specifies the integer interpolation exactly, so an independent implementation returns the same whole-meter height. Cells own their seam samples, so a position on a cell boundary has one answer.

Only obc-dem decodes the source DEM. The packer never touches GeoTIFF data.

Missing height is not zero#

The sampler returns nothing when terrain is unavailable or a required corner has no data, and no consumer may read that as zero, because zero meters is a valid height at the coast.

An unknown sample pauses ascent integration and the next valid run starts a new reference, so a gap in coverage contributes no climb instead of a false one. A route records which of its segments had complete coverage, and a profile leaves the gaps empty.

Navigation coupling#

The ascent stored on each navigation edge belongs to a terrain revision, and the catalog records which revision the network was baked from. The bake refuses a network whose terrain revision does not match, because routing costs and route profiles must describe one surface.

Crest lifts#

A 30 m grid cannot hold a rock tower. At Engelberg the Copernicus surface runs about a hundred meters below the summit of the Hahnen, and a panorama drawn from it loses the shape that makes the mountain recognisable.

The bakery corrects this with a crest lift. Where a finer national elevation model says the ground inside a sample's own cell stands well above our surface, and the ground there is convex, the baker raises that sample. The convexity test is what keeps a steep flat face and a mountain pass unchanged: only a crest moves.

The sample rises by the gap between the two models, not to the highest ground in its cell. On a slope most of that height is the cell's own fall, and lifting by it would raise the ground all around a summit. The highest measured ground stays the ceiling, so a lift never invents height.

Copernicus stays the base model, and a national model can only lift: it measures bare ground, so it sits below Copernicus over every forest and town. Coverage may stop at any sample, so a model that ends at a border is not a problem, and a cell with no finer coverage is identical to a cell baked without one.

The lift goes into the baked sample, so the contours, the ascent, the route profile, the altimeter, and Peak View all keep reading one surface. A change of finer model is therefore a new terrain revision, which also re-bakes the navigation graph. Only the cells the changed source reaches are baked again.

Each finer model keeps its own attribution, and it must travel with the map. See Attribution.

Terrain of the planner and TrailView#

The planner maps and TrailView read one pyramid of terrain tiles. A national model gives the height where it holds the ground, and GLO-30 gives it elsewhere. GLO-30 leaves the sea edge above sea level, spreads a sea cliff over a ramp up to 100 m wide, and rounds a summit into a dome.

Where GLO-30 gives the height, also in the 1 km band where it blends into a national model, the bake corrects these faults from OSM. The sea beside the coastline is at sea level, and a mapped sea cliff becomes sheer, no deeper than its mapped height. A mapped summit with a height rises toward the cone of its flanks, never above that height, but only where GLO-30 has its top next to the summit. Each correction uses only the mapped feature and the GLO-30 heights near it, so a summit node on a slope or a wrong tag changes little or nothing. An inland cliff and an unmapped cliff keep their GLO-30 heights. The rules are in shape.rs.

Peak View#

Peak View draws the horizon around the rider and names the summits on it. It uses the loaded map and the current position: the named summits come from the map's own summit category, and their visibility is tested against the terrain.

The bakery writes a surface index for this one purpose. Coarse height levels and conservative height bounds let the renderer skip terrain that cannot be visible and draw large smooth areas without visiting every sample. There are no stored viewpoints and no stored images: the panorama is computed where the rider stands.

A visibility test uses the greater of the summit's recorded height and the sampled surface, so a narrow peak is not rejected because the grid samples below its top. The four lattice corners under the rider are clamped to the rider's own height, so the ground the rider stands on never blocks the view.

Candidates are ranked by their angle above the horizon, and one place is reserved per sector for the tallest summit there, so a single high massif cannot take every label.

The view is 60° wide and starts below the horizon. A summit above the window opens the frame upward and widens it by the same ratio, because the horizontal and vertical scales must stay equal or the drawn shape of a mountain is wrong. Lighting has a fixed world direction, so turning changes neither the scale nor the shading.

Opening Peak View needs a recent fix, and it asks for one even when no ride is recording. The renderer prepares the current view first and fills the rest of the circle in the background, and a hatch shows what is not drawn yet. Turning toward an unfinished view gives it priority.

Live follows the compass. Select enters Browse on the most prominent visible peak, and further steps walk the visible peaks, turning the view only when there is no next peak. Where the map carries content for a summit, a mark appears. Content is text, a photo, or both: Select opens the text pages and the photo, and a summit with a photo and no text opens on the photo. The photo has to help the rider recognise the mountain, so the bakery prefers a picture taken at a distance and puts a picture taken on the summit last. Down + Back opens the sources, because credits must travel with the content. An article is linked by the summit's OSM identity alone: a name and a coordinate cannot prove that an article describes that summit.

Peak articles are reached only through Peak View. Other places are in Ride Assistant landmarks.

Terrain in the map#

The assembler verifies each selected cell against the catalog, copies the cells into one container, and puts that container in the map's terrain region. The device reads terrain from there and from nowhere else.

obc-pack --terrain is a different operation: it samples terrain input to trace contours and to integrate edge ascent. It does not copy that input into its output map.

The posting and the cell size are header fields, so changing them is a re-bake and not a format version. See the OBCT specification for the limits and the byte layout, and OBCC section 13 for the catalog contract.

Attribution#

The data requires an exact attribution. The text is stored once, as the copernicus-glo-30 entry of the source registry, and the bakery copies it into the catalog terrain block, so every consumer reads it from the catalog and none of them hard-codes it. The device About page is the one exception: it shows the text pre-wrapped, and a test compares it with the registry. A map with terrain-derived contours needs the attribution too.

A map with crest lifts also carries the attribution of each finer model it used. The catalog lists those models with their required credit and license, copied from the reference archive that holds the wording. A consumer that shows one shows all of them. The map builder shows them on the map summary card.

Map data carries the OpenStreetMap credit of the same registry.

Implementation#

• OBCT contract: OBCT_Spec.md
• Reader and sampler: obc-elevation
• DEM converter: obc-dem
• Terrain publisher: terrain.rs
• Map assembly: terrain.rs
• Edge ascent: nav.rs
• Altimeter fusion: altitude.rs