Offline data packs
Every cell Meld renders needs two kinds of source data: OpenStreetMap for what is there, and elevation for how high it sits. Fetched live, both come from public servers that rate-limit hard, which is what turns a country-sized render into a stalled one. Data packs flip that: download or bake a region’s data once, into a shared cache, and every render after that is fast, repeatable, and fully offline.
Why bake instead of fetch
The live Overpass servers are shared infrastructure and they throttle heavy users, correctly. A thousand-cell render is a heavy user. A baked region never asks them anything: the cells read tiles straight from your disk, so nothing is rate-limited, nothing times out, and a re-render of the same region downloads nothing at all. Baked OSM tiles are kept for 365 days, so a region you prepared this spring is still warm next winter. The same goes for elevation: a truncated or missing terrain tile is the documented cause of flat seams in a world, and a verified local pack removes the failure mode entirely.
How it works
One button: “Get the data for this region”
You do not have to know what Geofabrik is. Meld reads your selection, asks Geofabrik’s own index which OSM extract fits it, the smallest one that contains it, or the set of neighbouring countries when the selection crosses a border, and downloads into the default .pbf folder with progress and a Stop button. The choice is priced first: the by-country set is recommended with its combined size, and a continent-sized file is offered last with its true download size, the memory baking it needs on this machine, and a warning when that will not fit.
The OSM bake
Baking reads the .pbf files and writes OSM tiles onto the same fixed grid the live fetch fills, so baked and live tiles are interchangeable and overlapping selections reuse each other’s tiles. The bake prices itself before it starts: memory, disk, time and worker count, checked against what your machine actually has. The measured constants behind that: a baked tile averages about 15.7 MB, the disk briefly holds roughly double the final size while merging, and the OSM index costs about 2.2 GB of memory per GB of .pbf, per worker.
A bake that cannot fit is refused with the fix named (use the country extract, not the continent). Files that cannot contain your region are skipped instead of read, sea areas get truthful empty tiles so coverage really means 100%, a scan warns when an extract is more than six months old, and Stop lands in about a second, keeping every tile already written.
Elevation packs
The elevation side works the same way: Bake Mapterhorn elevation pre-downloads exactly the terrain tiles the default provider stack will use for your selection, at your project’s scale, so the cached zoom matches what the cells request. A coverage check tells you how complete the pack is, and the height preview overlays the cached terrain on the map as hillshade, with uncached tiles marked, so you can see the terrain before you build it and spot holes before they become flat cells. A Cached elevation only toggle then guarantees the render uses the bake and nothing else; a tile that was never baked still produces flat ground for that cell, but Meld flags those cells as suspect instead of leaving the failure silent. More in Elevation & terrain.
Buildings too
If Overture buildings are on, a Buildings data-pack card pre-warms their download cache in parallel up front, so a buildings-on build never stalls on a cold fetch mid-render.
How to use it
- Draw your selection, then press Get the data for this region and accept the recommended extract set.
- Run the OSM bake and the elevation bake from the data-pack cards. Both show live progress and cost estimates; both can be stopped and resumed without losing finished tiles.
- Check coverage. When both packs read complete, the region renders with zero network calls, and keeps doing so for every re-render, retry and queued project that touches it.
One note for source installs: the packaged app ships the .pbf reader built in, but running from the repository needs pip install osmium for the bake. If a render still stalls on data, the troubleshooting guide covers Overpass rate limits and going fully offline step by step.