On-the-fly Mosaics, titiler-cmr and Planetary Computer

Three medium-resolution series on the historical timeline are not tile pyramids anyone published. They are composited the moment a tile is asked for, by a dynamic tiler that searches a scene catalog for the window, reads the matching Cloud-Optimized GeoTIFFs by range request, and renders the tile:

Both tilers are public and keyless from the browser, and both are services someone else runs for their own catalog, not a contract: a slow tile or an outage is theirs to fix. Google Earth Timelapse is the opposite kind of source, one finished frame a year, and sits next to the Planetary Computer catalogs so a place can be read both ways.

titiler-cmr, behind the NASA HLS pillCopy Anchor Link

titiler-cmr is Development Seed's titiler with NASA's Common Metadata Repository (CMR) as its backend: a tile request names a CMR collection by concept id and a time range, the server runs the granule search, and reads the granules' assets, GeoTIFFs with the rasterio backend or NetCDF and HDF5 with the xarray one. NASA VEDA runs a public instance at openveda.cloud/api/titiler-cmr, with a staging twin at staging.openveda.cloud; Development Seed's titiler-cmr browser is a front end on that same endpoint, configured for HLS and NISAR.

The data behind HLS lives at NASA's LP DAAC and needs an Earthdata login to read; the VEDA instance holds those credentials server-side, which is why the browser needs none. The instance is a demo of VEDA's, not an SLA.

The pill (lib/hls.ts) asks for one true-colour mosaic per date:

https://openveda.cloud/api/titiler-cmr/rasterio/tiles/WebMercatorQuad/{z}/{x}/{y}
  ?collection_concept_id=C2021957295-LPCLOUD      HLSS30 v2.0, the Sentinel-2 half of HLS
  &temporal=<date - 15 d>/<date + 15 d>
  &assets=B04&assets=B03&assets=B02                red, green, blue
  &rescale=0,3000 ×3                               surface reflectance ×10000: the usual stretch
  &color_formula=Gamma RGB 1.3 Saturation 1.5 Sigmoidal RGB 8 0.35

• HLSS30, not HLSL30. The Sentinel-2 half revisits every 5 days against Landsat's 16, so a 30 day window nearly always has cloud-free pixels. Both halves are 30 m; the HLS algorithm resamples Sentinel-2 onto Landsat's grid, which is why "10 m Sentinel-2" is not what HLS gives.
• The window, not a date. A tile is whatever granules fall in the 30 days, in catalog order: no cloud filter and no cloud-first sort are sent today, titiler-cmr accepts cloud_cover and sort_key for that. Scene seams show where two granules meet.
• Ticks are synthetic. One a month from January 2016, the date at the middle of its own window; nothing checks that a granule exists at the view's centre. HLS exists from 2013 for the Landsat half; the ticks start where Sentinel-2 coverage is dense.
• Zoom. Served down to zoom 0, but a low-zoom tile means many granules read; the source is declared to zoom 16 and the tiles are 256 px.

Planetary Computer, behind the two World catalogsCopy Anchor Link

Microsoft Planetary Computer keeps a STAC API over its catalog (Landsat Collection 2, Sentinel-2 L2A, NAIP, HLS 2.0, DEMs and more, the assets in Azure Blob Storage) and a data API that tiles it: titiler-pgstac behind planetarycomputer.microsoft.com/api/data/v1. Reading an asset normally needs a SAS token from the SAS API; the data API signs its own reads, so the browser sends none, and the register and tiles endpoints answer with an open CORS header.

A mosaic is a registered search:

POST /api/data/v1/mosaic/register
{ "collections": ["landsat-c2-l2"],
  "datetime": "1990-04-01T00:00:00Z/1990-06-30T23:59:59Z",
  "filter-lang": "cql2-json",
  "filter": { "op": "<=", "args": [{ "property": "eo:cloud_cover" }, 10] },
  "sortby": [{ "field": "eo:cloud_cover", "direction": "asc" }] }
→ { "searchid": "5dee14b3…" }

GET /api/data/v1/mosaic/5dee14b3…/tiles/WebMercatorQuad/{z}/{x}/{y}@1x.png
  ?assets=red&assets=green&assets=blue&nodata=0&collection=landsat-c2-l2
  &color_formula=gamma RGB 2.7, saturation 1.5, sigmoidal RGB 15 0.55

The search id is a hash of the search, so registering the same window again is free. For a tile the API lists the search's items that intersect it, in the sort order, and fills pixels from the first item that has data there (pixel_selection=first): the least cloudy scene wins where scenes overlap, and the scene edges, marked by nodata=0, let the next scene show through. That is a least-cloudy-scene mosaic, not the per-pixel median of a Timelapse year: seams follow Landsat paths and Sentinel-2 granules, and a scene that is 8 % cloudy brings its 8 % along. Below a zoom of 8 for Landsat and 9 for Sentinel-2 the API answers 204, empty, rather than read hundreds of scenes.

The protocol (lib/pc-mosaic.ts) exists because the registration is a POST, which a MapLibre tiles template cannot send:

pcmosaic://landsat-c2-l2/{z}/{x}/{y}?from=1990-04-01&to=1990-06-30
pcmosaic://sentinel-2-l2a/{z}/{x}/{y}?from=2024-08-01&to=2024-08-31

• Two searches a window. Scenes with at most 10 % cloud first; when no such scene covers the tile (an overcast quarter, a rainy month: June 2024 over Paris has none) the handler registers the same window at 60 % and tries again, so the tile draws, cloudier, rather than stays blank. Both ids are cached for the session.
• Landsat is rendered from the red, green and blue surface-reflectance assets with Planetary Computer's own "Natural color" formula, 30 m, declared to zoom 13. Landsat 4 TM's first scene is 22 August 1982; Landsat 1 to 5 MSS (1972 to 1992, 60 m, no blue band) sits in landsat-c2-l1 and is not offered.
• Sentinel-2 uses the L2A product's own true-colour image, the visual asset, 10 m, declared to zoom 15. Planetary Computer holds Level-2A from 2016 for most of the world, with 2015 scenes in places.
• In the catalog (lib/national-historical.ts) each window is a layer of a complete source: every layer covers the globe, so the timeline lists them all without probing a tile at the view's centre. The Landsat catalog has a layer a quarter, the Sentinel-2 one a layer a month, and both sit under the Medium res pill.
• Not in Export Multi. The export dialog lists the timeline's own pills; catalog layers, these included, are picked on the map.

What to expect of eachCopy Anchor Link