COG (Cloud-Optimized GeoTIFF)

A GeoTIFF image organized so software can read just the part it needs over the network, without downloading the whole file.

Why it matters

COG is NASA’s recommended raster format for the cloud era. It lets a browser map or a script grab one tile of a 10 GB image in milliseconds.

Where you’ll meet it

• HLS surface-reflectance is distributed as COG — a browser map reads one tile without pulling the whole scene.
• A web map’s tile server (like the titiler services NASA runs) reads just the byte ranges it needs from a COG in cloud storage to render each map tile on demand.
• In a notebook, rasterio or rioxarray opening a COG over HTTPS fetches only the windows you read, so clipping to an AOI never downloads the full image.
• COGs are the raster assets that STAC items point to and that DAACs increasingly publish, replacing the old “download the entire GeoTIFF first” workflow.

In plain terms

Like a PDF you can jump to page 400 of without loading pages 1–399 — the internal index makes partial reads cheap.