Cloud-Optimized GeoTIFF

Cloud-Optimized GeoTIFF is an ordinary GeoTIFF, but laid out inside in such a way that it can be read piece by piece straight from the cloud. The file has internal tiles and a pyramid of reduced copies, so a client makes an HTTP request with a byte range and takes only the fragment it needs and only the scale it needs, without downloading the whole file.

The format is chosen when the map lives in cloud storage and is served to many consumers: a basemap for a web map, a layer in a cloud GIS, a raster for analytics. A single file on S3 replaces a whole tile server — readers take from it what they need themselves.

The output is a single .tif file: the map of the selected area is drawn into a raster and repacked into COG with GDAL. The internal tiles are 512 pixels, overview levels are built automatically with bilinear resampling, and the default compression is DEFLATE (LZW, ZSTD, JPEG and WebP are also available). Georeferencing is in WGS84, the resolution is chosen by a quality preset, and the raster side is capped so the file stays workable.

It is important to understand the nature of the file: these are pixels, not objects. There are no OSM attributes in it, and you cannot click a building and see its tags — for that you need vector formats.