The European Space Agency
Contact us

File format recommendations

Within the APEx interoperability and compliance guidelines, PROV-REQ-01 specifies the file formats that projects should use when publishing static data. The information on this page complement that requirement with more detailed recommendations on the use of these formats. Projects are encouraged to consult with the APEx team if they have a use case that does not fit within these guidelines.

Cloud Optimised GeoTIFF

Cloud Optimised GeoTIFF (COG) is the most widely supported format for geospatial raster data, and also one of the most efficient in terms of access costs. It is recommended as the default option to consider when publishing static data products. When combined with STAC metadata, COG can produce self-describing, FAIR-compliant datasets that are easily consumable by various services.

Generating Cloud Optimised GeoTIFF

If you are not familiar with COG or GeoTIFF generation, it is recommended to format your GeoTIFF files using a recent version of GDAL to ensure that they are compliant:

gdal_translate world.tif world_webmerc_cog.tif -of COG

Organizing spatiotemporal multi-band datasets

Many datasets have multiple bands (or ‘variables’), or have a date associated with them. The general recommendation is to store a single band per GeoTIFF file and to create STAC items with one asset per band. This layout is commonly used by many other datasets and avoids the complexities of multi-band GeoTIFF files, which can be challenging to use.

An exception to this are the ‘RGB’ style products, where three bands are used to represent a single image. In this case, creating a Cloud Optimised GeoTIFF with three bands is an option.

For associating time information, create one GeoTIFF per timestamp, and one STAC item per timestamp. The GeoTIFF format has not built-in support for conveying time information, but STAC metadata is supporting this very well.

Visualisation in APEx Geospatial Explorer

To optimise visualisation in the APEx Geospatial Explorer, additional guidelines have been established. Adhering to these guidelines will ensure that the data is effectively optimised for visualisation on a map. Please refer to this page for more information.

(Geo-)Zarr

Zarr is a format that is gaining traction in the geospatial community, although it is not yet as widely supported as Cloud Optimised GeoTIFF. Its main advantage lies in its ability to store complex multi-dimensional datasets that go beyond a simple 4D (x, y, time, bands) structure. Just like COG, Zarr allows for efficient cloud access.

At the time of writing, there are, however these important caveats:

  • GeoZarr aims to define how to store geospatial data in Zarr format, but this standard is not fully developed and lacks widespread tooling support.
  • Overview pyramids for fast online rendering are not yet supported.
  • By design, Zarr allows for many degrees of freedom, which requires the data producer to have a good understanding of the associated trade-offs.
  • By design, Zarr stores data as separate files in a directory structure, optimising cloud access but making direct downloads less convenient.

NetCDF

NetCDF is a self-describing format with some properties similar to Zarr, but less optimised for cloud access. It can be useful for exchanging data cubes as single files through traditional methods. However, it is less recommended for convenient sharing of large datasets, for which either COG or Zarr provide better options.