- Cloud-free collections key for...
Cloud-free collections key for land monitoring
14 Sep 2023
Satellite imagery is a crucially important tool for monitoring the land, but cloud cover can often block parts of Earth’s surface from view, limiting subsequent analyses. To address this challenge, ESA disseminates collections composed of images from different time points to deliver a mostly unobstructed view of the planet.
In this summary article, Earth Online highlights key cloud-free collections and how to access them.
These data typically cover large areas, which make them an important tool for scientists completing a wide range of regional and national analyses, including land cover monitoring, biosphere research, mapping and cartography.
To develop the products, data are pulled from a large timeframe before cloud-free images are selected and assimilated to create a mostly unobscured perspective of the land.
ALOS-1 delivers unobstructed view of Europe
The recently released ALOS European cloud-free collection is based on a subset of data from Japan’s Advanced Land Observing Satellite (ALOS-1), which is part of ESA’s Third Party Missions programme, through which ESA has an agreement with the Japanese Aerospace Exploration Agency (JAXA) to distribute products from the mission.
The collection is composed of Level-1C products from ALOS-1’s Panchromatic Remote-sensing Instrument for Stereo Mapping (PRISM) sensor. Around 70% of the scenes included in the collection have zero cloud coverage, while the remaining 30% have cloud coverage of no more than 20%.
More information on the collection can be found online.
Cloud-free data from Landsat
Two cloud-free open access collections based on data delivered by the US Landsat programme are being disseminated by ESA.
The cloud-free Landsat-7 Enhanced Thematic Mapper (ETM) collection covers Europe, as well as parts of North Africa and the middle East. For each scene one ETM product is selected that has minimal cloud coverage.
The collection covers between July 1999 and April 2003, and each scene typically covers an area of 185 km by 170 km.
The second cloud-free collection – which also covers Europe, North Africa and the middle East – draws on products from the Landsat-5 Thematic Mapper (TM). It covers April 1987 to August 1995 and individual scenes cover an area of 183 km by 173 km.
High resolution data with less than 10% cloud cover
Part of ESA’s Third Party Missions programme, GEOSAT-2 is an Earth-imaging multispectral satellite capable of delivering sub-metric data.
GEOSAT-2 data collected across 2021 underpin a mostly cloud-free collection covering Portugal and its islands. The collection has cloud coverage of less than 10% and a spatial resolution of down to 1m, with geometric accuracy of less than 6m.
Cloud-free data over 38 European countries
The ‘Image 2006 collection’ is composed of Level-1 data delivered by SPOT-4’s High-Resolution Visible and InfraRed (HRVIR) sensor, SPOT-5’s High-Resolution Geometric (HRG) sensor, and ResourceSat-1’s Linear Imaging Self-Scanning Sensor-III (LISS-3).
In addition to these Level-1 data, the collection provides the same data geometrically corrected towards a European map projection with a resolution of 25 m.
You can check the spatial coverage of the collection using a map and request access through ESA’s Data Dissemination Service.
The collection is available via fast registration with approval and it can take up to 2 days to gain access to the data.