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The Earth in mosaic

Beautiful and meaningful imagery covering vast areas by Airbus Defence and Space's SPOTMaps service

Over and above the market for raw satellite imaging datasets and the many levels at which they can be processed and interpreted, there is also a demand for accurate, beautiful and meaningful imagery covering vast areas. This is where Airbus Defence and Space's SPOTMaps service comes in.

 The Earth as seen by SPOTMaps: coverage of over 100 countries, or 95 million km², and increasing every month.
The Earth as seen by SPOTMaps: coverage of over 100 countries, or 95 million km², and increasing every month. © Airbus Defence and Space

 

A quarter of a century after the first SPOT satellite entered service, satellite imagery is now present in all aspects of our daily lives, through services such as Google Earth, the computer graphics used in every TV news programme, or even road maps, whether on paper or via the digitised data used by GPS receivers.

With an initial resolution of 10 metres, associated with a broad swath able to cover large areas with a high level of detail, SPOT satellites were very much at the forefront of this revolution. Just think back to the intensive use of its images on the world’s TV screens at the time of the Chernobyl accident in 1986, or during the First Gulf War in 1991.

Since its 2002 launch, the SPOT 5 satellite has been acquiring scenes covering 3600km2 with a resolution of 2.5m. Four years later, the first tangible applications arrived, in particular through the signing of a contract with Google for the supply of SPOT imaging mosaics intended among other things for its Google Earth and Google Maps geographical display applications. The mosaics were created by the then Spot Image and Infoterra – using the Pixel FactoryTM production line to produce digital geographical data from SPOT imagery.

An artist's impression of SPOT 5, designed and built by Airbus Defence and Space. Since 2002, this satellite has enabled the acquisition of 3,600km2 swaths at a 2.5m resolution.  © Airbus Defence and Space / D. Apikian This first major contract demonstrated the strong market demand for this type of uniform, large-scale imaging coverage and marked the birth of the SPOTMaps product, which was to be brought to market the following year. Development and marketing of the mosaics is now being managed by the GEO-Information division of Airbus Defence and Space, which comprises Spot Image and Infoterra, now Airbus Defence and Space subsidiaries.
An artist's impression of SPOT 5, designed and built by Airbus Defence and Space. Since 2002, this satellite has enabled the acquisition of 3,600km2 swaths at a 2.5m resolution. © Airbus Defence and Space / D. Apikian

 

“To start with, most demand was for coverage of entire countries, to help with large-scale planning,” explains Laurent Pradère, technical manager of the SPOTMaps programme. “But these images can also be used as a working basis for survey work in countries where today this doesn’t exist.”

Four months for mosaic production

 Australia by SPOTMaps, covering the whole of the 7,686,850km2 surface of the island-continent. (© CNES 2010 – Distribution Airbus Defence and Space / Spot Image)
Australia by SPOTMaps, covering the whole of the 7,686,850km2 surface of the island-continent. © CNES 2010 – Distribution Airbus Defence and Space / Spot Image

 

To create a mosaic, the images are programmed according to criteria specific to the SPOTMaps product, by minimising saturated areas and those with mist and clouds.

They then undergo geometrical corrections to minimise distortion due to relief and viewing angles, and are assembled into mosaics.

In areas where the images overlap, optimised cut-lines are defined – generally along a riverbank, the side of a building, a field, a road, or other clearly defined formation or construction – rendering them virtually invisible. Finally, the colours are adjusted so that they are as uniform as possible across the entire mosaic – even though the images may have been taken at different seasons and changes may have occurred on the ground between the times the views were taken – so that the overall colours and contrasts are as close as possible to the actual landscape.


SPOTMaps detail: Culburra Beach on the Australian coast. (© CNES 2010 – Distribution Airbus Defence and Space / Spot Image)


 

SPOTMaps detail: Culburra Beach on the Australian coast. © CNES 2010 – Distribution Airbus Defence and Space / Spot Image

 

“Our aim is to create a beautiful image of a large area, with which people can identify.”

But beauty isn't everything! To address the needs of professional users, the image also has to be accurate, with excellent geometrical quality, which is no mean feat. These mosaics are compiled in batches of 300,000–900,000km2 (150–800 images, covering an area far larger than France!), with a resolution of 2.5m and positioning of each pixel to within 10–15m. The orbitographic precision offered by the satellite’s Doris system, along with the extreme pointing accuracy of its instruments, means the images acquired by SPOT 5 are geo-positioned to within 25m. This accuracy is then doubled through rectification with a precise digital terrain model, using the 3D Reference product which was jointly developed by Airbus Defence and Space GEO-Information Services and IGN Espace.

 Each SPOTMaps block (300,000–900,000km2, or 150–800 images), represents four months work by a range of operators expert in fields as diverse as mapping, photogrammetry, GIS and SPOT satellite data interpretation.
Each SPOTMaps block (300,000–900,000km2, or 150–800 images), represents four months work by a range of operators expert in fields as diverse as mapping, photogrammetry, GIS and SPOT satellite data interpretation. © Airbus Defence and Space

 

It is the extremely high radiometric precision of the SPOT instruments which enables the colour adjustment to work so well. This is something which the SPOTMaps teams have experienced at first hand when, in certain specific cases, they have been asked to match SPOT images with those from other sensors – sometimes with higher resolution – and seen that the colour of these other images just wasn’t up to scratch.

Overall, each batch requires four months of work by several operators deploying their mapping, photogrammetry and geographical information system (GIS) skills, allied with their expertise acquired over the past two decades turning the data collected by the SPOT satellites into marketable products.

20 million km² of SPOTMaps are produced every year. To date, more than 100 countries and a total of 95 million km2, or the equivalent of Africa, Asia and Oceania combined, have already been processed as SPOTMaps mosaics. These images can be purchased by the square kilometre from the SPOTMaps website to which updates and additions are regularly made.

Earth ObservationSPOTGeoinformation Services