SPIRIT & SPIRIT2
The SPIRIT (SPOT 5 stereoscopic survey of Polar Ice: Reference Images and Topographies) project took place during the Fourth International Polar Year (2007/09) and built up a large archive of SPOT 5 stereoscopic images covering most polar ice masses. It allowed the free delivery of high resolution (5-m) ortho-images and 40-m digital elevation models (see "The Product"), data which have already contributed to >100 publications. These data are freely available for the glaciological community (see "Get Data").
In 2013, CNES, Airbus D&S and LEGOS decided to launch a new acquisition campaign, SPIRIT2. SPOT5-HRS will acquire new stereo-pairs from November 2013 to April 2014 in Antarctica and July-October 2014 in the Arctic regions.
Obtaining a uniform and precise topography of the remote Polar Regions is important to characterize their response to recent climate change, constrain ice modelling,quantify their contribution to sea-level rise and detect future evolutions. However, the topography of glaciers, ice caps and ice shelves is still poorly understood because in situ observations are sparse and difficult to obtain. Furthermore, spaceborne measurements of ice elevation are also challenging and not always a priority. To build a reference ice topography during the fourth International Polar Year (IPY), the French space agency CNES, Spot Image (now Airbus Defence & Space), IGN Espace and LEGOS launched the SPIRIT project.
The aims of the SPIRIT project were to build a comprehensive archive of SPOT 5 high-resolution stereoscopic (HRS) imagery of polar ice and, for selected regions, to produce a DEM and orthoimages that were delivered for free to the scientific community involved in IPY projects. Over 40 scientists in 20 different countries benefited from SPIRIT data.
The SPOT 5 HRS sensor is designed to generate DEMs by acquiring stereopairs of images along the satellite track. It is composed of two telescopes, one pointing 20° forward and one pointing 20° aft relative to nadir. The short time interval between the two stereopair scenes (90 seconds) ensures very small changes at the glacier surface between acquisitions and nearly identical solar illumination so that the radiometry of the two images is similar.
The target areas were the margins of the Greenland and Antarctic ice sheets and most glaciers, icefields
and ice caps surrounding the Arctic Ocean and Antarctica. One major constraint was the 81.15° North – 81.15° South acquisition limits of the SPOT 5 satellite. The flat, uniform, snow-covered central regions of the Antarctic and Greenland ice sheets were deliberately excluded because DEMs produced by matching optical stereoimages generally contain large data gaps over uniform regions and would not attain the 10-metre accuracy possible with spaceborne radar or laser altimeters.
About 75% of the targeted areas were covered with cloud-free images. Cloud-free images were acquired over 1.6x106 km² in the Arctic (Fig. 1) and 4.4x106 km² in Antarctica. DEMs were derived over 1.3x106 km² in each hemisphere. Images showing the IPY stereoimages and DEM coverage are shown below.
Korona J., Berthier E., Bernard M., Rémy F. & Thouvenot E. SPIRIT. SPOT 5 stereoscopic survey of Polar Ice: Reference Images and Topographies during the fourth International Polar Year (2007-2009). ISPRS Journal of Photogrammetry & Remote Sensing, 64, 204-212, doi: 10.1016/j.isprsjprs.2008.10.005, 2009 (pdf)