Abstract
High resolution (10 m and 20 m) optical imagery satellite Sentinel-2 brings a new perspective to Earth observation. Its frequent revisit time enables monitoring the Earth surface with high reliability. Since Sentinel-2 data is provided free of charge by the European Space Agency, its mass use for variety of purposes is expected. Quality evaluation of Sentinel-2 data is thus necessary. Quality analysis in this experiment is based on comparison of Sentinel-2 imagery with reference data (orthophoto). From the possible set of features to compare (point features, texture lines, objects, etc.) line segments were chosen because visual analysis suggested that scale differences matter least for these features. The experiment was thus designed to compare long line segments (e.g. airstrips, roads, etc.) in both datasets as the most representative entities. Edge detection was applied to both images and corresponding edges were manually selected. The statistical parameter which describes the geometrical relation between different images (and between datasets in general) covering the same area is calculated as the distance between corresponding curves in two datasets. The experiment was conducted for two different test sites, Austria and Serbia. From 21 lines with a total length of ca. 120 km the average offset of 6.031 m (0.60 pixel of Sentinel-2) was obtained for Austria, whereas for Serbia the average offset of 12.720 m (1.27 pixel of Sentinel-2) was obtained out of 10 lines with a total length of ca. 38 km.
Highlights
Modern satellite missions whose aim is Earth observation provide researchers, organizations and individuals with new possibilities
Quality assessment is essential for Sentinel-2 orthorectified data since it is important as for any other orthophoto to be evaluated for geometric accuracy before acceptance (Greenfeld, 2001)
Having the whole territory of Austria covered by the orthophoto data, entities to be used in geometry analysis were chosen based on Sentinel-2 data covering only certain parts of Austria
Summary
Modern satellite missions whose aim is Earth observation provide researchers, organizations and individuals with new possibilities. Nowadays, when some of the distributors of satellite images have changed their distribution policy towards offering all data to the public free of charge, people do not need to worry about these issues anymore. People must be aware of the restrictions of the available products not to get blindfolded by a chance of a free access to them. Quality assessment is essential for Sentinel-2 orthorectified data since it is important as for any other orthophoto to be evaluated for geometric accuracy before acceptance (Greenfeld, 2001). Providers of satellite images often give very limited information about their product quality (Novák and Baltsavias, 2009). That is why testing of satellite image quality, especially by independent groups, is needed
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