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Abstract
The spatial and temporal distribution of trees has a large impact on human health and the environment through contributions to important climate mechanisms as well as commercial, recreational and social activities in society. A range of tree mapping methodologies has been presented in the literature, but tree cover estimates still differ widely between the individual datasets, and comparisons of the thematic accuracy of the resulting tree maps are rather scarce. The Copernicus Sentinel-2 satellites, which were launched in 2015 and 2017, have a combination of high spatial and temporal resolution. Given that this is a new satellite, a substantial amount of research on development of tree mapping algorithms as well as accuracy assessment of said algorithms have to be done in the years to come. To contribute to this process, a tree map produced through unsupervised classification was created for six Sentinel-2 tiles. The agreement between the tree map and the corresponding national forest inventory, as a function of the band combination chosen, was analysed and the thematic accuracy was assessed for two out of the six tiles. The results show that the highest agreement between the present tree map and the national forest inventory was found for bands 2, 3, 6 and 12. The present tree map has a relative difference in tree cover between 8% and 79% compared to previous estimates, but results are characterised by large scatter. Lastly, it is shown that the overall thematic accuracy of the present map is up to 90%, with the user’s accuracy ranging from 34.85% to 92.10%, and the producer’s accuracy ranging from 23.80% to 97.60% for the various thematic classes. This demonstrates that tree maps with high thematic accuracy can be produced from Sentinel-2. In the future the thematic accuracy can be increased even more through the use of temporal averaging in the mapping procedure, which will enable an accurate estimate of the European tree cover.
Highlights
Trees serve as a major carbon pool contributing to important feedback mechanisms to the earth’s climate (Bonan, 2008)
It is evident that the spatial distribution of trees and changes in the spatial distribution of trees over time has a large impact on human health and the environment
The calculation of the wall-to-wall kappa coefficient with the corresponding national forest inventory for all 8100 band combinations shows that the highest summed kappa coefficients generally are 2.7 to 2.8, where the theoretical maximum is 5.0 and the highest kappa-coefficient is found when using a combination of four bands (Table 2)
Summary
Trees serve as a major carbon pool contributing to important feedback mechanisms to the earth’s climate (Bonan, 2008). It has been demonstrated that location and abundance of trees are important in relation to the release of VOC (Arneth et al, 2011; van Meeningen et al., 2016) and their contribution towards production of secondary organic aerosols (Oderbolz et al, 2013; Tchepel et al, 2014) or PBA (Hernandez-Ceballos et al, 2011; Pauling et al, 2012). Despite reported high thematic accuracy for Corine Land Cover (Büttner and Maucha, 2006; Caetano et al, 2006) and Globcover (Defourny et al, 2009), large biases were found in these compared to the national dataset. Similar map comparison exercises have been carried out at European scale (Seebach et al, 2011a; Seebach et al, 2011b). A remote sensing approach was used in Kempeneers et al (2011), who mapped
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