Evaluating the potential of multi-temporal Sentinel-1 and Sentinel-2 data for regional mapping of olive trees

ABSTRACT

Olives are a crucial economic crop in Mediterranean countries. Detailed spatial information on the distribution and condition of crops at regional and national scales is essential to ensure the continuity of crop quality and yield efficiency. However, most earlier studies on olive tree mapping focused mainly on small parcels using single-sensor, very high resolution (VHR) data, which is time-consuming, expensive and cannot feasibly be scaled up to a larger area. Therefore, we evaluated the performance of Sentinel-1 and Sentinel-2 data fusion for the regional mapping of olive trees for the first time, using the Izmir Province of Türkiye, an ancient olive-growing region, as a case study. Three different monthly composite images reflecting the different phenological stages of olive trees were selected to separate olive trees from other land cover types. Seven land-cover classes, including olives, were mapped separately using a random forest classifier for each year between 2017 and 2021. The results were assessed using the k-fold cross-validation method, and the final olive tree map of Izmir was produced by combining the olive tree distribution over two consecutive years. District-level areas covered by olive trees were calculated and validated using official statistics from the Turkish Statistical Institute (TUIK). The K-fold cross-validation accuracy varied from 94% to 95% between 2017 and 2021, and the final olive map achieved 98% overall accuracy with 93% producer accuracy for the olive class. The district-level olive area was strongly related to the TUIK statistics (R² = 0.60, NRMSE = 0.64). This study used Sentinel data and Google Earth Engine (GEE) to produce a regional-scale olive distribution map that can be scaled up to the entire country and replicated elsewhere. This map can, therefore, be used as a foundation for other scientific studies on olive trees, particularly for the development of effective management practices.

KEYWORDS:

• Olive classification
• Sentinel optical and radar data
• data fusion
• random forest
• Google Earth Engine

Acknowledgements

The corresponding author appreciates the financial support from TÜBİTAK during his one-year research at the University of Southampton, UK. The authors would like to thank the Izmir Olive Research Institute for providing phenological information on olive trees, and Xuerui Guo for her contribution to the determination of the phenological stages of olive trees from Sentinel-2 data. The research provided in this paper is part of the corresponding author’s Ph.D. thesis work at the Graduate School of Istanbul Technical University (İTÜ).

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

Google Earth Engine (https://code.earthengine.google.com/) is a free and open platform. All the data, models and codes generated or used during this study are available from the corresponding author upon request.

Additional information

Funding

The corresponding author conducted this study as a result of his research at the University of Southampton, which was funded by the Turkish Scientific and Technological Research Council (TÜBİTAK) 2214-A fellowship programme with the number 1059B142000675.