In this study, we reported the inter-comparison of image fusion products against in-situ spectral measurements over a heterogeneous, cloudy rice paddy landscape during the whole growing season in 2017. The evaluation of fusion products against pixel-level ground measurements revealed: 1) fusion NDVI products in the mixed land cover were overestimated due to the positive bias of input data against ground NDVI, 2) fusion NDVI products in homogeneous rice paddy cover type were underestimated due to the negative bias of input data against ground NDVI, 3) Temporal dependence of blended fusion NDVI products is not clear in mixed land cover type, and 4) fusion products NDVI could underestimate the variations in vegetation activity. Moreover, 5) the strength of each fusion products were confirmed by inter-comparison of fusion products. 6) We expect our in-situ spectral measurement could be useful in quantifying uncertainty in image fusion products, improving fine resolution cropland mapping and monitoring, and choosing the algorithm depending on the research purpose.
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Supplementary Materials
Supplementary 1. Data Day-of-year (DOY) for time-series of NDVI (Except DOY 268 (Sentinel-2B), all Sentinel-2 data observed from Sentinel-2A)
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Supplementary 2. Day-of-year in the comparison of satellite products against ground NDVI (Underlined date: data was replaced to another data within +- 3 days / ‘*’: no data or cloud contaminated within +-3 days)
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239 243 238 * 243
248 246 246 * *
291 290 290 286 293
Supplementary 3. Day-of-year in the comparison of fusion products against ground NDVI (Underlined date: data was replaced to another data within +- 3 days / ‘*’: no data or cloud contaminated within +-3 days)
Ground FSDAF CESTEM STAIR ESTARFM
155 155 155 155 155
187 187 187 187 187
194 195 * 194 195
206 206 206 206 *
215 213 * 215 213
223 223 223 223 223
239 243 238 239 243
248 246 247 248 246
291 290 290 291 290
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Supplementary 4. Simple linear regression analysis of (a) ESTARFM NDVI, (b) FSDAF NDVI, (c) STAIR NDVI, and (d) CESTEM NDVI, against ground NDVI (Ground NDVI dates distinguished by color) with the same number of data points on same date.
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Supplementary 5. Scatter plot of fusion product NDVI to ground NDVI (Rice paddy land cover type: red dots, Mixed land cover type: green dots, and Plot 5: yellow dots) with the same number of data points on same date
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요약(국문 초록)
지표면이 이질적인 농경지를 대상으로 영상합성자료와 현장스펙트 럼측정 간의 상호비교
공주원(Juwon Kong) 생태조경학과(Landscape Architecture major)
다양한 규모의 생태계 역동성을 관찰하기 위해 중요한 시공간 해상도가 높은 영상에 대한 기술의 발전과 함께 그 정확도 평가가 요구된다. 기존의 영상합성 자료를 평가하는 연구들은 지표면이 이질적인 경관에서 체계적인 평가를 하기 알맞은 픽셀단위의 현장 스펙트럼 측정값이 결여되었다. 본 연구는 식물의 생장기 동안 지표면이 이질적인 벼논 경관에서 현존하는 네가지 영상합성 자료- ESTARFM 와 FSDAF, STAIR, CESTEM -를 현장 스펙트럼
측정값과 상호비교하였다. 영상합성의 NDV (정규식생지수)가
시공간적으로 변화하는 것에 대해 실험하였다. 영상합성의 NDVI 는 현장 NDVI 에 대해 알맞거나 높은 선형성을 보였고, 그 값들이 음의 방향으로 편향되었다 (0.73< R2 <0.93, -8 %<bias <-1.6%).
영상합성들의 NDVI 는 NDVI 변동성을 포착하였지만, 현장
NDVI 보다 낮은 변동성을 예측하였다. 이러한 결과로 현장자료에 비해 영상합성에 입력하는 자료의 값이 편향된 경우 영상합성 자료에 주는 영향은 표지피복의 분류에 따라 다르다는 것을 도출하였다. 또한 영상합성의 NDVI 자료는 식생활동의 변동성을 과소평가 할 수 있음을
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알 수 있었다. 기존의 연구결과와 달리 ESTARFM 과 FSDAF, STAIR 는 입력자료에 대한 시간적 의존도가 명확하게 관찰되지 않은 토지피복이 있었다. 현장 NDVI 와 비교를 통해 각 영상합성의 NDVI 의 강점을 확증하였다. 본 연구에 사용된 현장 스펙트럼 자료는 영상합성 자료의 불확실성을 평가하고, 경작지를 모니터링하기 적합한 영상자료를 향상시키고, 연구자의 목적에 맞게 영상합성기법을 선택하기에 유용할 것으로 전망된다.
주요어: 영상합성, 현장스펙트럼측정, 상호비교, 높은 시공간 영상, 정확도 평가, 정규식생지수
학번: 2017-26230
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Acknowledgements
First, I would like to thank my advisor, Prof. Youngryel Ryu of Department of Landscape Architecture and Rural System Engineering at Seoul National University. Whenever I ask about the writings to Prof. Ryu, he always provide not only the writing guidelines but also the decent comments about my research. He led me to the science world and the scientific network for next step of my career. I also acknowledge Prof. Xiaolin Zhu, Ph.D Rasmus Houborg, and Prof. Kaiyu Guan to provide fusion products for this study.
The chairman and member of the committee, Prof. Dongkun Lee and Prof. Heeyeun Yoon, encouraged me insightful comments and question throughout 2018. I would also like to acknowledge Ph.D.
Huang Yan as the second reader of my research results, and I am gratefully indebted to her comments on this study. I also want to thank all my current and previous lab members – Ph.D. Benjamin Dechant, Soyoun Kim, Jongmin Kim, Yourum Huang, Jeehwan Bae, Kaige Yang, Ph.D. Chongya Jiang, Yulin Yan, Jane Lee, and Yunah Lee – for constructive discussions and field works during the past two years.
Last, and the most importantly, I must express gratitude to my parents –Ph.D. Dukam Kong, Ph.D. Haesun Song- and my sister - Hyunwook Kong – for their support through prayers and
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encouragement. With their love and guidance, I could accomplish every step of my life as a son and student.
This research was funded by the National Research Foundation of Korea (NRF-2016M1A5A1901789).