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Integrating a Hybrid Back Propagation Neural Network and Particle Swarm Optimization for Estimating Soil Heavy Metal Contents Using Hyperspectral Data

by Piao Liu 1, Zhenhua Liu 1,2,*, Yueming Hu 1,2,*, Zhou Shi 3, Yuchun Pan 4, Lu Wang 1,2 and Guangxing Wang 1,5
1
College of Natural Resources and Environment, South China Agricultural University, Guangzhou 510642, China
2
Guangdong Provincial Key Laboratory of Land Use and Consolidation, Guangzhou 510642, China
3
Institute of Agricultural Remote Sensing and Information Technology Application, Zhejiang University, Hangzhou 310058, China
4
National Engineering Research Center for Information Technology in Agriculture, Beijing 100089, China
5
Department of Geography and Environmental Resources, Southern Illinois University, Carbondale, IL 62901, USA
*
Authors to whom correspondence should be addressed.
Sustainability 2019, 11(2), 419; https://doi.org/10.3390/su11020419
Received: 5 November 2018 / Revised: 8 January 2019 / Accepted: 11 January 2019 / Published: 15 January 2019
Soil heavy metals affect human life and the environment, and thus, it is very necessary to monitor their contents. Substantial research has been conducted to estimate and map soil heavy metals in large areas using hyperspectral data and machine learning methods (such as neural network), however, lower estimation accuracy is often obtained. In order to improve the estimation accuracy, in this study, a back propagation neural network (BPNN) was combined with the particle swarm optimization (PSO), which led to an integrated PSO-BPNN method used to estimate the contents of soil heavy metals: Cd, Hg, and As. This study was conducted in Guangdong, China, based on the soil heavy metal contents and hyperspectral data collected from 90 soil samples. The prediction accuracies from BPNN and PSO-BPNN were compared using field observations. The results showed that, 1) the sample averages of Cd, Hg, and As were 0.174 mg/kg, 0.132 mg/kg, and 9.761 mg/kg, respectively, with the corresponding maximum values of 0.570 mg/kg, 0.310 mg/kg, and 68.600 mg/kg being higher than the environment baseline values; 2) the transformed and combined spectral variables had higher correlations with the contents of the soil heavy metals than the original spectral data; 3) PSO-BPNN significantly improved the estimation accuracy of the soil heavy metal contents, with the decrease in the mean relative error (MRE) and relative root mean square error (RRMSE) by 68% to 71%, and 64% to 67%, respectively. This indicated that the PSO-BPNN provided great potential to estimate the soil heavy metal contents; and 4) with the PSO-BPNN, the Cd content could also be mapped using HuanJing-1A Hyperspectral Imager (HSI) data with a RRMSE value of 36%, implying that the PSO-BPNN method could be utilized to map the heavy metal content in soil, using both field spectral data and hyperspectral imagery for the large area. View Full-Text
Keywords: heavy metal; PSO-BPNN Method, soil sample; HJ-1A Hyper Spectral Imager; Guangdong heavy metal; PSO-BPNN Method, soil sample; HJ-1A Hyper Spectral Imager; Guangdong
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Liu, P.; Liu, Z.; Hu, Y.; Shi, Z.; Pan, Y.; Wang, L.; Wang, G. Integrating a Hybrid Back Propagation Neural Network and Particle Swarm Optimization for Estimating Soil Heavy Metal Contents Using Hyperspectral Data. Sustainability 2019, 11, 419.

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