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A special issue of Remote Sensing (ISSN 2072-4292).

Deadline for manuscript submissions: closed (30 November 2020) | Viewed by 72151

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Special Issue Editors

Prof. Dr. Francisco Javier García-Haro

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Guest Editor

Earth Physics and Thermodynamics Department, Faculty of Physics, Universitat de València, 46100 Burjassot, València, Spain
Interests: biophysical parameters estimation; canopy reflectance models; spectral mixture analysis; classification; data fusion; validation
Special Issues, Collections and Topics in MDPI journals

Prof. Dr. Hongliang Fang

E-Mail Website
Guest Editor

LREIS, Institute of Geographic Sciences and Natural Resources Research Chinese Academy of Sciences, 11A Datun Road, Beijing 100101, China
Interests: optical remote sensing; biophysical parameter estimation; vegetation; radiative transfer modeling; calibration/validation; in situ measurements
Special Issues, Collections and Topics in MDPI journals

Dr. Manuel Campos-Taberner

E-Mail Website
Guest Editor

Earth Physics and Thermodynamics Department, Faculty of Physics, Universitat de València, 46100 Burjassot, València, Spain
Interests: biophysical parameter retrieval; machine learning; radiative transfer modelling; calibration/validation; classification; in situ measurements

Special Issue Information

Dear Colleagues,

Vegetation plays an essential role in the study of the environment through plant respiration and photosynthesis. Therefore, the assessment of the current vegetation status is critical to modeling terrestrial ecosystems and energy cycles. Canopy structure (LAI, fCover, plant height, biomass, leaf angle distribution) and biochemical parameters (leaf pigmentation and water content) have been employed to assess vegetation status and its dynamics at scales ranging from kilometric to decametric spatial resolutions thanks to methods based on remote sensing (RS) data.

Optical RS retrieval methods are based on radiative transfer processes of sunlight in vegetation, determining the amount of radiation measured by passive sensors in the visible and infrared channels. The increased availability of active RS (radar and LiDAR) data has fostered their use in many applications for analysis of land surface properties and processes, thanks also to their insensitivity to weather conditions and the capability to exploit rich structural and texture information. Optical and radar data fusion and multi-sensor integration approaches are pressing topics to fully exploit the information conveyed by both optical and microwave parts of the electromagnetic spectrum.

This Special Issue will review the state of the art in biophysical parameters retrieval and its usage in a wide variety of applications (e.g., ecology, carbon cycle, agriculture, forestry and food security). Articles covering recent research about the following topics are invited for this Special Issue:

Field methods to measure vegetation biophysical parameters.
Methods for the retrieval of canopy (e.g., LAI, fCover, fAPAR, plant height, biomass)and leaf biochemical (e.g., leaf chlorophyll/water and fuel moisture contents), as well as fuel/vegetation water content parameters from satellite and airborne sensors.
Construction and improvement of radiative transfer models, and input data needed for the retrieval of vegetation parameters.
Evaluations of recent missions (e.g., Sentinel-1, -2, and -3) to improve the spatial and temporal resolutions of retrieved biophysical maps.
LiDAR and microwave Remote Sensing.
Vegetation parameter retrieval from Unmanned Autonomous Vehicle (UAV) and high-resolution data.
Processing of big remote sensing data, e.g., data fusion and multi-sensor data integration techniques
Development of operational biophysical products and services using remote sensing.
Calibration/validation activities for biophysical parameter products.
Methods to estimate vegetation status and condition (e.g., vegetation diseases, stress condition, forest disturbance, degradation and regrowth).
Assimilation of biophysical parameters derived from remote sensing for agriculture and forestry applications.

Review articles covering one or more of these topics

Prof. Francisco Javier García-Haro
Prof. Dr. Hongliang Fang
Dr. Manuel Campos-Taberner
Guest Editors

Manuscript Submission Information

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Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Remote Sensing is an international peer-reviewed open access semimonthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2700 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

Biophysical parameters estimation
Radiative transfer modelling
Optical and microwave
Calibration/Validation

Published Papers (12 papers)

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18 pages, 2054 KiB

Open AccessArticle

Urban Fire Severity and Vegetation Dynamics in Southern California

by Lauren E. H. Mathews and Alicia M. Kinoshita

Remote Sens. 2021, 13(1), 19; https://0-doi-org.brum.beds.ac.uk/10.3390/rs13010019 - 23 Dec 2020

Cited by 7 | Viewed by 3224

Abstract

A combination of satellite image indices and in-field observations was used to investigate the impact of fuel conditions, fire behavior, and vegetation regrowth patterns, altered by invasive riparian vegetation. Satellite image metrics, differenced normalized burn severity (dNBR) and differenced normalized difference vegetation index (dNDVI), were approximated for non-native, riparian, or upland vegetation for traditional timeframes (0-, 1-, and 3-years) after eleven urban fires across a spectrum of invasive vegetation cover. Larger burn severity and loss of green canopy (NDVI) was detected for riparian areas compared to the uplands. The presence of invasive vegetation affected the distribution of burn severity and canopy loss detected within each fire. Fires with native vegetation cover had a higher severity and resulted in larger immediate loss of canopy than fires with substantial amounts of non-native vegetation. The lower burn severity observed 1–3 years after the fires with non-native vegetation suggests a rapid regrowth of non-native grasses, resulting in a smaller measured canopy loss relative to native vegetation immediately after fire. This observed fire pattern favors the life cycle and perpetuation of many opportunistic grasses within urban riparian areas. This research builds upon our current knowledge of wildfire recovery processes and highlights the unique challenges of remotely assessing vegetation biophysical status within urban Mediterranean riverine systems. Full article

(This article belongs to the Special Issue Remote Sensing of Biophysical Parameters)

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17 pages, 3929 KiB

Open AccessArticle

Evaluation of the MODIS LAI/FPAR Algorithm Based on 3D-RTM Simulations: A Case Study of Grassland

by Jiabin Pu, Kai Yan, Guohuan Zhou, Yongqiao Lei, Yingxin Zhu, Donghou Guo, Hanliang Li, Linlin Xu, Yuri Knyazikhin and Ranga B. Myneni

Remote Sens. 2020, 12(20), 3391; https://0-doi-org.brum.beds.ac.uk/10.3390/rs12203391 - 16 Oct 2020

Cited by 30 | Viewed by 3565

Abstract

Uncertainty assessment of the moderate resolution imaging spectroradiometer (MODIS) leaf area index (LAI) and the fraction of photosynthetically active radiation absorbed by vegetation (FPAR) retrieval algorithm can provide a scientific basis for the usage and improvement of this widely-used product. Previous evaluations generally depended on the intercomparison with other datasets as well as direct validation using ground measurements, which mix the uncertainties from the model, inputs, and assessment method. In this study, we adopted the evaluation method based on three-dimensional radiative transfer model (3D RTM) simulations, which helps to separate model uncertainty and other factors. We used the well-validated 3D RTM LESS (large-scale remote sensing data and image simulation framework) for a grassland scene simulation and calculated bidirectional reflectance factors (BRFs) as inputs for the LAI/FPAR retrieval. The dependency between LAI/FPAR truth and model estimation serves as the algorithm uncertainty indicator. This paper analyzed the LAI/FPAR uncertainty caused by inherent model uncertainty, input uncertainty (BRF and biome classification), clumping effect, and scale dependency. We found that the uncertainties of different algorithm paths vary greatly (−6.61% and +84.85% bias for main and backup algorithm, respectively) and the “hotspot” geometry results in greatest retrieval uncertainty. For the input uncertainty, the BRF of the near-infrared (NIR) band has greater impacts than that of the red band, and the biome misclassification also leads to nonnegligible LAI/FPAR bias. Moreover, the clumping effect leads to a significant LAI underestimation (−0.846 and −0.525 LAI difference for two clumping types), but the scale dependency (pixel size ranges from 100 m to 1000 m) has little impact on LAI/FPAR uncertainty. Overall, this study provides a new perspective on the evaluation of LAI/FPAR retrieval algorithms. Full article

(This article belongs to the Special Issue Remote Sensing of Biophysical Parameters)

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29 pages, 769 KiB

Open AccessArticle

Assessment of Workflow Feature Selection on Forest LAI Prediction with Sentinel-2A MSI, Landsat 7 ETM+ and Landsat 8 OLI

by Benjamin Brede, Jochem Verrelst, Jean-Philippe Gastellu-Etchegorry, Jan G. P. W. Clevers, Leo Goudzwaard, Jan den Ouden, Jan Verbesselt and Martin Herold

Remote Sens. 2020, 12(6), 915; https://0-doi-org.brum.beds.ac.uk/10.3390/rs12060915 - 12 Mar 2020

Cited by 39 | Viewed by 4871

Abstract

The European Space Agency (ESA)’s Sentinel-2A (S2A) mission is providing time series that allow the characterisation of dynamic vegetation, especially when combined with the National Aeronautics and Space Administration (NASA)/United States Geological Survey (USGS) Landsat 7 (L7) and Landsat 8 (L8) missions. Hybrid retrieval workflows combining non-parametric Machine Learning Regression Algorithms (MLRAs) and vegetation Radiative Transfer Models (RTMs) were proposed as fast and accurate methods to infer biophysical parameters such as Leaf Area Index (LAI) from these data streams. However, the exact design of optimal retrieval workflows is rarely discussed. In this study, the impact of five retrieval workflow features on LAI prediction performance of MultiSpectral Instrument (MSI), Enhanced Thematic Mapper Plus (ETM+) and Operational Land Imager (OLI) observations was analysed over a Dutch beech forest site for a one-year period. The retrieval workflow features were the (1) addition of prior knowledge of leaf chemistry (two alternatives), (2) the choice of RTM (two alternatives), (3) the addition of Gaussian noise to RTM produced training data (four and five alternatives), (4) possibility of using Sun Zenith Angle (SZA) as an additional MLRA training feature (two alternatives), and (5) the choice of MLRA (six alternatives). The features were varied in a full grid resulting in 960 inversion models in order to find the overall impact on performance as well as possible interactions among the features. A combination of a Terrestrial Laser Scanning (TLS) time series with litter-trap derived LAI served as independent validation. The addition of absolute noise had the most significant impact on prediction performance. It improved the median prediction Root Mean Square Error (RMSE) by 1.08 m² m⁻² when 5% noise was added compared to inversions with 0% absolute noise. The choice of the MLRA was second most important in terms of median prediction performance, which differed by 0.52 m² m⁻² between the best and worst model. The best inversion model achieved an RMSE of 0.91 m² m⁻² and explained 84.9% of the variance of the reference time series. The results underline the need to explicitly describe the used noise model in future studies. Similar studies should be conducted in other study areas, both forest and crop systems, in order to test the noise model as an integral part of hybrid retrieval workflows. Full article

(This article belongs to the Special Issue Remote Sensing of Biophysical Parameters)

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19 pages, 3430 KiB

Open AccessArticle

Assessment of UAV-Onboard Multispectral Sensor for Non-Destructive Site-Specific Rapeseed Crop Phenotype Variable at Different Phenological Stages and Resolutions

by Sadeed Hussain, Kaixiu Gao, Mairaj Din, Yongkang Gao, Zhihua Shi and Shanqin Wang

Remote Sens. 2020, 12(3), 397; https://0-doi-org.brum.beds.ac.uk/10.3390/rs12030397 - 26 Jan 2020

Cited by 16 | Viewed by 3118

Abstract

Unmanned aerial vehicles (UAVs) equipped with spectral sensors have become useful in the fast and non-destructive assessment of crop growth, endurance and resource dynamics. This study is intended to inspect the capabilities of UAV-onboard multispectral sensors for non-destructive phenotype variables, including leaf area index (LAI), leaf mass per area (LMA) and specific leaf area (SLA) of rapeseed oil at different growth stages. In addition, the raw image data with high ground resolution (20 cm) were resampled to 30, 50 and 100 cm to determine the influence of resolution on the estimation of phenotype variables by using vegetation indices (VIs). Quadratic polynomial regression was applied to the quantitative analysis at different resolutions and growth stages. The coefficient of determination (R²) and root mean square error results indicated the significant accuracy of the LAI estimation, wherein the highest R² values were attained by RVI = 0.93 and MTVI2 = 0.89 at the elongation stage. The noise equivalent of sensitivity and uncertainty analyses at the different growth stages accounted for the sensitivity of VIs, which revealed the optimal VIs of RVI, MTVI2 and MSAVI in the LAI estimation. LMA and SLA, which showed significant accuracies at (R² = 0.85, 0.81) and (R² = 0.85, 0.71), were estimated on the basis of the predicted leaf dry weight and LAI at the elongation and flowering stages, respectively. No significant variations were observed in the measured regression coefficients using different resolution images. Results demonstrated the significant potential of UAV-onboard multispectral sensor and empirical method for the non-destructive retrieval of crop canopy variables. Full article

(This article belongs to the Special Issue Remote Sensing of Biophysical Parameters)

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19 pages, 4039 KiB

Open AccessArticle

Retrieval of the Fraction of Radiation Absorbed by Photosynthetic Components (FAPAR_green) for Forest Using a Triple-Source Leaf-Wood-Soil Layer Approach

by Siyuan Chen, Liangyun Liu, Xiao Zhang, Xinjie Liu, Xidong Chen, Xiaojin Qian, Yue Xu and Donghui Xie

Remote Sens. 2019, 11(21), 2471; https://0-doi-org.brum.beds.ac.uk/10.3390/rs11212471 - 23 Oct 2019

Cited by 10 | Viewed by 2891

Abstract

The fraction of absorbed photosynthetically active radiation (FAPAR) is generally divided into the fraction of radiation absorbed by the photosynthetic components (

F A P A R_{g r e e n}

) and the fraction of radiation absorbed by the non-photosynthetic components [...] Read more.

The fraction of absorbed photosynthetically active radiation (FAPAR) is generally divided into the fraction of radiation absorbed by the photosynthetic components (

F A P A R_{g r e e n}

) and the fraction of radiation absorbed by the non-photosynthetic components (

F A P A R_{w o o d y}

) of the vegetation. However, most global FAPAR datasets do not take account of the woody components when considering the canopy radiation transfer. The objective of this study was to develop a generic algorithm for partitioning

F A P A R_{c a n o p y}

into

F A P A R_{g r e e n}

and

F A P A R_{w o o d y}

based on a triple-source leaf-wood-soil layer (TriLay) approach. The LargE-Scale remote sensing data and image simulation framework (LESS) model was used to validate the TriLay approach. The results showed that the TriLay

F A P A R_{g r e e n}

had higher retrieval accuracy, as well as a significantly lower bias (R² = 0.937, Root Mean Square Error (RMSE) = 0.064, and bias = −6.02% for black-sky conditions; R² = 0.997, RMSE = 0.025 and bias = −4.04% for white-sky conditions) compared to the traditional linear method (R² = 0.979, RMSE = 0.114, and bias = −18.04% for black-sky conditions; R² = 0.996, RMSE = 0.106 and bias = −16.93% for white-sky conditions). For FAPAR that did not take account of woody components (

F A P A R_{n o W A I}

), the corresponding results were R² = 0.920, RMSE = 0.071, and bias = −7.14% for black-sky conditions, and R² = 0.999, RMSE = 0.043, and bias = −6.41% for white-sky conditions. Finally, the dynamic

F A P A R_{g r e e n}

F A P A R_{w o o d y}

F A P A R_{c a n o p y}

and

F A P A R_{n o W A I}

products for a North America region were generated at a resolution of 500 m for every eight days in 2017. A comparison of the results for

F A P A R_{g r e e n}

against those for

F A P A R_{n o W A I}

and

F A P A R_{c a n o p y}

showed that the discrepancy between

F A P A R_{g r e e n}

and other FAPAR products for forest vegetation types could not be ignored. For deciduous needleleaf forest, in particular, the black-sky

F A P A R_{g r e e n}

was found to contribute only about 23.86% and 35.75% of

F A P A R_{c a n o p y}

at the beginning and end of the year (from January to March and October to December, JFM and OND), and 75.02% at the peak growth stage (from July to September, JAS); the black-sky

F A P A R_{n o W A I}

was found to be overestimated by 38.30% and 28.46% during the early (JFM) and late (OND) part of the year, respectively. Therefore, the TriLay approach performed well in separating

F A P A R_{g r e e n}

from

F A P A R_{c a n o p y}

, which is of great importance for a better understanding of the energy exchange within the canopy. Full article

(This article belongs to the Special Issue Remote Sensing of Biophysical Parameters)

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26 pages, 8780 KiB

Open AccessFeature PaperArticle

Climate Data Records of Vegetation Variables from Geostationary SEVIRI/MSG Data: Products, Algorithms and Applications

by Francisco Javier García-Haro, Fernando Camacho, Beatriz Martínez, Manuel Campos-Taberner, Beatriz Fuster, Jorge Sánchez-Zapero and María Amparo Gilabert

Remote Sens. 2019, 11(18), 2103; https://0-doi-org.brum.beds.ac.uk/10.3390/rs11182103 - 09 Sep 2019

Cited by 21 | Viewed by 3732

Abstract

The scientific community requires long-term data records with well-characterized uncertainty and suitable for modeling terrestrial ecosystems and energy cycles at regional and global scales. This paper presents the methodology currently developed in EUMETSAT within its Satellite Application Facility for Land Surface Analysis (LSA SAF) to generate biophysical variables from the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) on board MSG 1-4 (Meteosat 8-11) geostationary satellites. Using this methodology, the LSA SAF generates and disseminates at a time a suite of vegetation products, such as the leaf area index (LAI), the fraction of the photosynthetically active radiation absorbed by vegetation (FAPAR) and the fractional vegetation cover (FVC), for the whole Meteosat disk at two temporal frequencies, daily and 10-days. The FVC algorithm relies on a novel stochastic spectral mixture model which addresses the variability of soils and vegetation types using statistical distributions whereas the LAI and FAPAR algorithms use statistical relationships general enough for global applications. An overview of the LSA SAF SEVIRI/MSG vegetation products, including expert knowledge and quality assessment of its internal consistency is provided. The climate data record (CDR) is freely available in the LSA SAF, offering more than fifteen years (2004-present) of homogeneous time series required for climate and environmental applications. The high frequency and good temporal continuity of SEVIRI products addresses the needs of near-real-time users and are also suitable for long-term monitoring of land surface variables. The study also evaluates the potential of the SEVIRI/MSG vegetation products for environmental applications, spanning from accurate monitoring of vegetation cycles to resolving long-term changes of vegetation. Full article

(This article belongs to the Special Issue Remote Sensing of Biophysical Parameters)

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21 pages, 3298 KiB

Open AccessArticle

Lead-Induced Changes in Fluorescence and Spectral Characteristics of Pea Leaves

by Marlena Kycko, Elżbieta Romanowska and Bogdan Zagajewski

Remote Sens. 2019, 11(16), 1885; https://0-doi-org.brum.beds.ac.uk/10.3390/rs11161885 - 12 Aug 2019

Cited by 8 | Viewed by 3497

Abstract

Chlorophyll fluorescence parameters can provide useful indications of photosynthetic performance in vivo. Coupling appropriate fluorescence measurements with other noninvasive techniques, such as absorption spectroscopy or gas exchange, can provide insights into the limitations to photosynthesis under given conditions. Chlorophyll content is one of the dominant factors influencing the conditions of a vegetation growing season, and can be tested using both fluorescence and remote sensing methods. Hyperspectral remote sensing and recording the narrow range of the spectrum can be used to accurately analyze the parameters and properties of plants. The aim of this study was to analyze the influence of lead ions (Pb, 5 mM Pb(NO₃)₂) on the growth of pea plants using spectral properties. Hyperspectral remote sensing and chlorophyll fluorescence measurements were used to assess the physiological state of plants seedlings treated by lead ions during the experiment. The plants were growing in hydroponic cultures supplemented with Pb ions under various conditions (control, complete Knop + phosphorus (+P); complete Knop + phosphorus (+P) + Pb; Knop (-P) + Pb, distilled water + Pb) affecting lead uptake via the root system. Spectrometric measurements allowed us to calculate the remote sensing indices of vegetation, which were compared with chlorophyll and carotenoids content and fluorescence parameters. The lead contents in the leaves, roots, and stems were also analyzed. Spectral characteristics and vegetation properties were analyzed using statistical tests. We conclude that: (1) pea seedlings grown in complete Knop (with P) and in the presence of Pb ions were spectrally similar to the control plants because lead was not transported to the shoots of plants; (2) lead most influenced plants that were grown in water, according to the highest lead content in the leaves; and (3) the effects of lead on plant growth were confirmed by remote sensing indices, whereas fluorescence parameters identified physiological changes induced by Pb ions in the plants. Full article

(This article belongs to the Special Issue Remote Sensing of Biophysical Parameters)

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17 pages, 2201 KiB

Open AccessArticle

Estimating Forest Leaf Area Index and Canopy Chlorophyll Content with Sentinel-2: An Evaluation of Two Hybrid Retrieval Algorithms

by Luke A. Brown, Booker O. Ogutu and Jadunandan Dash

Remote Sens. 2019, 11(15), 1752; https://0-doi-org.brum.beds.ac.uk/10.3390/rs11151752 - 25 Jul 2019

Cited by 38 | Viewed by 5150

Abstract

Estimates of biophysical and biochemical variables such as leaf area index (LAI) and canopy chlorophyll content (CCC) are a fundamental requirement for effectively monitoring and managing forest environments. With its red-edge bands and high spatial resolution, the Multispectral Instrument (MSI) on board the Sentinel-2 missions is particularly well-suited to LAI and CCC retrieval. Using field data collected throughout the growing season at a deciduous broadleaf forest site in Southern England, we evaluated the performance of two hybrid retrieval algorithms for estimating LAI and CCC from MSI data: the Scattering by Arbitrarily Inclined Leaves (SAIL)-based L2B retrieval algorithm made available to users in the Sentinel Application Platform (SNAP), and an alternative retrieval algorithm optimised for forest environments, trained using the Invertible Forest Reflectance Model (INFORM). Moderate performance was associated with the SNAP L2B retrieval algorithm for both LAI (r² = 0.54, RMSE = 1.55, NRMSE = 43%) and CCC (r² = 0.52, RMSE = 0.79 g m⁻², NRMSE = 45%), while improvements were obtained using the INFORM-based retrieval algorithm, particularly in the case of LAI (r² = 0.79, RMSE = 0.47, NRMSE = 13%), but also in the case of CCC (r² = 0.69, RMSE = 0.52 g m⁻², NRMSE = 29%). Forward modelling experiments confirmed INFORM was better able to reproduce observed MSI spectra than SAIL. Based on our results, for forest-related applications using MSI data, we recommend users seek retrieval algorithms optimised for forest environments. Full article

(This article belongs to the Special Issue Remote Sensing of Biophysical Parameters)

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24 pages, 4361 KiB

Open AccessArticle

A Simplified and Robust Surface Reflectance Estimation Method (SREM) for Use over Diverse Land Surfaces Using Multi-Sensor Data

by Muhammad Bilal, Majid Nazeer, Janet E. Nichol, Max P. Bleiweiss, Zhongfeng Qiu, Evelyn Jäkel, James R. Campbell, Luqman Atique, Xiaolan Huang and Simone Lolli

Remote Sens. 2019, 11(11), 1344; https://0-doi-org.brum.beds.ac.uk/10.3390/rs11111344 - 04 Jun 2019

Cited by 63 | Viewed by 12474

Abstract

Surface reflectance (SR) estimation is the most critical preprocessing step for deriving geophysical parameters in multi-sensor remote sensing. Most state-of-the-art SR estimation methods, such as the vector version of the Second Simulation of the Satellite Signal in the Solar Spectrum (6SV) radiative transfer (RT) model, depend on accurate information on aerosol and atmospheric gases. In this study, a Simplified and Robust Surface Reflectance Estimation Method (SREM) based on the equations from 6SV RT model, without integrating information of aerosol particles and atmospheric gasses, is proposed and tested using Landsat 5 Thematic Mapper (TM), Landsat 7 Enhanced Thematic Mapper plus (ETM+), and Landsat 8 Operational Land Imager (OLI) data from 2000 to 2018. For evaluation purposes, (i) the SREM SR retrievals are validated against in situ SR measurements collected by Analytical Spectral Devices (ASD) from the South Dakota State University (SDSU) site, USA; (ii) cross-comparison between the SREM and Landsat spectral SR products, i.e., Landsat Ecosystem Disturbance Adaptive Processing System (LEDAPS) and Landsat 8 Surface Reflectance Code (LaSRC), are conducted over 11 urban (2013–2018), 13 vegetated (2013–2018), and 11 desert/arid (2000 to 2018) sites located over different climatic zones at a global scale; (iii) the performance of the SREM spectral SR retrievals for low to high aerosol loadings is evaluated; (iv) spatio-temporal cross-comparison is conducted for six Landsat paths/rows located in Asia, Africa, Europe, and the United States of America from 2013 to 2018 to consider a large variety of land surfaces and atmospheric conditions; (v) cross-comparison is also performed for the Normalized Difference Vegetation Index (NDVI), the Enhanced Vegetation Index (EVI), and the Soil Adjusted Vegetation Index (SAVI) calculated from both the SREM and Landsat SR data; (vi) the SREM is also applied to the Sentinel-2A and Moderate Resolution Imaging Spectrometer (MODIS) data to explore its applicability; and (vii) errors in the SR retrievals are reported using the mean bias error (MBE), root mean squared deviation (RMSD), and mean systematic error (MSE). Results depict significant and strong positive Pearson’s correlation (r), small MBE, RMSD, and MSE for each spectral band against in situ ASD data and Landsat (LEDAPS and LaSRC) SR products. Consistency in SREM performance against Sentinel-2A (r = 0.994, MBE = −0.009, and RMSD = 0.014) and MODIS (r = 0.925, MBE = 0.007, and RMSD = 0.014) data suggests that SREM can be applied to other multispectral satellites data. Overall, the findings demonstrate the potential and promise of SREM for use over diverse surfaces and under varying atmospheric conditions using multi-sensor data on a global scale. Full article

(This article belongs to the Special Issue Remote Sensing of Biophysical Parameters)

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22 pages, 4333 KiB

Open AccessArticle

A Comparison of Hybrid Machine Learning Algorithms for the Retrieval of Wheat Biophysical Variables from Sentinel-2

by Deepak Upreti, Wenjiang Huang, Weiping Kong, Simone Pascucci, Stefano Pignatti, Xianfeng Zhou, Huichun Ye and Raffaele Casa

Remote Sens. 2019, 11(5), 481; https://0-doi-org.brum.beds.ac.uk/10.3390/rs11050481 - 26 Feb 2019

Cited by 96 | Viewed by 11396

Abstract

This study focuses on the comparison of hybrid methods of estimation of biophysical variables such as leaf area index (LAI), leaf chlorophyll content (LCC), fraction of absorbed photosynthetically active radiation (FAPAR), fraction of vegetation cover (FVC), and canopy chlorophyll content (CCC) from Sentinel-2 satellite data. Different machine learning algorithms were trained with simulated spectra generated by the physically-based radiative transfer model PROSAIL and subsequently applied to Sentinel-2 reflectance spectra. The algorithms were assessed against a standard operational approach, i.e., the European Space Agency (ESA) Sentinel Application Platform (SNAP) toolbox, based on neural networks. Since kernel-based algorithms have a heavy computational cost when trained with large datasets, an active learning (AL) strategy was explored to try to alleviate this issue. Validation was carried out using ground data from two study sites: one in Shunyi (China) and the other in Maccarese (Italy). In general, the performance of the algorithms was consistent for the two study sites, though a different level of accuracy was found between the two sites, possibly due to slightly different ground sampling protocols and the range and variability of the values of the biophysical variables in the two ground datasets. For LAI estimation, the best ground validation results were obtained for both sites using least squares linear regression (LSLR) and partial least squares regression, with the best performances values of R² of 0.78, rott mean squared error (RMSE) of 0.68 m² m⁻² and a relative RMSE (RRMSE) of 19.48% obtained in the Maccarese site with LSLR. The best results for LCC were obtained using Random Forest Tree Bagger (RFTB) and Bagging Trees (BagT) with the best performances obtained in Maccarese using RFTB (R² = 0.26, RMSE = 8.88 μg cm⁻², RRMSE = 17.43%). Gaussian Process Regression (GPR) was the best algorithm for all variables only in the cross-validation phase, but not in the ground validation, where it ranked as the best only for FVC in Maccarese (R² = 0.90, RMSE = 0.08, RRMSE = 9.86%). It was found that the AL strategy was more efficient than the random selection of samples for training the GPR algorithm. Full article

(This article belongs to the Special Issue Remote Sensing of Biophysical Parameters)

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17 pages, 6356 KiB

Open AccessArticle

Physically-Based Retrieval of Canopy Equivalent Water Thickness Using Hyperspectral Data

by Matthias Wocher, Katja Berger, Martin Danner, Wolfram Mauser and Tobias Hank

Remote Sens. 2018, 10(12), 1924; https://0-doi-org.brum.beds.ac.uk/10.3390/rs10121924 - 30 Nov 2018

Cited by 38 | Viewed by 4775

Abstract

Quantitative equivalent water thickness on canopy level (EWT_canopy) is an important land surface variable and retrieving EWT_canopy from remote sensing has been targeted by many studies. However, the effect of radiative penetration into the canopy has not been fully understood. Therefore, in this study the Beer-Lambert law is applied to inversely determine water content information in the 930 to 1060 nm range of canopy reflectance from measured winter wheat and corn spectra collected in 2015, 2017, and 2018. The spectral model was calibrated using a look-up-table (LUT) of 50,000 PROSPECT spectra. Internal model validation was performed using two leaf optical properties datasets (LOPEX93 and ANGERS). Destructive in-situ measurements of water content were collected separately for leaves, stalks, and fruits. Correlation between measured and modelled water content was most promising for leaves and ears in case of wheat, reaching coefficients of determination (R²) up to 0.72 and relative RMSE (rRMSE) of 26% and in case of corn for the leaf fraction only (R² = 0.86, rRMSE = 23%). These findings indicate that, depending on the crop type and its structure, different parts of the canopy are observed by optical sensors. The results from the Munich-North-Isar test sites indicated that plant compartment specific EWT_canopy allows us to deduce more information about the physical meaning of model results than from equivalent water thickness on leaf level (EWT) which is upscaled to canopy water content (CWC) by multiplication of the leaf area index (LAI). Therefore, it is suggested to collect EWT_canopy data and corresponding reflectance for different crop types over the entire growing cycle. Nevertheless, the calibrated model proved to be transferable in time and space and thus can be applied for fast and effective retrieval of EWT_canopy in the scope of future hyperspectral satellite missions. Full article

(This article belongs to the Special Issue Remote Sensing of Biophysical Parameters)

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Estimation of LAI with the LiDAR Technology: A Review

by Yao Wang and Hongliang Fang

Remote Sens. 2020, 12(20), 3457; https://0-doi-org.brum.beds.ac.uk/10.3390/rs12203457 - 21 Oct 2020

Cited by 55 | Viewed by 11059

Abstract

Leaf area index (LAI) is an important vegetation parameter. Active light detection and ranging (LiDAR) technology has been widely used to estimate vegetation LAI. In this study, LiDAR technology, LAI retrieval and validation methods, and impact factors are reviewed. First, the paper introduces types of LiDAR systems and LiDAR data preprocessing methods. After introducing the application of different LiDAR systems, LAI retrieval methods are described. Subsequently, the review discusses various LiDAR LAI validation schemes and limitations in LiDAR LAI validation. Finally, factors affecting LAI estimation are analyzed. The review presents that LAI is mainly estimated from LiDAR data by means of the correlation with the gap fraction and contact frequency, and also from the regression of forest biophysical parameters derived from LiDAR. Terrestrial laser scanning (TLS) can be used to effectively estimate the LAI and vertical foliage profile (VFP) within plots, but this method is affected by clumping, occlusion, voxel size, and woody material. Airborne laser scanning (ALS) covers relatively large areas in a spatially contiguous manner. However, the capability of describing the within-canopy structure is limited, and the accuracy of LAI estimation with ALS is affected by the height threshold and sampling size, and types of return. Spaceborne laser scanning (SLS) provides the global LAI and VFP, and the accuracy of estimation is affected by the footprint size and topography. The use of LiDAR instruments for the retrieval of the LAI and VFP has increased; however, current LiDAR LAI validation studies are mostly performed at local scales. Future research should explore new methods to invert LAI and VFP from LiDAR and enhance the quantitative analysis and large-scale validation of the parameters. Full article

(This article belongs to the Special Issue Remote Sensing of Biophysical Parameters)

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