Surface Reflectance Measurements Research Articles

Continuous Wavelet Analysis for Spectroscopic Determination of Subsurface Moisture and Water-Table Height in Northern Peatland Ecosystems

Climate change is altering the water-table (WT) height and near-surface moisture conditions in northern peatlands, which in turn both increases the susceptibility to fire and reduces the carbon sink capacity of these ecosystems. To further develop remote sensing-based measurements of peatland moisture characteristics, we employed coincident surface reflectance and moisture measurements in two Sphagnum moss-dominated peatland sites. We applied the Mexican hat continuous wavelet transform to the measured spectra to generate wavelet features and coefficients across a range of scales. Overall, wavelet analysis was an improvement over the previously tested spectral indices at both the study sites. Linear mixed effect models for WT height using wavelet features accounted for more of the variance with both an improved marginal $R^{\mathrm { {2}}}$ (29% greater) and a larger conditional $R^{\mathrm { {2}}}$ (21% greater) compared to the best performing spectral index. While spectral indices performed similarly with wavelet coefficients for moisture content measured at 3 cm depth, they performed poorly for volumetric moisture content measured at 7 cm depth. The current study also revealed the advantage of selecting the best subsets of wavelet features based upon genetic algorithm over a more widely used technique that selects features based on correlation scalograms. It also provided new insights into the significance of various spectral regions to detect WT alteration-induced vegetation change.

Online Monitoring of Laser-Generated XUV Radiation Spectra by Surface Reflectivity Measurements with Particle Detectors

In this contribution, we present a wavelength-sensitive method for the detection of extreme ultraviolet (XUV) photon energies between 30 eV and 120 eV. The method is based on 45° reflectivity from either a cesium iodide-coated or an uncoated metal surface, which directs the XUV beam onto an electron or ion detector and its signal is used to monitor the XUV beam. The benefits of our approach are a spectrally sensitive diagnosis of the XUV radiation at the interaction place of time-resolved XUV experiments and the detection of infrared leak light though metal filters in high-harmonic generation (HHG) experiments. Both features were tested using spectrally shaped XUV pulses from HHG in a capillary, and we have achieved excellent agreement with XUV spectrometer measurements and reflectivity calculations. Our obtained results are of interest for time-resolved XUV experiments presenting an additional diagnostic directly in the interaction region and for small footprint XUV beamline diagnostics.

Editors' Choice—Reaction Kinetics of Metal Deposition via Surface Limited Redox Replacement of Underpotentially Deposited Monolayer Studied by Surface Reflectivity and Open Circuit Potential Measurements

Presented work studies the relation between kinetics of metal deposition via surface limited redox replacement (SLRR) of underpotentially deposited (UPD) monolayer (ML) and experimental parameters of reaction solution such as meal ions concentrations and supporting electrolyte concentration. The model system is Au deposition on Au(111) via SLRR of Pb UPD ML. The rate constant of the SLRR reaction for different solution designs is determined from temporal change of electrode surface reflectivity and from the open circuit potential transients’ analysis. The obtained results show clearly that reaction kinetics of metal deposition via SLRR of UPD ML is significantly affected by the design of the reaction solution i.e. the UPD metal ion, depositing metal ion, and supporting electrolyte concentrations. The ten-fold change of concentration of either solution parameter produces approximately the same change in the value of the rate constants. The presented results have fundamental importance for the future development and application of the metal deposition via SLRR of UPD ML. They offer a link between the reaction solution design and expected trend in SLRR reaction rate, which transposes to successful control of deposition flux, nucleation density and resulting morphology of the deposit.

Perceptually Motivated BRDF Comparison using Single Image

Surface reflectance of real-world materials is now widely represented by the bidirectional reflectance distribution function (BRDF) and also by spatially varying representations such as SVBRDF and the bidirectional texture function (BTF). The raw surface reflectance measurements are typically compressed or fitted by analytical models, that always introduce a certain loss of accuracy. For its evaluation we need a distance function between a reference surface reflectance and its approximate version. Although some of the past techniques tried to reflect the perceptual sensitivity of human vision, they have neither optimized illumination and viewing conditions nor surface shape. In this paper, we suggest a new image-based methodology for comparing different anisotropic BRDFs. We use optimization techniques to generate a novel surface which has extensive coverage of incoming and outgoing light directions, while preserving its features and frequencies that are important for material appearance judgments. A single rendered image of such a surface along with simultaneously optimized lighting and viewing directions leads to the computation of a meaningful BRDF difference, by means of standard image difference predictors. A psychophysical experiments revealed that our surface provides richer information on material properties than the standard surfaces often used in computer graphics, e.g., sphere or blob.

Surface and edge electroluminescence study of as-grown VCSEL structures

We present a simple quick evaluation method on vertical-cavity-surface-emitting laser (VCSEL) structures, which is designed for atomic clock working at high temperature (>350K), to determine the temperature at which the wavelength of the VCSEL cavity mode (CM) aligns with that of the quantum well (QW) gain peak. The surface and edge electroluminescence (EL) measurements were performed non-destructively on pieces of as-grown VCSEL structures by employing soldered indium (In) contacts. The surface EL spectra determine the CM position, while the edge EL spectra are used to identify the wavelength of ground-state emission from the QW in the active region (QW gain peak). The room temperature EL measurements from a cleaved edge of the VCSEL structures indicate that the QW gain peak is at ∼780.5nm, while the CM measured in the VCSEL surface emission EL peak (sample B) is at ∼793nm. When the sample is heated up, the amount of CM shift with temperature can be fitted with ΔλCM=T×0.068 nm/K, also corroborated by temperature dependence surface reflectivity measurement, and the gain peak can be fitted withλQWpeak=780.5nm + T×0.26 nm/K. The CM and QW gain peak of sample B will be brought into alignment at ∼365K by fitting curve and this was confirmed by measurement. In summary, the surface and edge EL measurements using soldered In as electrodes can be very useful for post growth non-destructive characterization of VCSELs at different temperature.

Improved Vegetation Profiles with GOCI Imagery Using Optimized BRDF Composite

The purpose of this study was to optimize a composite method for the Geostationary Ocean Color Imager (GOCI), which is the first geostationary ocean color sensor in the world. Before interpreting the sensitivity of each composite with ground measurements, we evaluated the accuracy of bidirectional reflectance distribution function (BRDF) performance by comparing modeled surface reflectance from BRDF simulation with GOCI-measured surface reflectance according to composite period. The root mean square error values for modeled and measured surface reflectance showed reasonable accuracy for all of composite days since each BRDF composite period includes at least seven cloud-free angular sampling for all BRDF performances. Also, GOCI-BRDF-adjusted NDVIs with four different composite periods were compared with field-observation NDVI and we interpreted the sensitivity of temporal crop dynamics of GOCI-BRDF-adjusted NDVIs. The results showed that vegetation index seasonal profiles appeared similar to vegetation growth curves in both field observations from crop scans and GOCI normalized difference vegetation index (NDVI) data. Finally, we showed that a 12-day composite period was optimal in terms of BRDF simulation accuracy, surface coverage, and real-time sensitivity.

Adaption of the MODIS aerosol retrieval algorithm using airborne spectral surface reflectance measurements over urban areas: a case study

Abstract. MODIS (MOderate-resolution Imaging Spectroradiometer) retrievals of aerosol optical depth (AOD) are biased over urban areas, primarily because the reflectance characteristics of urban surfaces are different than that assumed by the retrieval algorithm. Specifically, the operational "dark-target" retrieval is tuned towards vegetated (dark) surfaces and assumes a spectral relationship to estimate the surface reflectance in blue and red wavelengths. From airborne measurements of surface reflectance over the city of Zhongshan, China, were collected that could replace the assumptions within the MODIS retrieval algorithm. The subsequent impact was tested upon two versions of the operational algorithm, Collections 5 and 6 (C5 and C6). AOD retrieval results of the operational and modified algorithms were compared for a specific case study over Zhongshan to show minor differences between them all. However, the Zhongshan-based spectral surface relationship was applied to a much larger urban sample, specifically to the MODIS data taken over Beijing between 2010 and 2014. These results were compared directly to ground-based AERONET (AErosol RObotic NETwork) measurements of AOD. A significant reduction of the differences between the AOD retrieved by the modified algorithms and AERONET was found, whereby the mean difference decreased from 0.27±0.14 for the operational C5 and 0.19±0.12 for the operational C6 to 0.10±0.15 and -0.02±0.17 by using the modified C5 and C6 retrievals. Since the modified algorithms assume a higher contribution by the surface to the total measured reflectance from MODIS, consequently the overestimation of AOD by the operational methods is reduced. Furthermore, the sensitivity of the MODIS AOD retrieval with respect to different surface types was investigated. Radiative transfer simulations were performed to model reflectances at top of atmosphere for predefined aerosol properties. The reflectance data were used as input for the retrieval methods. It was shown that the operational MODIS AOD retrieval over land reproduces the AOD reference input of 0.85 for dark surface types (retrieved AOD = 0.87 (C5)). An overestimation of AOD = 0.99 is found for urban surfaces, whereas the modified C5 algorithm shows a good performance with a retrieved value of AOD = 0.86.

Rapid, high-resolution detection of environmental change over continental scales from satellite data – the Earth Observation Data Cube

ABSTRACTThe effort and cost required to convert satellite Earth Observation (EO) data into meaningful geophysical variables has prevented the systematic analysis of all available observations. To overcome these problems, we utilise an integrated High Performance Computing and Data environment to rapidly process, restructure and analyse the Australian Landsat data archive. In this approach, the EO data are assigned to a common grid framework that spans the full geospatial and temporal extent of the observations – the EO Data Cube. This approach is pixel-based and incorporates geometric and spectral calibration and quality assurance of each Earth surface reflectance measurement. We demonstrate the utility of the approach with rapid time-series mapping of surface water across the entire Australian continent using 27 years of continuous, 25 m resolution observations. Our preliminary analysis of the Landsat archive shows how the EO Data Cube can effectively liberate high-resolution EO data from their complex sensor-specific data structures and revolutionise our ability to measure environmental change.

Hyperspectral Surface Reflectance Data Detect Low Moisture Status of Pecan Orchards during Flood Irrigation

For large fields, remote sensing might permit plant low moisture status to be detected early, and this may improve drought detection and monitoring. The objective of this study was to determine whether canopy and soil surface reflectance data derived from a handheld spectroradiometer can detect moisture status assessed using midday stem water potential (ψsmd) in pecan (Carya illinoinensis) during cyclic flood irrigations. We conducted the study simultaneously on two mature pecan orchards, one in a sandy loam (La Mancha) and the other in a clay loam (Leyendecker) soil. We were particularly interested in detecting moisture status in the −0.90 to −1.5 MPa ψsmd range because our previous studies indicated this was the critical range for irrigating pecan. Midday stem water potential, photosynthesis (A) and canopy and soil surface reflectance measurements were taken over the course of irrigation dry-down cycles at ψsmd levels of −0.40 to −0.85 MPa (well watered) and −0.9 to −1.5 MPa (water deficit). The decline in A averaged 34% in La Mancha and 25% in Leyendecker orchard when ψsmd ranged from −0.9 to −1.5 MPa. Average canopy surface reflectance of well-watered trees (ψsmd −0.4 to −0.85 MPa) was significantly higher than the same trees experiencing water deficits (ψsmd −0.9 to −1.5 MPa) within the 350- to 2500-nm bands range. Conversely, soil surface reflectance of well-watered trees was lower than water deficit trees over all bands. At both orchards, coefficient of determinations between ψsmd and all soil and canopy bands and surface reflectance indices were less than 0.62. But discriminant analysis models derived from combining soil and canopy reflectance data of well-watered and water-deficit trees had high classification accuracy (overall and cross-validation classification accuracy >80%). A discriminant model that included triangular vegetation index (TVI), photochemical reflectance index (PRI), and normalized soil moisture index (NSMI) had 85% overall accuracy and 82% cross-validation accuracy at La Mancha orchard. At Leyendecker, either a discriminant model weighted with two soil bands (690 and 2430 nm) or a discriminant model that used PRI and soil band 2430 nm had an overall classification and cross-validation accuracy of 99%. In summary, the results presented here suggest that canopy and soil hyperspectral data derived from a handheld spectroradiometer hold promise for discerning the ψsmd of pecan orchards subjected to flood irrigation.

(Invited) Reaction Kinetics of Metal Deposition Via Slrr of UPD ML Studied By Surface Reflectivity Measurements

The deposition protocol based on Surface Limited Redox Replacement (SLRR) reaction of UPD monolayers [[i]] has gained a lot of attention in recent years [[ii]-,[iii],[iv],[v],[vi],[vii],[viii],[ix]]. The monolayers as well as thin films of arbitrary thickness can be deposited with great fidelity using this approach [9,[x]]. Their nucleation and morphology is found to be a direct function reaction kinetics of SLRR [[xi]]. Understanding of the interplay between the experimental conditions during SLRR on one side and resulting SLRR reaction kinetics on the other is necessary if the true control of the deposit morphology is to be assumed using this deposition protocol. Our work explores a fundamental relation between kinetics of Au deposition via SLRR of Pb UPD ML and experimental parameters of the SLRR process such as concentration of the UPD metal and depositing metal ions in the solution, effect of complexing agents and ions such as citrate, effect of temperature and effect of supporting electrolyte. The reaction solution in our kinetics studies contained both Pb2+ and Au3+ ions in 0.1 M HClO4 as a supporting electrolyte. The Pb coverage as a function of time during SLRR is calculated form the temporal change of the surface reflectivity. The typical results for Pb UPD ML coverage change during Au deposition via SLRR of Pb UPD ML obtained from reflectivity data are shown in Figure 1. Data such as one in Figure 1 are fitted with the model-rate equation of first order in terms of Pb UPD coverage which also takes into account the contribution of parallel SLRR of Pb UPD due to oxygen reduction reaction (ORR). This allowed a more realistic and more reproducible values of the rate constant, k’, for Au deposition process to be obtained. Our results suggest a linear relation between the rate constant k’ and Au3+ concentration. The increase in Pb2+ ion concentration for one order of magnitude increases the value of the fundamental rate constant for Au deposition via SLRR of Pb UPD ML for almost 100%. The presence of citrate ions in the solution lowers the value of rate constant but not significantly. The presented data show that effective manipulation of deposition kinetics can be achieved by proper choice of UPD and depositing metal ions concentration. This should have a strong implication when deposit morphology control is considered. The authors acknowledge the support from NSF Chemistry division under the contract # 0955922.

A Simplified Empirical Line Method of Radiometric Calibration for Small Unmanned Aircraft Systems-Based Remote Sensing

The use of small unmanned aircraft systems (sUAS) to acquire very high-resolution multispectral imagery has attracted growing attention recently; however, no systematic, feasible, and convenient radiometric calibration method has been specifically developed for sUAS remote sensing. In this research, we used a modified color infrared (CIR) digital single-lens reflex (DSLR) camera as the sensor and the DJI S800 hexacopter sUAS as the platform to collect imagery. Results show that the relationship between the natural logarithm of measured surface reflectance and image raw, unprocessed digital numbers (DNs) is linear and the y-intercept of the linear equation can be theoretically interpreted as the minimal possible surface reflectance that can be detected by each sensor waveband. The empirical line calibration equation for every single band image can be built using the y-intercept as one data point, and the natural log-transformed measured reflectance and image DNs of a gray calibration target as another point in the coordinate system. Image raw DNs are therefore converted to reflectance using the calibration equation. The Mann-Whitney U test results suggest that the difference between the measured and the predicted reflectance values of 13 tallgrass sampling quadrats is not statistically significant. The method theory developed in this study can be employed for other sUAS-based remote sensing applications.

Computer Calculations versus Field Measurements for Tunnel Lighting Design

ABSTRACTThe purpose of this article is to discuss the differences between the design (calculated) and field-measured luminance of a recent tunnel lighting project. Field-measured illuminance values agreed reasonably well with the design (calculated) values. However, the field-measured luminance values, which are based on the reflected light from surfaces, did not concur with the design (calculated) values. A roadway’s reflective characteristics are defined by its physical surface properties. Sufficient data have been collected on the reflective characteristics of different pavement types to allow them to be described by reflectance tables or “R” tables. Investigation and calculations for the tunnel lighting design were performed using the average luminance coefficient (Q0) from the roadway lighting standard RP-8-00 [ANSI/IES 2000], R-tables, and the Memphis (a mobile gonio-reflectometer) field measurements. An empirical factor was developed to account for the differences in the design and measured luminance results. Although established standard parameters are helpful to start a tunnel lighting design calculation, it is important to note that not all design parameters associated with new construction are appropriate. Some tunnel lighting systems are rehabilitated without new roadway paving and/or without newly painted walls and ceiling. Using an established standard R-table may be economical but may produce inaccurate results. Therefore, various factors must be taken into account based on existing conditions of the tunnel proper. Based on the findings from this project, it is critical to use the actual roadway and surface reflectance measurements within the tunnels to obtain meaningful computer calculations for tunnel rehabilitation projects; otherwise, the results can be substantially inaccurate and result in a design that does not satisfy the design criteria and/or industry standards.

A Multi-Temporal and Multi-Spectral Method to Estimate Aerosol Optical Thickness over Land, for the Atmospheric Correction of FormoSat-2, LandSat, VENμS and Sentinel-2 Images

The correction of atmospheric effects is one of the preliminary steps required to make quantitative use of time series of high resolution images from optical remote sensing satellites. An accurate atmospheric correction requires good knowledge of the aerosol optical thickness (AOT) and of the aerosol type. As a first step, this study compares the performances of two kinds of AOT estimation methods applied to FormoSat-2 and LandSat time series of images: a multi-spectral method that assumes a constant relationship between surface reflectance measurements and a multi-temporal method that assumes that the surface reflectances are stable with time. In a second step, these methods are combined to obtain more accurate and robust estimates. The estimated AOTs are compared to in situ measurements on several sites of the AERONET (Aerosol Robotic Network). The methods, based on either spectral or temporal criteria, provide accuracies better than 0.07 in most cases, but show degraded accuracies in some special cases, such as the absence of vegetation for the spectral method or a very quick variation of landscape for the temporal method. The combination of both methods in a new spectro-temporal method increases the robustness of the results in all cases.

MOCVD에 의한 Al 박막 증착 중의 표면 반사도 측정을 통한 박막 성장 메커니즘 분석

Al thin films were deposited on TiN/Si(100) via metal-organic chemical vapor deposition using N-methylpyrrolidine alane as a precursor. Characterization of the deposited films were investigated with SEM, XRD, -step, AFM, 4-point probe. The early stage of Al thin film deposition was analyzed by in-situ surface reflectance measurement with laser and photometer apparatus. The surface reflectance were changed greatly during the initial 30~40 seconds. There were two increases and two decreases in the surface reflectance, thus the sequence of Al films were deposited at 8 significant points of the surface reflectance change. Surface topograph and cross-sectional view of each film were analyzed with SEM. Al films were grown in the complex mechanism of Volmer-Weber and Stranski-Krastanov process.

Permanent target for optical payload performance and data quality assessment: spectral characterization and a case study for calibration

To regularly evaluate the optical payload performance (geometric, radiometric, and spatial resolution) and the data quality for high-resolution airborne and satellite imaging systems, two new permanent targets (the knife-edge target and the fan-shaped target) made of gravel and with the advantages of year-round availability, lower maintenance operations, and a long lifetime were established in the Academy of Opto-Electronics Baotou site in China. The spectral properties of these targets are investigated in this study. Note that the anisotropy factor at 550 nm for the white gravel is approximately 6%, 12.5% 16.5%, 17.5%, 11.5%, and 5% at the principal plane for the observer zenith angle of 60 deg, 50 deg, 40 deg, 30 deg, 20 deg, and 10 deg (backscatter), respectively. The corresponding value for the gray gravel is 20.8%, 24.8%, 29.4%, 23.8%, 13%, and 3.7%, respectively, and 62.8%, 65.7%, 59.2%, 40.3%, 22.3%, and 9.0%, respectively, for the black gravel. The anisotropy of the black gravel is larger than that of the gray and white gravel areas. The nonuniformity of the target reflectivity is within 2.5%. Furthermore, a calibration for the optical payloads onboard the GF-1 satellite is performed with the knife-edge target, and the uncertainty analysis demonstrates that the uncertainty for this calibration is <2.12% when the relative error for the surface reflectance measurement, the aerosol optical depth, and the total column water vapor are approximately 1%, 10%, and 10%, respectively.

Introduction of Atmospheric Pressure Plasma to Aqueous Detergent Processes.

The effects of exposure of polymer surfaces to atmospheric pressure plasma (APP) on detergency were investigated from the viewpoint of pretreatment to cleaning in aqueous systems using three PET substrates: film, mesh, and fabric. The PET substrates were soiled with stearic acid as a model oily contaminant, and were treated with the APP jet immediately before cleaning. Stir washing in aqueous solutions with and without alkali or anionic surfactant was performed, and then the detergency was evaluated from the microscopic image analysis or surface reflectance measurement. For all PET samples and detergent solutions, APP exposure was found to promote the removal of stearic acid. Contact angle measurements showed that APP exposure enhanced the hydrophilicity of PET and stearic acid. The increase in the surface oxygen concentration on PET and stearic acid due to the APP exposure was also observed by XPS analysis. The simultaneous oxidation of the PET substrate and stearic acid soil by the APP pretreatment resulted in detergency improvement via surface hydrophilization. Furthermore, microscopic observations suggested that the collapse of crystallized stearic acid deposited on the PET substrate by APP heating facilitated its removal. In situ detergency evaluation by a quartz crystal microbalance technique confirmed that the removal of stearic acid from the PET substrate was promoted by the APP exposure. The experimental findings of this study demonstrate the effectiveness of the APP exposure before cleaning in aqueous solutions.

Vicarious Calibration Data Screening Method Based on Variance of Surface Reflectance and Atmospheric Optical Depth Together with Cross Calibration

Vicarious calibration data screening method based on the measured atmospheric optical depth and the variance of the measured surface reflectance at the test sites is proposed. Reliability of the various calibration data has to be improved. In order to improve the reliability of the vicarious calibration data, some screenings have to be made. Through experimental study, it is found that vicarious calibration data screening would be better to apply with the measured atmospheric optical depth and variance of the measured surface reflectance due to the facts that thick atmospheric optical depth means that the atmosphere contains serious pollution sometime and that large deviation of the surface reflectance from the average means that the solar irradiance has an influence due to cirrus type of clouds. As the results of the screening, the uncertainty of vicarious calibration data from the approximated radiometric calibration coefficient is remarkably improved. Also, it is found that cross calibration uncertainty is poorer than that of vicarious calibration.

Hand-Held Blue Light 3D Measurement Technology and its Application in Hot Stamping

Over the last few years hot stamping for its high strength, lightweight characteristics has developed at an alarming rate, At the same time the measurement process a higher demand. Compared with the traditional three-dimensional coordinate measurement, Hand-held Blue structured Light 3D measurement technology for its fast, high precision, good flexibility, etc., has become the best choice of high-strength steel stamping in non-contact 3D measurement and accuracy detection. This paper developed a set of holding blue light 3D measurement system. The system project blue structured pattern to the surface of the object, and captured by two high-speed cameras synchronized. Then using the grabbed pictures reconstruct 3D topography of the object. Blue light projection technology can eliminate the use of influence of different surface reflectance measurement accuracy, So that a single measurement accuracy of 0.02mm, overall measurement accuracy of 0.05mm / m, Single measurement time is less than 0.12 seconds. This measuring system can be widely used in the rapid detection of various types of three-dimensional measurement and precision casting.

A normalisation framework for (hyper-)spectral imagery

It is well known that the topography has an influence on the observed reflectance spectra. This influence is not compensated by spectral ratios, i.e. the effect is wavelength dependent. In this work, we present a complete normalisation framework.The surface temperature is estimated based on the measured surface reflectance. To normalise the spectral reflectance with respect to a standard illumination geometry, spatially varying reflectance parameters are estimated based on a non-linear reflectance model. The reflectance parameter estimation has one free parameter, i.e. a low-pass function, which sets the scale of the spatial-variance, i.e. the lateral resolution of the reflectance parameter maps. Since the local surface topography has a major influence on the measured reflectance, often neglected shading information is extracted from the spectral imagery and an existing topography model is refined to image resolution.All methods are demonstrated on the Moon Mineralogy Mapper dataset. Additionally, two empirical methods are introduced that deal with observed systematic reflectance changes in co-registered images acquired at different phase angles. These effects, however, may also be caused by the sensor temperature, due to its correlation with the phase angle.Surface temperatures above 300K are detected and are very similar to a reference method. The proposed method, however, seems more robust in case of absorptions visible in the reflectance spectrum near 2000nm. By introducing a low-pass into the computation of the reflectance parameters, the reflectance behaviour of the surfaces may be derived at different scales. This allows for an iterative refinement of the local surface topography using shape from shading and the computation reflectance parameters. The inferred parameters are derived from all available co-registered images and do not show significant influence of the local surface topography. The results of the empirical correction show that both proposed methods greatly reduce the influence of different phase angles or sensor temperatures.

Evaluation of atmospheric correction models and Landsat surface reflectance product in an urban coastal environment

Precise atmospheric correction is important for applications where small differences in surface reflectance (SR) are significant, such as biomass estimation, crop phenology, and retrieval of water quality parameters. It also enables direct comparison between different image dates and different sensors. As a precursor to monitoring different parameters of water quality around the coastline of Hong Kong using medium-resolution sensors Landsat TM/ETM, and HJ-1A/B, this study evaluated the performance of five atmospheric correction methods. The estimated SR of the first four reflective bands of Landsat 7 ETM+ and of the identical bands of the HJ-1A/B satellites was compared with in situ multispectral radiometer (MSR) SR measurements over sand, artificial turf, grass, and water surfaces for the five atmospheric correction methods – second simulation of the satellite signal in the solar spectrum (6S), fast line-of-sight atmospheric analysis of spectral hypercubes (FLAASH), atmospheric correction (ATCOR), dark object subtraction (DOS), and the empirical line method (ELM). Among the five methods, 6S was observed to be consistently more precise for SR estimation, with significantly less difference from the in-situ-measured SR, especially over lower reflective water surfaces. Of the two image-based methods, DOS performed well over the darker surfaces of water and artificial turf, although still inferior to 6S, while ELM worked well for grass sites as compared to the DOS and equalled the good performance of 6S over the high reflective sand surfaces.The study also evaluated the new standard Landsat SR product Landsat ecosystem disturbance adaptive processing system (LEDAPS) using the in situ measured SR data for the three land surface types – sand, artificial turf, and grass. For the highly and moderately reflecting bright sand and artificial turf, LEDAPS performed poorly, while for the darker grass site it performed better, although still inferior to 6S and ELM methods. This is probably due to the variable aerosol types and atmospheric conditions of Hong Kong, as LEDAPS was mainly compiled with reference to larger continental landmass areas.