Computation of subpixel land surface temperature from MODIS satellite data

COMPUTATION OF SUBPIXEL LAND SURFACE TEMPERATURE FROM MODIS SATELLITE DATA AGNES LIM HUEI NI (B.Sc (Hons), NUS) A THESIS SUBMITTED FOR THE DEGREE OF MASTERS OF SCIENCE DEPARTMENT OF PHYSICS NATIONAL UNIVERSITY OF SINGAPORE 2005 Acknowledgment - Acknowledgment I wish to take this opportunity to express my most sincere gratitude to the following people whom without them, the completion of this piece of work will not be possible To my supervisors, Dr Liew Soo Chin and Professor Lim Hock for their invaluable guidance, patience and advice whenever I encounter problems in the course of my research To Dr Elvidge C D at NOAA National Geophysical Data Centre(NGDC) who provided the Landsat ETM+ data used for validation in this study To my colleagues at the Centre for Remote Imaging, Sensing and Processing for their suggestions, encouragements and support They are in deed a team that is always ready to share with me whatever they know and think that is going to be helpful to me in my research To my friends and family for the encouragement, care and concern they showered on me during these three and a half years I spent on this work i Contents Acknowledgment i Table of Contents ii Summary v List of Figures .ix List of Tables .xvi List of Symbols .xviii I Introduction .1 Section 1.1 Moderate Resolution Imaging Spectroradiometer Section 1.2 Review on Techniques on the Retrieval of Land Surface Temperature Section 1.3 Identification of Temperature Fields at Subpixel Resolution .13 Section 1.4 II Thesis Aim 15 The Earth’s Atmosphere and Atmospheric Correction 16 Section 2.1 The Earth's Atmosphere 17 Section 2.2 Standard Atmospheres 20 Section 2.3 Atmospheric Parameters 20 Section 2.3.1 Section 2.3.2 Atmospheric Temperature Profile 21 Atmospheric Pressure Profile .24 ii Section 2.3.3 Atmospheric Water Vapour profile 27 Section 2.4 Atmospheric Absorption 30 Section 2.5 Atmospheric Transmittance 36 Section 2.6 Atmospheric Correction 43 Section 2.7 Implementation of Atmospheric Correction 47 III Resolving A Mixed Pixel 55 IV Applications and Results 61 Section 4.1 Datasets 62 Section 4.2 Atmospheric Correction of MODIS data .68 Section 4.2.1 Method 68 Section 4.2.2 Section 4.3 Results .73 Subpixel Retrieval 88 Section 4.3.1 Method 88 Section 4.3.2 Results .92 Section 4.4 Validation using High resolution Landsat Data 108 Section 4.4.1 Method 108 Section 4.4.2 Results 115 Section 4.5 Validation of Subpixel Retrieval Hotspots using High Resolution SPOT Imagery 137 Section 4.6 V MODIS Fire Detection Limits .142 Conclusions 145 Bibliography I iii Appendices A Sensor Spectral Characteristics VI B Standard Tropical Atmosphere .VIII C Derivation of the Radiative Transfer Equations and its Solutions for Plane Parallel Atmosphere X D Central Difference Method and Trapezium Method XIV E Fitting Coefficients for Calculating Transmittance for MODIS Spectral Bands XVI F Subpixel fire area and fire temperature of MODIS Images XXII G Fitting Coefficients for Calculating Transmittance for Landsat Spectral Bands XXXIII iv Summary - Summary In this study, we investigate the possibility of calculating sub resolution forest fire burning area and temperature from atmospherically corrected thermal infrared data measured by a thermals sensor from space The Earth's surface is far from homogeneous The thermal radiation measured by the sensor is a contribution of thermal radiation emitted by different temperature components within the field of view (FOV) of the sensor There is a need to retrieve sub resolution information in order to obtain the temperature as well as the area occupies for different temperature components For example when a fire is detected within the FOV of the sensor, the fire does not necessarily occupy the whole FOV; giving rise to two temperatures within the FOV The radiance of a pixel is assumed to be contributed by the two temperature fields, one from the background and the other from the target fire area The problem then is to estimate the fire temperature and the fire area within the FOV Constant emissivity is assumed for both target and background Background temperature field is estimated from surrounding pixels, leaving two unknowns to be determined which are the target temperature field and the portion of the FOV that the target temperature field occupies Iterative method is used to solve for the unknown parameters from the radiances detected at two wavelength bands v Summary - Prior to retrieving the sub resolution information, the measured thermal data by the sensor is not the “true” thermal radiation emitted by the land surface, but a contribution from both the land surface and the atmosphere The thermal radiation detected by the sensor is a sum of contributions from the atmospheric emission, reflected component from atmospheric emission and surface emission attenuated by the atmosphere through absorption by atmospheric gases The process of removing atmospheric emission and atmospheric emission reflected from surface known as atmospheric correction is required to obtain the “true” ground leaving thermal infrared radiation To simulate various atmospheric conditions, MODTRAN, radiative transfer code is used to calculate the amount of atmospheric contributions Relationships between the simulation results and their respective atmospheric conditions are then derived Atmospheric contributions are then calculated from solutions of the radiative transfer equation for thermal infrared radiation and the simulation results The atmospheric contributions are then subtracted from the total thermal radiance measured by the senor to obtain the ground leaving radiance The atmospheric correction procedure is applied on different MODIS bands and the retrieved LST from the two bands are cross compared Good correlation between the LST retrieved from two MODIS bands for all datasets is obtained showing that the LSTs from different bands agreed with one another In addition, the root mean square differences between the LST of the two bands for all test data are below 1.5K The retrieved LST from atmospherically corrected MODIS data was also validated with the MODIS Standard LST Product They agreed with a root mean square difference of approximately 1K, considering that vi Summary - LST values are derived from two different methods with different assumptions Fire pixels identified using fire temperature retrieved by applying subpixel retrieval algorithm on atmospherically corrected MODIS thermal data are compared with the fire pixels identified using algorithms developed by NASA based on thresholding the brightness temperature Fires that are not detected using fire temperature retrieved by applying subpixel retrieval algorithm on atmospherically corrected MODIS data are generally due to cloud contamination where atmospheric correction is not applied A few other fires are missed as the data did not allow for a successful retrieval The atmospheric correction and subpixel retrieval algorithms developed in this study for the MODIS thermal data are validated using high resolution night time Landsat data The algorithms are applied to Landsat data to obtained sub resolution information The subpixel model was used to calculate the expected MODIS thermal radiances Good correlations results are obtained between actual MODIS radiance and that simulated from the high resolution Landsat data Good correlations also exist between fire temperature retrieved from the MODIS data and the Landsat simulated MODIS data The fire area correlation fair much weaker than that of the fire temperature Results are acceptable as both sets of satellite data have very different spatial and spectral resolutions Lastly, fires identified using the proposed algorithms are compared with daytime SPOT data at 20m spatial resolution Fires in SPOT are identified by visual inspection The proposed algorithm detected more fires as compared to visual inspection of SPOT because burnt areas as well as bare land are picked up as fires This is possible as the burnt area may not be totally cooled or even vii Summary - include smoldering fires that are not observed on the SPOT data and bare land's temperature may be elevated by the solar radiation viii List of Figures - List of Figures Figure Pressure and Temperature Profiles of the US 1976 Standard Atmosphere .17 Figure The global distribution of total atmospheric water vapour (precipitable water) above the Earth’s surface This depiction includes data from both satellite and weather balloon observations and represents an average for the period 1988– 1997 19 Figure Temperature Profile of Standard Tropical Atmosphere and best fit of the Temperature Profile 23 Figure Pressure Profile of the Standard Tropical Atmosphere and best fit of the Pressure Profile 26 Figure Water Vapour Density Profile of Standard Tropical Atmosphere, the best fit Water Vapour Density Profile and the Saturated Water Vapour Density Profile .28 Figure Low Resolution Infrared Absorption of the Major Atmospheric Gases 33 Figure Location of MODIS Band 21/22 on the Total Atmospheric Transmittance Spectrum 35 Figure Location if MODIS Bands 31 and 32 on the Total Atmospheric Transmittance Spectrum 35 Figure Atmospheric Transmittance due to Water Vapour as a function of Total Precipitable Water for MODIS Band 22 .38 Figure 10 Atmospheric Transmittance due to Water Vapour as a Function of Total Precipitable Water Vapour for MODIS Band 31 39 ix List of Figures - Figure 11 Atmospheric Transmittance due to Water Vapour as a Function of Total Precipitable Water Vapour for MODIS Band 32 39 Figure 12 Illustration of plane versus spherical geometry (a) In plane geometry, the slant path is the same for all layers of equal geometrical thickness (b) In spherical geometry, the slant path changes from layer to layer .41 Figure 13 Optical thickness computed with and without refraction for MODIS band 22 as function of total precipitable water 41 Figure 14 Optical thickness computed with and without refraction for MODIS band 31 as function of total precipitable water 42 Figure 15 Optical thickness computed with and without refraction for MODIS band 32 as function of total precipitable water 42 Figure 16 Sources of Radiant Energy received by the Satellite 44 Figure 17 Definition of terms for a Plane Parallel Atmosphere .45 Figure 18 Dozier Method for Subpixel Retrieval 55 Figure 19 Plot of S vs T 59 Figure 20 Flow Diagram showing the Iterative Computation of Tf 60 Figure 21 Coverage of MODIS Images and Landsat Images 64 Figure 22 Coverage of Landsat scenes .65 Figure 23 Thermal Infrared Images of Landsat scene 197 on February 2002 .65 Figure 24 Thermal Infrared Images of Landsat scene 196 on February 2002 .66 Figure 25 Rainfall and temperature distribution of Chiang Mai Thailand 67 Figure 26 Coverage of MODIS Images and SPOT Image .68 x List of Figures - Figure 27 Brightness Temperature of MODIS band 22 in kelvin 74 Figure 28 Brightness Temperature of MODIS band 31 in kelvin .74 Figure 29 LST retrieved from atmospherically corrected MODIS band 22 data in kelvin (T22) 75 Figure 30 LST Retrieved from atmospherically corrected MODIS band 31 data in kelvin (T31) 75 Figure 31 Plots of temperatures on January 2002 78 Figure 32 Plots of temperature for February 2002 79 Figure 33 Plots of temperature for 24 February 2002 .80 Figure 34 Plots of temperature for 12 March 2002 81 Figure 35 Cross Validation of LST retrieved from atmospherically corrected MODIS data with the Standard MODIS LST Product for the data acquired on January 2002 83 Figure 36 Cross Validation of LST retrieved from atmospherically corrected MODIS data with the Standard MODIS LST Product for the data acquired on February 2002 83 Figure 37 Cross Validation of LST retrieved from atmospherically corrected MODIS data with the Standard MODIS LST Product for the data acquired on 24 February 2002 .84 Figure 38 Cross Validation of LST retrieved from atmospherically corrected MODIS data with the Standard MODIS LST Product for the data acquired on 12 March 2002 84 Figure 39 Plot of Tf vs f for TERRA MODIS Pass on January 2002 96 Figure 40 Plot of Tf vs f for TERRA MODIS Pass on February 2002 96 Figure 41 Plot of Tf vs f for TERRA MODIS Pass on 24 February 2002 97 xi List of Figures - Figure 42 Plot of Tf vs f for TERRA MODIS Pass on 12 March 2002 97 Figure 43 Fire temperature histogram for fire pixels detected by thresholding the fire temperature obtained from applying subpixel algorithm on atmospherically corrected MODIS data and for the fire pixels that are detected by both the MODIS fire algorithm and the subpixel retrieval algorithm using MODIS day data .99 Figure 44 Fire temperature histogram for fire pixels detected by thresholding the fire temperature obtained from applying subpixel algorithm on atmospherically corrected MODIS data and for the fire pixels that are detected by both the MODIS fire algorithm and the subpixel retrieval algorithm using MODIS night data 99 Figure 45 Fire area histogram for fire pixels detected by thresholding the fire temperature obtained from applying subpixel algorithm on atmospherically corrected MODIS data and for the fire pixels that are detected by both the MODIS algorithm and the subpixel retrieval algorithm using MODIS day data 102 Figure 46 Fire area histogram for fire pixels detected by thresholding the fire temperature obtained from applying subpixel algorithm on atmospherically corrected MODIS data and for the fire pixels that are detected by both the MODIS algorithm and the subpixel retrieval algorithm using MODIS night data 102 Figure 47 Cumulative curves of fire temperature for MODIS day data for fire pixels detected by thresholding fire temperatures obtained from subpixel algorithm and fire pixels detected by both MODIS fire algorithm and the subpixel algorithm 103 Figure 48 Cumulative curves of fire area for MODIS day data for fire pixels detected by thresholding fire temperatures obtained from subpixel algorithm and fire pixels detected by both MODIS fire algorithm and the subpixel algorithm 103 xii List of Figures - Figure 49 Cumulative curves of fire temperature for MODIS night data for fire pixels detected by thresholding fire temperatures obtained from subpixel algorithm and fire pixels detected by both MODIS fire algorithm and the subpixel algorithm 104 Figure 50 Cumulative curves of fire area for MODIS night data for fire pixels detected by thresholding fire temperatures obtained from subpixel algorithm and fire pixels detected by both MODIS fire algorithm and the subpixel algorithm .104 Figure 51 Landsat Band data for Path Row 197 acquired on February 2002 109 Figure 52 Valid pixels of Landsat band for Path Row 197 acquired on February 2002 109 Figure 53 Flow Diagram for the Conversion of Landsat Data to MODIS spatial and spectral resolution 114 Figure 54 Comparison of Band 22 radiance for MODIS and Landsat simulated MODIS data at for Landsat scene with row 197 acquired on February 2002 116 Figure 55 Comparison of Band 22 radiance for MODIS and Landsat simulated MODIS data at for Landsat scene with row 196 acquired on February 2002 116 Figure 56 Comparison of Band 22 radiance for MODIS and Landsat simulated MODIS data at for Landsat scene with row 197 acquired on 24 February 2002 117 Figure 57 Comparison of Band 22 radiance for MODIS and Landsat simulated MODIS data at for Landsat scene with row 196 acquired on 24 February 2002 117 Figure 58 Comparison of Band 22 radiance for MODIS and Landsat simulated MODIS data at for Landsat scene with row 197 acquired on 12 March 2002 118 Figure 59 Comparison of Band 22 radiance for MODIS and Landsat simulated MODIS data at for Landsat scene with row 196 acquired on 12 March 2002 118 xiii List of Figures - Figure 60 Scatter Plots of atmospherically corrected radiance at 3.959µm (L22) and 11.03µm (L31) for MODIS data and Landsat simulated MODIS data for scene with row number 197 acquired on February 2002 120 Figure 61 Scatter Plots of atmospherically corrected radiance at 3.959µm (L22) and 11.03µm (L31) for MODIS data and Landsat simulated MODIS data for scene with row number 196 acquired on February 2002 .121 Figure 62 Scatter Plots of atmospherically corrected radiance at 3.959µm (L22) and 11.03µm (L31) for MODIS data and Landsat simulated MODIS data for scene with row number 197 acquired on 24 February 2002 .122 Figure 63 Scatter Plots of atmospherically corrected radiance at 3.959µm (L22) and 11.03µm (L31) for MODIS data and Landsat simulated MODIS data for scene with row number 196 acquired on 24 February 2002 .123 Figure 64 Scatter Plots of atmospherically corrected radiance at 3.959µm (L22) and 11.03µm (L31) for MODIS data and Landsat simulated MODIS data for scene with row number 197 acquired on 12 March 2002 .124 Figure 65 Scatter Plots of atmospherically corrected radiance at 3.959µm (L22) and 11.03µm (L31) for MODIS data and Landsat simulated MODIS data for scene with row number 196 acquired on 12 March 2002 125 Figure 66 Scatter plots of retrieved fire fraction and fire temperature of MODIS and Landsat simulated MODIS for scene 196 on February 2002 128 Figure 67 Scatter plots of retrieved fire fraction and fire temperature of MODIS and Landsat simulated MODIS for scene 197 on February 2002 130 xiv List of Figures - Figure 68 Scatter plots of retrieved fire fraction and fire temperature of MODIS and Landsat simulated MODIS for scene 196 on 24 February 2002 131 Figure 69 Scatter plots of retrieved fire fraction and fire temperature of MODIS and Landsat simulated MODIS for scene 197 on 24 February 2002 132 Figure 70 Scatter plots of retrieved fire fraction and fire temperature of MODIS and Landsat simulated MODIS for scene 196 on 12 March 2002 133 Figure 71 Scattered plots of retrieved fire fraction and fire temperature of MODIS and Landsat simulated MODIS for scene 197 on 12 March 2002 134 Figure 72 Mosiac Images of 11 SPOT scenes acquired on 17 August 2002 137 Figure 73 Examples of SPOT fires with fire fronts and smoke plumes 138 Figure 74 Examples of False Alarms 141 Figure 75 Minimum and maximum Fire Fraction before a MODIS pixel with a background temperature of 300K can be detected as a fire or saturates the sensor for different fire temperatures .143 xv List of Tables - List of Tables Table MODIS Bands Specifications .8 Table Key Uses of MODIS Thermal Emissive Bands Table Temperature Profile of Standard Tropical Atmosphere 22 Table Pressure Profile of Standard Tropical Atmosphere 25 Table Water Vapour Density Profile of Standard Tropical Atmosphere 28 Table Date and time of MODIS passes 63 Table Date and time of Landsat passes 63 Table rms difference of LST retrieved from MODIS bands 22 and 31 before and after atmospheric correction 77 Table rms difference between retrieved L:ST from atmospherically corrected MODIS data and MODIS Standard LST Product 85 Table 10 Percentage of data points whose difference between LST retrieved from atmospherically corrected MODIS data and the MODIS standard LST Product is within ±1 K 86 Table 11 Number of hotspots detected by MODIS fire algorithm and that by thresholding retrieved Tf from subpixel retrieval using a threshold of 400K .92 Table 12 Number of fires detected by both NASA and subpixel algorithm and those detected by NASA algorithm but not by subpixel retrieval algorithm 93 Table 13 Breakdown on the undetected fire pixels 95 xvi List of Tables - Tab;e 14 Minimum and Maximum of f and Tf 98 Table 15 Number of fire pixels detected and undetected by of threshold is increased .101 Table 16 Median, Lower and Upper quartiles of the cumulative curves: Blue curve corresponds to fire pixels detected by subpixel retrieval algorithm and red curve corresponds to fire pixels detected by both NASA algorithm and subpixel algorithm 105 Table 17 Thresholds for detecting fire pixels for MODIS data and Landsat Simulated MODIS data 126 Table 18 Number of Common hotspots between MODIS data and Landsat Simulated MODIS data 127 Table 19 Breakdown of the fire pixels detected using subpixel algorithm for both MODIS and simulated MODIS data 135 Table 20 Minimum and Maximum of f and Tf for fire pixels detected from both the MODIS and simulated MODIS data and the correlations of f and Tf 136 Table 21 Summary of Validation Results using Active Fires detected on SPOT Imagery acquired on 17 August 2002 over Sumatra 140 xvii List of Symbols - List of Symbols λ Wavelength Γ Lapse rate h Planck's constant kB Boltzmann constant c Speed of light b Extinction coefficient βa Absorption coefficient of a bands ka Absorption coefficient of a single absorption line W Total Precipitable water τ Optical thickness θ Zenith angle φ Azimuth angle ρwater Density of liquid water ρw Density of water vapour ρ Molecular number density ω Single scattering albedo z Height xviii List of Symbols - s Slant Path ε Emissivity t Transmittance ν Wavenumber T Temperatures P Pressure g Acceleration due to gravity M Effective molecular mass of air f Subpixel fire area Tf Subpixel fire temperature Tb Background temperature xix ... pixels for MODIS data and Landsat Simulated MODIS data 126 Table 18 Number of Common hotspots between MODIS data and Landsat Simulated MODIS data 127 Table 19 Breakdown of the fire... actual MODIS radiance and that simulated from the high resolution Landsat data Good correlations also exist between fire temperature retrieved from the MODIS data and the Landsat simulated MODIS data. .. fire temperature of MODIS and Landsat simulated MODIS for scene 196 on 12 March 2002 133 Figure 71 Scattered plots of retrieved fire fraction and fire temperature of MODIS and Landsat