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- 1. Estimation of subpixel land surface temperature usingan endmember index technique based on ASTER andMODIS products over a heterogeneous area Dr. Yang Guijun Beijing Research Center for Information Technology in Agriculture, Beijing, China guijun.yang@163.com 2011-07-27
- 2. Contents 1 Introduction 2 Study area and data sets 3 Methodology 4 Results and analysis 5 Discussion and conclusions
- 3. 1 Introduction When referring to land observation from space, the landsurface temperature (LST) is a physical parameter veryimportant to a wide variety of scientific studies (Sobrino etal., 1991; Wan & Li, 1997; Qin et al., 2001). A common use of thermal infrared (TIR) data is to derivesurface energy budgets (Kustas et al., 2003; Liang, 2004)from high-resolution thermal data, providing assessments ofevapotranspiration (ET) down to scales of individualagricultural fields. However, surface heterogeneity inducesuncertainty in estimating subpixel temperature.
- 4. 1 Introduction As a result, LST products retrieved from satellite TIR dataare often composed of a mixture of different temperaturecomponents (Lakshmi & Zehrfuhs, 2002; Liu et al., 2006).Therefore, there is a need for enhancing the spatial resolutionof the current LST products. In recent years, many studies have focused on theestimation of subpixel LST and developed a number ofcorresponding approaches at either thermal digital number(DN) level, or thermal radiance level or LST level: Statistical regression methods (e.g., Sandholt et al., 2002; Kustas et al., 2003; Agam et al., 2007; Yang et al., 2010) Modulation methods (e.g., Liu and Moore, 1998; Nichol, 2009; Stathopoulou & Cartalisa, 2009) Hybrid methods (e.g., Pu et al., 2006; Liu & Pu, 2008)
- 5. 1 Introduction Statistical regression methodsDevelopment of the relationship between a vegetation index/variable (e.g.,the Normalized Difference Vegetation Index, NDVI; vegetation fraction, )and radiometric surface temperature by a polynomial or power function. Modulation methodsConcentrate more on how to distribute the thermal radiance of a thermalpixel block and to balance it among those visible pixels in such a big block;many scaling factors, such as effective emissivity, or combining emissivityand LST altogether. Hybrid methodsHybrid algorithms combine statistical regression and modulation methods,while straightforwardly connecting subpixel information with thermal blockpixel radiance on the basis of the scale invariance of radiance.
- 6. 1 IntroductionIn this research, we propose a new hybrid disaggregationmethod, which is using the endmember index basedtechnique (DisEMI) to estimate 90 m resolution subpixeltemperature (Ts90) from 990 m resolution MODIS LSTproduct (Ts990) and 30 m resolution ASTER reflectance data(VNIR30). MODIS LST-990 m Subpixel LST DisEMI 90m ASTER products VNIR-30 m/TIR-90 m
- 7. 2 Study area and data setsChangping District, locatedin the northeast of BeijingCity, China, was chosen asthe study area. Therepresentative land use/landcover (LULC) types in thestudy area typically consist ofthe vegetation, bare soil,impervious (including villagesand towns), and water.
- 8. 2 Study area and data setsData sets:ASTER-ASTER surface reflectance (AST_07) and surface kinetictemperature (AST_08) products, observed on 19 May 2001, were collectedfrom the Earth Observing System Data Gateway (EDG).MODIS-MOD02_QKM, MOD02_HKM, MOD02_1KM, MOD03_L1A,MOD07_L2, MOD11B1, and MOD11_L2 data acquired also on 19 May 2001,were collected from the Land Processes Distributed Active Archive Center(LP DAAC) of U.S. Geological Survey (USGS). They are geolocation,atmospheric profile, reflectance and LST products, respectively. MODIS LST-990 m
- 9. 3 Methodology 1 2 3 4 10 5 6 7 8 9A workflow of estimating subpixel temperature based on the endmember indices
- 10. 3.1 Selection of remote sensing indices As the heart of the DisEMI technique, to discover which index is best for the prediction of LST from available nine ASTER bands, the normalized difference spectral indices (NDSI) was employed, examined and compared based on their linear relationships with “pure pixels” temperature at the 90 m resolution for each land cover type: r −r LST ~ NDSI ( x , y ) = x y 9 9 rx + ry 8 8 7 7 0.26 ( ASTER3 − ASTER2 ) 0.50 0.24ASTER bands 6 6 SAVI = (1 + L) 0.45 0.22 Vegetation: 0.40 0.20 ( ASTER3 + ASTER2 + L) 5 5 0.18 0.35 0.16 0.30 0.14 4 0.25 4 0.12 ASTER3 − ( ASTER4 − ASTER5 ) 0.20 0.10 NMDI = 3 3 0.08 Bare soil: 0.15 (a) 0.10 (b) 0.06 ASTER3 + ( ASTER4 − ASTER5 ) 0.04 2 Vegetation 0.05 2 Bare soil 0.02 0.00 0.00 1 1 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 ASTER4 − ASTER2 9 9 Impervious: NDBI = 8 8 ASTER4 + ASTER2 7 7 Water surface: NDWI = ASTER1 − ASTER3 0.65 0.18 0.60 0.55ASTER bands 6 0.16 6 0.50 ASTER1 + ASTER3 0.14 0.45 5 0.12 5 0.40 0.35 0.10 4 0.30 4 0.08 0.25 0.20 The contour maps of coefficients of determination (R2) 0.06 3 (c) 0.04 3 (d) 0.15 0.10 Impervious Water between NDSI and LST at 90 m resolution. The symbol 0.05 2 0.02 2 0.00 0.00 “+” represents the maximal R2. 1 1 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 ASTER bands ASTER bands
- 11. 3.2 Classification of land cover typesWith a supervised classification algorithm-support vector machines (SVM) and theremote sensing indices calculated from the ASTER data (SAVI, NMDI, NDBI, andNDWI), the land cover in the experimental area was classified into four types,namely, vegetation, bare soil, impervious surface and water surface.
- 12. 3.3 Statistics of area ratio and EMI MODIS LST In this study, the area proportion of 990 m ASTER LST each of the four land cover types within 90 m 990 m and 90 m resolution pixels were calculated to obtain AR990 and AR90. maximum, minimum, mean and variance of the remote sensing indices in the individual pixels at 990 m and 90 m resolutions were used as change indicators of remote sensing indices within the individual pixels.Class map 30 m
- 13. 3.4 GA-SOFM-ANN training and output A network with the selected SOFM of three hidden layerswas designed.Each vector computing unit in the networkcorresponded to a group of input parameters (4 for arearatios and 16 for statistical parameters of the endmemberindices of the four land cover types) and an output value oftemperature. The SOFM network biggest drawbacks are the slowness ofconvergence and the lack of effective evaluation mechanismsto terminate the iteration. However GA (genetic algorithms) isable to solve these problems and possesses thecharacteristics of adaptation, global optimization. To take theadvantages of both GA and SOFM, we combined the twoalgorithms and used GA to optimize the parameters’ value inthe process of rough and fine regulation.
- 14. 4 Results and analysis 3 2 (a) The percent of total pixels at 990 m resolution(%)(1)The analysis of area ratios 1 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 5 4 (b) 3 2 1 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 6 (c) 4 2 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 60 (d) 40 20 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 40 30 (a) The percent of total pixels at 90 m resolution(%) 20 10 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 50 40 (b) 30 20 10 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 100 80 (c) 60 40 20 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 100 80 (d) 60 40 20 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
- 15. 4 Results and analysis (1)The analysis of area ratios AR90 and AR990 are generally similar in spatial distribution. However, since the spatial resolution of AR990 is lower 11 times than that of AR90, the AR90, compared to AR990, can show more detailed information of area ratio change of the four land cover types than AR90. The progression of AR990 of the four land cover types was relatively large, resulting in a relatively continuous change process of area ratios. Meanwhile, the occurrence probability of pure pixel was greatly reduced. Conversely, AR90 show the opposite results.Area ratio Vegetation (%) Bare soil (%) Impervious (%) Water (%)MODIS 990, n=448 pixels at 990 m resolution≥0.25 86.61 55.13 22.10 3.35≥0.50 47.77 20.09 5.13 0.22≥0.75 10.71 2.01 0.89 0.00≥0.90 1.34 0.22 0.22 0.00ASTER 90, n=54208 pixels at 90 m resolution≥0.25 61.16 40.39 23.75 3.67≥0.50 48.19 29.11 14.10 2.27≥0.75 35.93 20.29 7.38 1.48≥0.95 24.91 12.88 2.45 0.83
- 16. 4 Results and analysis (2)The analysis of endmember remote sensing indices 1 1 990 m 990 m 990 m 0.9 90 m 0.9 90 m The normalized histogram frequency of pixels 90 m The normalized histogram frequency of pixels 0.8 0.8 0.7 SAVI~ 0.7 NMDI~ 0.6 0.6 0.5 0.5 The histogram distributions at the 0.4 0.4 0.3 0.3 0.2 0.2 two resolutions for 0.1 0.1 SAVI~ were very 0 0 0.05 0.1 0.15 0.2 0.25 SAVI~ 0.3 0.35 0.4 0.45 0.5 0 0.66 0.67 0.68 0.69 0.7 0.71 0.72 0.73 0.74 similar, but for the other three remote NMDI~ 1 1 990 m sensing index means, 0.9 990 m 90 m 0.9The normalized histogram frequency of pixels The normalized histogram frequency of pixels 90 m their histogram 0.8 0.8 0.7 NDBI~ 0.7 NDWI~ 0.6 0.6 distributions at the 0.5 0.5 990 m resolution 0.4 0.3 0.4 were more continuous. 0.3 0.2 0.2 0.1 0.1 0 0 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 -0.35 -0.3 -0.25 -0.2 -0.15 -0.1 -0.05 0 NDBI~ NDWI~ Histograms of the four remote sensing index means at the 990 m and 90 m resolutions.
- 17. 4 Results and analysis (2)The analysis of endmember remote sensing indices 2.5 6 RMSE(K) RMSE(K) 990 m 90 m RMSE(K) MAE(K) MAE(K) MAE(K) 5 2 4 1.5 Values(K)Values(K) 3 1 2 0.5 1 0 0 AR1 SAVI SAVI SAVI~ SAVI* AR2 NMDI NMDI NMDI~ NMDI* AR3 NDBI NDBI NDBI~ NDBI* AR4 NDWI NDWI NDWI~ NDWI* The errors of temperature estimated by a linear regression model between temperature and AR1 SAVI SAVI SAVI~ SAVI* AR2 NMDI NMDI NMDI~ NMDI* AR3 NDBI NDBI NDBI~ NDBI* AR4 NDWI NDWI NDWI~ NDWI* individual parameters of area ratios and remote sensing indices at the 990m(left) and 90 m (right) resolution. 990 m resolution-RMSE and MAE, were about 2 K and 1.5 K, respectively;90 m resolution-RMSE and MAE, were about 5 K and 3 K, respectively. The result indicated that at the 90 m resolution, the spatial variability of surface temperature was very obvious, and considerable error might be expected if single independent variables and single statistical regression models were adopted for subpixel temperature estimation.
- 18. 4 Results and analysis (3)Profile analysis of LST images 330 330 325 325 320 320LST(K) LST(K) 315 315 310 310 eTs 90 eTs 90 Ts 90 Ts 90 305 Ts 990 305 Ts 990 Ta990 Ta990 300 0 50 100 150 200 250 300 300 0 20 40 60 80 100 120 140 160 180 Distance steps(1 step=90 m) Distance steps(1 step=90 m) LST profiles for L1(a) and L2(b) locations Along the L1 and L2 transects, the LST profiles (curves) have a generally varying tendency. They have consistent “peaks” and “low points” along transects. The slight difference of LST between them might be caused by subpixel registration error and calibration difference of the two sensor systems. However, as expected, LST variability reduced with resolution decreasing.
- 19. 4 Results and analysis(4)Evaluating the accuracy of downscaled temperature An estimation error for the 73.82% of total pixels over the study area was within ± 3K. The overall estimated accuracy of the subpixel temperature at the 90 m resolution MODIS LST 90 m was relatively high (R2 = 0.709, MAE = 2.217 K and RMSE = 2.702 K). ASTER LST 90 mEstimated LST 90 m
- 20. 4 Results and analysis (5) DisEMI V.S. TsHARP techniqueIn order to assess advantages of our method, it is necessary to compare thisnew method with one or more present or widely used methods. Given it is awidely used regressive method, The TsHARP technique (Agam et al., 2007),a follow up improvement to the disaggregation procedure for radiometricsurface temperature (DisTrad, Kustas et al., 2003), was selected to comparewith our method. The accuracy of estimating 90 m resolution temperature produced by using the TsHARP method in this study was lower than that by using our DisEMI method. DisEMI TsHARP
- 21. 5 Discussion and conclusions The great difference between the spatial resolutions of MODIS LST and ASTERVNIR / SWIR data used in this study led to certain errors in geometrical registration,which directly influenced the accuracy of subpixel temperature estimation. MODIS and ASTER have some differences in the thermal infrared wavelengthrange, waveband response and temperature inversion algorithm, so there exist someproblems in verifying subpixel temperature by ASTER LST product. If estimating temperature in a large area, the DisEMI method must be optimizedto improve the computational efficiency based on prior knowledge. The increase of spatial resolution will increase the proportion of pure pixels, whichmay make a wider variation range of area ratios. The higher resolution also reducesthe variation progression of area ratios and leads to an approximately uniformdistribution. Various remote sensing indices related to pixel temperatures should befully used to ensure a high accuracy of subpixel temperature estimation. ASTER LST product was used to verify the estimated subpixel temperatures, andthe final estimated temperature distribution was basically consistent with that ofASTER LST product (R2 = 0.709, MAE = 2.217 K and RMSE = 2.702 K). Future work should emphasize on sensitivity analysis of other remote sensingindices to subpixel LST retrieval and absolute validation with ground measurements.
- 22. For more details, please refer to ScienceDirect website:http://www.sciencedirect.com/science/article/pii/S0034425711000174 Thanks for your attention!

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