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4 chenIGARSS_presentation.pptx


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4 chenIGARSS_presentation.pptx

  1. 1. Scale Effect of Vegetation Index Based Thermal Sharpening: A Simulation Study Based on ASTER Data<br />X.H. Chena, Y. Yamaguchia, J. Chenb, Y.S. Shia<br />a Graduate School of Environmental Studies, Nagoya University, Nagoya, 464-8601, Japan<br />b State Key Laboratory of Earth Surface Processes and Resource Ecology, Beijing Normal University, Beijing, 100875, China<br />
  2. 2. Outlines<br />1<br />3<br />Introduction<br />Scale Effect of NDVI-T Relationship<br />2<br />TsHARP<br />4<br />Improved TsHARP Method<br />5<br />Discussion and Conclusion<br />6<br />
  3. 3. 1. INTRODUCTION<br />Thermal infrared (TIR) band imagery has been widely applied in many studies (e.g. evapotranspiration esitimation; urban heat island; drought monitoring, etc.)<br />Unfortunately, the spatial resolution of TIR bands is usually coarser than that of visble-near infrared (VNIR) bands<br />Several thermal sharpening methods have been developed for sharpening spatial resolution of TIR band by using VNIR band<br />
  4. 4. Vegetation Index Based Thermal Sharpening<br />TsHARP(Kutas et al, 2003) was intensively studied<br />Negative correlation between NDVI and surface temperature (T)<br />NDVI-T Relationship established on coarse resolution is applied on fine resolution.<br />Previous studies found that the spatial resolution does not affect NDVI-T relationship largely;<br />However, another factor, spatial extent, was largely neglected in the previous studies.<br />Our study aims to: <br />Investigate the scale effect of NDVI-T<br />Improve TsHARP by considering the effect of spatial extent<br />
  5. 5. 2. TsHARP<br /><ul><li>Establish relationship between T and NDVI on the coarse resolution
  6. 6. The regression relationship is applied to the NDVI at their finer resolution (NDVIhigh).
  7. 7. Then, the divergence of the retrieved temperatures from the observed temperature field is due to spatial variability in T driven by factors other than vegetation cover, and can be assessed at the coarse resolution
  8. 8. This coarse-resolution residual field is added back into the sharpened map</li></ul>The slope is key parameter for sharpening result<br />
  9. 9. 3.SCALE EFFECT OF NDVI-T<br /><ul><li>3.1 Data
  10. 10. A subset image (256×256 pixels) with 90m resolution of ASTER captured in the grassland in Inner Mongolia, China (44.6ºN, 116.0ºE), on the date of July 16th, 2010, was used for study.
  11. 11. A subset image (256)</li></ul>VNIR band<br />NDVI<br />Surface Temperature<br />
  12. 12. SCALE EFFECT OF NDVI-T<br /><ul><li>Two aspects of “Scale”
  13. 13. Spatial Resolution (size of a pixel)
  14. 14. Spatial Extent (size of study area)</li></ul>90m<br />720m<br />1440m<br />
  15. 15. NDVI-T Relationship on Different Resolutions<br /><ul><li>NDVI and T images were resampled to different spatial resolutions (90m to 2880m) by linear aggregation.
  16. 16. Slope (a) of NDVI-T on different resolutions were investigated</li></ul>The regressed slope increases slightly with increasing of spatial resolution<br />
  17. 17. Spatial Extent of m pixels<br />Original image is divided into N/(m×m) windows.<br />Average the values of the pixels in each window<br />Local difference image is derived by subtracting the original image with the averaged image<br />Regression is conducted on the local difference images of NDVI and T<br />Local Difference Image<br />NDVI-T Relationship on Different Extents<br />
  18. 18. Regressed slope (a) increases with the increasing of spatial extent following a power function<br />Compared with spatial resolution, spatial extent affects regressed slope more largely. <br />(b)<br />(a)<br />Spatial extent (m)<br />
  19. 19. 4. IMPROVED TsHARP<br />Sharpening T image is equal to retrieving the local difference image of T on extent of a thermal pixel.<br />The regression relationship should be established on the spatial extent of one thermal pixel<br />Slope on extent of whole image (a)<br />We use the power function of (spatial extent -regressed slope) to estimate the slope (alocal) on the extent of one thermal pixel;<br />Improved TsHARP replaces a with alocal<br />Slope<br />Slope on extent of one thermal pixel (alocal):Unkown without high resolution T image <br />Slope on extent of 2×2 thermal pixels<br />Spatial Extent<br />
  20. 20. Algorithm Test<br />T image with 900m resolution was generated.<br />The coarse T image was sharpened to 90m using TsHARP and improved TsHARP respectively <br />TsHARP (a)<br />(23040m, 38.1)<br />Improved TsHARP (alocal)<br />Spatial extent (m)<br />
  21. 21. Sharpened Result<br />Coarse T image<br />TsHARP<br />Image sharpened by Improved TsHARP is smoother than that by original TsHARP<br />Improved TsHARP<br />True T image<br />(c)<br />℃<br />
  22. 22. Accuracy Assessment<br />Actual T image with 90m is used for accuracy assessment<br />Improved TsHARP<br />Best slope<br />TsHARP<br />The best value of slope is around 15.9<br />Improved method acquired higher sharpening accuracy<br />Original TsHARP over-sharpens the T image <br />
  23. 23. 5. DISCUSSION and CONCLUSION<br />Why spatial extent affects the NDVI-T relationship?<br />Other than NDVI, soil moisture also affects surface temperature. Assuming that<br />Since NDVI is somehow positively correlated with soil moisture, when T is regressed with only NDVI, the regressed slope becomes (for convenience, we assume the data is standardized)<br />As spatial pattern of moisture is smoother than NDVI, when spatial extent increases, the correlation between NDVI and Moisture increases, consequently the regressed slope also increases.<br />15<br />
  24. 24. Conclusion<br />Spatial extent is an important factor affecting the NDVI-T relationship, and should not be neglected in the related studies<br />Improved TsHARP considers the effect of spatial extent and can acquire better sharpening result in this case of study.<br />
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