October 15, 2012 - Global Land Survey Collections Processed to Surface Reflectance

Optimizations to the LaSRC aerosol retrieval method will reduce Landsat 8 Surface Reflectance data processing times.  Instead of performing aerosol retrieval on a per-pixel basis, the new method performs the retrieval for a 3x3-pixel window. The aerosol value is computed for the center pixel of the window and is used to interpolate the aerosol values for the remaining clear pixels in the window. If the center pixel of the 3x3 window is found to be cloud or water, the algorithm will search the window for the closest non-cloudy, land pixel to compute the aerosol value for the 3x3 window. Any cloud, cloud shadow, or water pixels are assigned the median aerosol value of all 3x3 windows. The SR_AEROSOL QA band has been updated with new bit description .  

Pixels within the PIXEL_QA band will be modified as follows:

• Pixels flagged as high confidence cirrus (Bits 8-9) will no longer unset the clear bit (Bit 1).
• Pixels flagged as terrain occluded (Bit 10) will also no longer unset the clear bit (Bit 1).
• NOTE: Pixels flagged as high confidence cloud (Bits 6-7) will continue to unset the clear bit.
• Pixels flagged as high confidence cirrus (Bits 8-9) will no longer unset the clear bit (Bit 1).
• Pixels flagged as terrain occluded (Bit 10) will also no longer unset the clear bit (Bit 1).
• NOTE: Pixels flagged as high confidence cloud (Bits 6-7) will continue to unset the clear bit.

Optimizations to the LaSRC aerosol retrieval method will reduce Landsat 8 Surface Reflectance data processing times. Instead of performing aerosol retrieval on a per-pixel basis, the new method performs the retrieval for a 3x3-pixel window. The aerosol value is computed for the center pixel of the window and is used to interpolate the aerosol values for the remaining clear pixels in the window. If the center pixel of the 3x3 window is found to be cloud or water, the algorithm will search the window for the closest non-cloudy, land pixel to compute the aerosol value for the 3x3 window. Any cloud, cloud shadow, or water pixels are assigned the median aerosol value of all 3x3 windows. The SR_AEROSOL QA band has been updated with new bit descriptions.

• Added the option of NetCDF output file format
• The "blockiness" or image artifacts resulted from inputs to the aerosol retrieval which remained at the global climate model grid resolution versus being interpolated to the Landsat pixel resolution. These updates will eliminate the blockiness issue. A second level of blockiness occurred over areas where aerosol retrieval was not successful. The interpolation of those aerosols resulted in blocks of the same aerosol value. This has also been fixed with the implementation of a better aerosol interpolation method.
• Image extents are now restricted to 200,000,000 pixels, doubling the previous limit
• This new version of LaSRC tries to retrieve aerosols over all pixels, except cirrus pixels. The retrievals are then tested based first on the model residual and then on the NDVI combined with Band 5 reflectance. If these tests fail, then the pixel is marked as failed for aerosol retrieval. Any pixel which failed retrieval is ultimately attempted to be interpolated. Water pixels (flagged if the NDVI < 0.01), cloud pixels, and cirrus pixels are not interpolated. The final reflectance corrections are applied using the retrieved/interpolated aerosols.
• Previous interpolation issues along coastal water bodies led us to implement a land/water mask to better identify coastal waters, since aerosols were not being retrieved over coastal waters and this resulted in significant blockiness along the coastal areas. Given the change in the new version of LaSRC to attempt aerosol retrieval over all pixels, the higher resolution land/water mask has been removed.

• While interpolation solves the data continuity issues, it introduces additional uncertainty to the reflectance values in areas where aerosols were interpolated. Therefore, we are adding an additional band (unsigned 8-bit integer) with flags to indicate where aerosol measurements were successful, failed (likely water or clouds), or interpolated.
  • CFMask may now be ordered as a standalone product
  • Image extents are now restricted to 200,000,000 pixels, doubling the previous limit
  • Original Input Products (untouched from the source) have been removed as a product option. Input products (formerly Customized Input Products) are still available
• The "name" variable will be "cfmask" instead of "fmask"
• The "short_name" variable will be "LXXCFMASK" instead of "LXXFMSK", where LXX could be LT4, LT5 or LE7.
• The "long_name" variable will be "cfmask_band" instead of "fmask_band"
• Re-projection options for Polar Stereographic
• Re-projection and subset functionality for L1T products

User-defined output formats for HDF-EOS2 (.hdf) , GeoTIFF (.tif), and Binary (.img)