NASA Global Web-Enabled Landsat Data Annual Global 30 m V032

The NASA Making Earth System Data Records for Use in Research Environments ([MEaSUREs](https://earthdata.nasa.gov/about/competitive-programs/measures)) Global Web-Enabled Landsat Data Annual (GWELDYR) Version 3.2 data product provides Landsat data at 30 meter (m) resolution for terrestrial non-Antarctica locations over annual reporting periods for the 2005 epoch. GWELD products are generated from all available Landsat 5 Thematic Mapper (TM) and Landsat 7 Enhanced Thematic Mapper Plus (ETM+) data in the U.S. Geological Survey (USGS) Landsat archive. The GWELD suite of products provides a consistent data source to derive land cover as well as geophysical and biophysical information for regional assessment of land surface dynamics.

The GWELD products include Nadir Bidirectional Reflectance Distribution Function (BRDF)-Adjusted Reflectance (NBAR) for the reflective wavelength bands and top of atmosphere (TOA) brightness temperature for the thermal bands. The products are defined in the Sinusoidal coordinate system to promote continuity of NASA's Moderate Resolution Imaging Spectroradiometer (MODIS) land tile grid.

Provided in the GWELDYR product are layers for surface reflectance bands 1-5 and 7, TOA brightness temperature for thermal bands, Normalized Difference Vegetation Index (NDVI), day of year, ancillary angle, and data quality information. A low-resolution red, green, blue (RGB) browse image of bands 5, 4, 3 is also available for each granule.

Known Issues
* GWELDYR Version 3.2 known issues can be found in Section 4 of the Algorithm Theoretical Basis Document (ATBD).
* Within the CFMask_State layer, the valid range is listed in the granule metadata as 0 to 65534 which includes the fill value. To avoid inclusion of the fill value, the valid range should be considered 1 to 65534 as reflected in the Data Layers Characteristics table above.
* Within the CFMask_State layer, the bit flag descriptions include a description for bit 0 that contradicts the listed fill value. In the description, bit 0 is stated as 1 when fill value is present, which prevents the use of 0 as the fill value. To avoid errors when using the CFMask_State layer, bit 0 should be ignored as an indicator of fill value.

美国国家航空航天局（National Aeronautics and Space Administration, NASA）面向研究环境构建地球系统数据记录项目（[MEaSUREs](https://earthdata.nasa.gov/about/competitive-programs/measures)）旗下的全球网络化陆地卫星（Landsat）年度数据（GWELDYR）3.2版产品，为2005年观测纪元的年度报告周期内的南极以外陆地区域提供30米分辨率的陆地卫星数据。GWELD系列产品由美国地质调查局（United States Geological Survey, USGS）陆地卫星档案库中所有可用的陆地卫星5号专题制图仪（Thematic Mapper, TM）与陆地卫星7号增强型专题制图仪Plus（Enhanced Thematic Mapper Plus, ETM+）数据生成。GWELD系列产品提供了一致的数据源，可用于提取土地覆盖以及地球物理和生物物理信息，以开展陆地表面动态的区域评估。

GWELD产品包含反射波段的天底方向双向反射分布函数校正反射率（Nadir Bidirectional Reflectance Distribution Function-Adjusted Reflectance, NBAR），以及热红外波段的大气顶层（Top of Atmosphere, TOA）亮温数据。该产品采用正弦坐标系进行定义，以适配美国国家航空航天局（NASA）中分辨率成像光谱仪（Moderate Resolution Imaging Spectroradiometer, MODIS）陆地瓦片网格的连续性要求。

GWELDYR产品包含以下数据层：地表反射率波段1-5与7、热红外波段的大气顶层亮温、归一化差分植被指数（Normalized Difference Vegetation Index, NDVI）、年积日、辅助角度数据以及数据质量信息。每个数据颗粒还附带波段5、4、3组成的低分辨率红绿蓝（RGB）预览图像。

已知问题
* GWELDYR 3.2版的已知问题可参见《算法理论基础文档（Algorithm Theoretical Basis Document, ATBD）》第4节。
* 在CFMask_State数据层中，数据颗粒元数据内所列的有效范围为0至65534，其中包含填充值。为避免引入填充值，有效范围应取1至65534，如前文《数据层特征表》所示。
* 在CFMask_State数据层中，位标志说明中对第0位的描述与所列填充值存在矛盾：文档中称当存在填充值时第0位为1，这使得0无法被用作填充值。为避免使用CFMask_State数据层时出现错误，应忽略将第0位作为填充值指示符的说明。