Landsat 5 Surface Reflectance Analysis Ready Data 1993

Data for 1993
This dataset is the Landsat 5 Surface Reflectance Analysis Ready Data (ARD) collection archive for Switzerland, generated by USGS (https://www.usgs.gov/land-resources/nli/landsat/landsat-surface-reflectance).

Detailed description about the Landsat 5 mission can be found in the CEOS Earth Observation Handbook: http://database.eohandbook.com/database/missionsummary.aspx?missionID=226.

The Landsat archive is extremely interesting because this is the longest EO program, initiated in 1972, it has provided continual and consistent observations for almost 50 years. Since 2008, the complete data archive has been provided under a free and open access policy. This has enabled dense time-series analysis, moving beyond simple diachronic comparison of a set of images, therefore dramatically improving capabilities to monitor environmental changes. To cover the whole of Switzerland, it requires eight Landsat 5 scenes (Path/Row: 193/027, 194/027, 195/027, 196/027, 193/028, 194/028, 195/028, 196/028) representing an area of latitude 44.9 to 48.7 and longitude 4.1 to 12.8. Data are downloaded as Collection 1/Tier 1 – Level 2 Surface Reflectance encompassing a surface of approximately 185km by 180km. Collection 1/Tier 1 scenes are data with the highest available data quality (e.g., geometric and radiometric corrections) and considered suitable for time-series analysis (https://www.usgs.gov/land-resources/nli/landsat/landsat-collection-1). Level 2 corresponds to surface reflectance (i.e., the estimate based on Landsat sensor observations of the fraction of incoming solar radiation reflected from Earth’s surface). These data are corrected for atmospheric perturbations (e.g., aerosol scattering, thin clouds) enabling direct comparison between multiple images and dates. This corresponds to the ARD level.

Landsat data are provided by USGS with Quality Assessment (QA) information to help users to determine their suitability for specific applications. An 8-bit LandsatLook Quality Image and 16-bit Quality Assessment Band40 are also included. Details on each file are described at: https://www.usgs.gov/land-resources/nli/landsat/landsat-collection-1-level-1-quality-assessment-band. Level 2 products are generated by the USGS from level 1 product and using the official LEDAPS/LASRC algorithm. "A Pixel Quality Assurance (pixel_qa) band is provided with all Landsat Surface Reflectance-derived Spectral Indices. The band is in unsigned 16-bit format, values are bit-packed and provide information pertaining to a pixel condition of fill, clear, water, cloud shadow, snow, cloud (yes/no), cloud confidence and cirrus cloud confidence (Landsat 8 only)" https://www.usgs.gov/land-resources/nli/landsat/landsat-sr-derived-spectral-indices-pixel-quality-band.

1993年数据集
本数据集为美国地质调查局（USGS）生成的瑞士地区Landsat 5地表反射率分析就绪数据（Analysis Ready Data，ARD）合集档案，数据来源链接：https://www.usgs.gov/land-resources/nli/landsat/landsat-surface-reflectance。
关于Landsat 5任务的详细说明可查阅CEOS地球观测手册，链接：http://database.eohandbook.com/database/missionsummary.aspx?missionID=226。
Landsat档案极具科研价值：该项目是全球持续时间最长的地球观测（EO）计划，始于1972年，近50年来始终提供连续且一致的观测数据。自2008年起，完整的Landsat档案已实行免费开放获取政策，这使得密集时序分析成为可能，不再局限于单组影像的简单历时对比，极大提升了环境变化监测能力。
为覆盖瑞士全境，需使用8景Landsat 5影像（条带号/行号：193/027、194/027、195/027、196/027、193/028、194/028、195/028、196/028），对应纬度范围44.9°至48.7°、经度范围4.1°至12.8°的区域。下载的数据为Collection 1/Tier 1——二级地表反射率产品，单景影像覆盖面积约185km×180km。
Collection 1/Tier 1影像是当前可用的最高质量数据（已完成几何校正与辐射校正等处理），适用于时序分析，详细说明链接：https://www.usgs.gov/land-resources/nli/landsat/landsat-collection-1。
二级产品对应地表反射率：即基于Landsat传感器观测数据，对地球表面反射的入射太阳辐射比例进行估算的结果。该类数据已针对大气扰动（如气溶胶散射、薄云干扰）完成校正，可实现不同影像与不同日期间的直接对比，符合ARD标准。
Landsat数据由USGS提供，并附带质量评估（QA）信息，以帮助用户判断数据是否适用于特定应用场景。数据集还包含8位LandsatLook质量影像与16位质量评估波段40（Quality Assessment Band40）。各文件的详细说明链接：https://www.usgs.gov/land-resources/nli/landsat/landsat-collection-1-level-1-quality-assessment-band。二级产品由USGS基于一级产品，通过官方LEDAPS/LASRC算法生成。所有Landsat地表反射率衍生光谱指数均附带像素质量评估（pixel_qa）波段：该波段采用无符号16位格式，数据以比特打包方式存储，可提供像元状态信息，包括填充值、清晰像元、水体、云阴影、积雪、云存在与否、云置信度以及卷云置信度（仅Landsat 8适用），详细说明链接：https://www.usgs.gov/land-resources/nli/landsat/landsat-sr-derived-spectral-indices-pixel-quality-band。