Landsat 5 Surface Reflectance Analysis Ready Data 2007

Data for 2007
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.

2007年数据集
本数据集为瑞士区域的Landsat 5地表反射率可直接分析数据（Analysis Ready Data，ARD）存档集，由美国地质调查局（United States Geological Survey，USGS）生成，相关信息可访问：https://www.usgs.gov/land-resources/nli/landsat/landsat-surface-reflectance。
关于Landsat 5任务的详细说明，可参阅CEOS地球观测手册：http://database.eohandbook.com/database/missionsummary.aspx?missionID=226。
Landsat存档极具研究价值：该项目是全球运行时间最长的地球观测（Earth Observation，EO）计划，始于1972年，近50年来持续提供稳定一致的观测数据。自2008年起，完整的Landsat存档已采用免费开放的共享政策，这使得密集时间序列分析成为可能，不再局限于对单组影像的简单历时对比，大幅提升了环境变化监测能力。
若要覆盖瑞士全境，需使用8景Landsat 5影像（轨道号/行号：193/027、194/027、195/027、196/027、193/028、194/028、195/028、196/028），对应覆盖纬度范围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——Level 2地表反射率产品，单景影像覆盖面积约185km×180km。Collection 1/Tier 1影像为当前最高质量的Landsat数据（例如已完成几何校正与辐射校正），适用于时间序列分析，相关说明可访问：https://www.usgs.gov/land-resources/nli/landsat/landsat-collection-1。
Level 2对应地表反射率产品，即基于Landsat传感器观测数据估算的地球表面反射入射太阳辐射的比例。该类数据已针对大气扰动（例如气溶胶散射、薄云干扰）完成校正，可实现不同影像与成像日期间的直接对比，符合ARD标准。
USGS提供的Landsat数据附带质量评估（Quality Assessment，QA）信息，可帮助用户判断数据是否适用于特定应用场景。本数据集同时包含8位LandsatLook质量影像与16位质量评估波段40。各文件的详细说明可访问：https://www.usgs.gov/land-resources/nli/landsat/landsat-collection-1-level-1-quality-assessment-band。
Level 2产品由USGS基于Level 1产品，通过官方LEDAPS/LASRC算法生成。所有基于Landsat地表反射率得到的光谱指数产品均附带像素质量保证（Pixel Quality Assurance，pixel_qa）波段。该波段采用无符号16位格式，数据以位打包形式存储，可提供像元状态相关信息，包括填充值、无云清晰区、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）。相关详细说明可访问：https://www.usgs.gov/land-resources/nli/landsat/landsat-sr-derived-spectral-indices-pixel-quality-band。