之前将笔记零碎的记录在Google Docs里，现在把它整理一下。
一、关于MODIS命名
我们知道，Terra和Aqua卫星都搭载有MODIS传感器，在MODIS产品名和文件名中则反映了数据来源。
• 以MOD开头表示是用Terra卫星数据生成的产品，
• 以MYD开头表示是用Aqua卫星数据生成的产品，
• 以MCD开头则表示是结合了两颗卫星数据合成的产品，字母C表示Combined。
完整的MODIS陆面产品列表参考以下网址：
二、关于数据格式
一般使用的MODIS数据产品有两种：
• .hdf文件：其文件扩展名为.hdf，但严格的说，它的文件格式是HDF-EOS。HDF-EOS是HDF的扩展，包含了geo-location，投影信息等。以.hdf文件提供MODIS的数据产品，每个文件就是经过Sinusoidal Tiling System分片后的一个tile。一个tile覆盖1200km*1200km。.hdf文件的陆面数据产品可以从https://lpdaac.usgs.gov获得。
• .txt文件：MODIS数据产品的子集，覆盖Selected Sites（一般是通量站点）7km*7km的大小。.hdf文件是根据产品的发布时间，如MOD09A1每个tile每8天提供一个HDF文件，而.txt则是一个站点一个产品一个文件，因此.txt文件可以更方便的得到时间序列。.txt文件的陆面数据产品可以从http://daac.ornl.gov获得。
三、 关于QC或是QA
MODIS每种数据产品都提供了质量控制（QC: Quality Control）或质量保证（QA: Quality Assurance）字段。QC信息是像元级的，标识了数据质量的高低，用户根据实际需求，使用QC信息筛选出质量足够好的数据。下面分别对MOD15A2和MOD09A1产品的QC字段进行说明。
• MOD15A2产品有两个QC字段：FparLai_QC和FparExtra_QC，FparLai_QC（8位）的描述如下：
Bit No. | Parameter Name | Bit Comb. | FparLai_QC |
|---|
| 0 | MODLAND_QC bits | 0 | Good quality (main algorithm with or without saturation) |
| 1 | Other Quality (back-up algorithm or fill values) |
| 1 | Sensor | 0 | Terra |
| 1 | Aqua |
| 2 | DeadDetector | 0 | Detectors apparently fine for up to 50% of channels 1,2 |
| 1 | Dead detectors caused >50% adjacent detector retrieval |
| 3–4 | CloudState (inherited from Aggregate_QC bits {0,1} cloud state) | 00 | 0 Significant clouds NOT present (clear) |
| 01 | 1 Significant clouds WERE present |
| 10 | 2 Mixed cloud present on pixel |
| 11 | 3 Cloud state not defined, assumed clear |
| 5–7 | SCF_QC (five level confidence score) | 000 | 0, Main (RT) method used, best result possible (no saturation) |
| 001 | 1, Main (RT) method used with saturation. Good,very usable |
| 010 | 2, Main (RT) method failed due to bad geometry, empirical algorithm used |
| 011 | 3, Main (RT) method failed due to problems other than geometry, empirical algorithm used |
| 100 | 4, Pixel not produced at all, value coudn't be retrieved (possible reasons: bad L1B data, unusable MODAGAGG data) |
（此表引用自：https://lpdaac.usgs.gov/lpdaac/products/modis_products_table/leaf_area_index_fraction_of_photosynthetically_active_radiation/8_day_l4_global_1km/mod15a2）
MOD15 ATBD中指出：The key indicator of retrieval quality of the LAI/FPAR product is SCF_QC bitfield.
MOD09A1产品也包含了两个QC字段：500m Reflectance Band Quality和500m State Flags，500m Reflectance Band Quality（32位）的描述如下：
Bit No. | Parameter Name | Bit Comb. | Sur_refl_qc_500m |
|---|
| 31 | adjacency correction performed | 1 | yes |
| 0 | no |
| 30 | atmospheric correction performed | 1 | yes |
| 0 | no |
| 26–29 | band 7 data quality four bit range | 0000 | highest quality |
| 1000 | dead detector; data interpolated in L1B |
| 1001 | solar zenith >= 86 degrees |
| 1010 | solar zenith >= 85 and <> |
| 1011 | missing input |
| 1100 | internal constant used in place of climatological data for at least one atmospheric constant |
| 1101 | correction out of bounds pixel constrained to extreme allowable value |
| 1110 | L1B data faulty |
| 1111 | not processed due to deep ocean or clouds |
| 22–25 | band 6 data quality four bit range | | SAME AS BAND ABOVE |
| 18–21 | band 5 data quality four bit range | | SAME AS BAND ABOVE |
| 14–17 | band 4 data quality four bit range | | SAME AS BAND ABOVE |
| 10–13 | band 3 data quality four bit range | | SAME AS BAND ABOVE |
| 6–9 | band 2 data quality four bit range | | SAME AS BAND ABOVE |
| 2–5 | band 1 data quality four bit range | | SAME AS BAND ABOVE |
| 0–1 | MODLAND QA bits | 00 | corrected product produced at ideal quality all bands |
| 01 | corrected product produced at less than ideal quality some or all bands |
| 10 | corrected product not produced due to cloud effects all bands |
| 11 | corrected product not produced due to other reasons some or all bands may be fill value [Note that a value of (11) overrides a value of (01)]. |
（此表引用自：https://lpdaac.usgs.gov/lpdaac/products/modis_products_table/surface_reflectance/8_day_l3_global_500m/mod09a1）
在我的工作中，只使用了MODLAND QA bits来进行质量控制。
这里需要特别指出的是：在QC字段中第0位是低位，即最右边那一位。
四、 地表覆盖类型（Land Cover Type）数据产品MCD12Q1
MCD12Q1是结合了Terra和Aqua两颗卫星的数据生成的地表覆盖类型数据产品，时间分辨率1年，空间分辨率500m。
MCD12Q1中包含5种分类体系（classification schemes）的分类结果，5种分类体系如下：
- • Land Cover Type 1: IGBP global vegetation classification scheme
- • Land Cover Type 2: University of Maryland (UMD) scheme
- • Land Cover Type 3: MODIS-derived LAI/fPAR scheme
- • Land Cover Type 4: MODIS-derived Net Primary Production (NPP) scheme
- • Land Cover Type 5: Plant Functional Type (PFT) scheme
| Class | IGBP (Type 1) | UMD (Type 2) | LAI/fPAR (Type 3) | NPP (Type 4) |
| 0 | Water | Water | Water | Water |
| 1 | Evergreen Needleleaf forest | Evergreen Needleleaf forest | Grasses/Cereal crops | Evergreen Needleleaf vegetation |
| 2 | Evergreen Broadleaf forest | Evergreen Broadleaf forest | Shrubs | Evergreen Broadleaf vegetation |
| 3 | Deciduous Needleleaf forest | Deciduous Needleleaf forest | Broadleaf crops | Deciduous Needleleaf vegetation |
| 4 | Deciduous Broadleaf forest | Deciduous Broadleaf forest | Savanna | Deciduous Broadleaf vegetation |
| 5 | Mixed forest | Mixed forest | Evergreen Broadleaf forest | Annual Broadleaf vegetation |
| 6 | Closed shrublands | Closed shrublands | Deciduous Broadleaf forest | Annual grass vegetation |
| 7 | Open shrublands | Open shrublands | Evergreen Needleleaf forest | Non-vegetated land |
| 8 | Woody savannas | Woody savannas | Deciduous Needleleaf forest | Urban |
| 9 | Savannas | Savannas | Non-vegetated | |
| 10 | Grasslands | Grasslands | Urban | |
| 11 | Permanent wetlands | | | |
| 12 | Croplands | Croplands | | |
| 13 | Urban and built-up | Urban and built-up | | |
| 14 | Cropland/Natural vegetation mosaic | | | |
| 15 | Snow and ice | | | |
| 16 | Barren or sparsely vegetated | Barren or sparsely vegetated | | |
| 254 | Unclassified | Unclassified | Unclassified | Unclassified |
| 255 | Fill Value | Fill Value | Fill Value | Fill Value |
（此表引用自：https://lpdaac.usgs.gov/lpdaac/products/modis_products_table/land_cover/yearly_l3_global_500_m/mcd12q1）
