MODIS产品笔记整理

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：

https://lpdaac.usgs.gov/lpdaac/products/modis_products_table

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）