Cloud Product (06_L2) File Format Basics

Cloud Mask (35_L2) Product files are stored in Hierarchical Data Format (HDF). HDF is a multi-object file format for sharing scientific data in multi-platform distributed environments. HDF files should only be accessed through HDF library subroutine and function calls, which can be downloaded from the HDF web site. Each of the parameters listed below is stored as a Scientific Data Set (SDS) within the HDF file.

Important Notes: 

• The Cloud Mask and Quality Assurance SDS's are stored at 1 kilometer pixel resolution. All other SDS's (those relating to time, geolocation, and viewing geometry) are stored at 5 kilometer pixel resolution. 

• An updated version of the MODIS Atmosphere QA Plan is available. This plan contains corrections to the to the Cloud Mask "Quality_Assurance" Bit Flags listed on pages 32-34. If you have any software that reads the Quality_Assurance bitflag SDS in 35_L2 files, review pages 32-34 of this updated document to make sure you are reading and interpreting these bit flags correctly. For more detailed information, see QA Plan. 

Cloud Mask Product (35_L2) Scientific Data Sets 
(Primary SDS's only, not a complete list)

Dimension List 

• Cell_Along_Swath_5km = 406 (typical)
• Cell_Across_Swath_5km = 270 (typical)
• Cell_Along_Swath_1km = 2030 (typical)
• Cell_Across_Swath_1km = 1354 (typical)
• Byte_Segment = 6 (array values are 1 to 6 in sequence)
• QA_Dimension = 10
• Number_of_Instrument_Scans = 203 (typical)
• Maximum_Number_of_1km_Frames = 1354 (typical)

35_L2 Scientific Data Set (SDS) List 

Geolocation and Time Parameters

• Latitude
  Description: Geodetic Latitude
  Dimensions: (Cell_Along_Swath_5km, Cell_Across_Swath_5km)
  Valid Range: -90 to +90 degrees north
• Longitude
  Description: Geodetic Longitude
  Dimensions: (Cell_Along_Swath_5km, Cell_Across_Swath_5km)
  Valid Range: -180 to +180 degrees east
• Scan_Start_Time
  Description: International Atomic Time at Start of Scan replicated across the Swath
  Dimensions: (Cell_Along_Swath_5km, Cell_Across_Swath_5km)
  Valid Range: 0.0 to 3.1558E+9 seconds since 1 January 1993 00:00:00

Solar and Viewing Geometry Parameters

• Solar_Zenith
  Description: Solar Zenith Angle, Cell to Sun
  Dimensions: (Cell_Along_Swath_5km, Cell_Across_Swath_5km)
  Valid Range: 0 to +180 degrees
• Solar_Azimuth
  Description: Solar Azimuth Angle, Cell to Sun
  Dimensions: (Cell_Along_Swath_5km, Cell_Across_Swath_5km)
  Valid Range: -180 to +180 degrees
• Sensor_Zenith
  Description: Sensor Zenith Angle, Cell to Sensor
  Dimensions: (Cell_Along_Swath_5km, Cell_Across_Swath_5km)
  Valid Range: 0 to 180 degrees
• Sensor_Azimuth
  Description: Sensor Azimuth Angle, Cell to Sensor
  Dimensions: (Cell_Along_Swath_5km, Cell_Across_Swath_5km)
  Valid Range: -180 to 180 degrees

Cloud Mask 

• Cloud_Mask
  Description: MODIS Cloud Mask and Spectral Test Results
  Dimensions: (6 Bytes, Cell_Along_Swath_1km, Cell_Across_Swath_1km)
  Valid Range: bit field
  Noteworthy: Internal 35_L2 documentation notes that Unobstructed FOV Flag = 1 means "Uncertain" it should read "Probably Cloudy" as noted below
  Bit-Field Interpretation: The Cloud_Mask SDS is represented in a 6 byte (48 bit) array. Individual bits or groups of bits are set to denote various cloud conditions and characteristics for that pixel. To get details on individual Cloud Mask bit flags see the Cloud Mask Users Guide View PDF.
  Shown below is the first byte only, of the six byte Cloud Mask:
   

Quality Flags

• Quality_Assurance
  Description: Quality Assurance for Cloud Mask
  Dimensions: (Cell_Along_Swath_1km, Cell_Across_Swath_1km, 10 Bytes)
  Valid Range: bit field
  Bit-Field Interpretation: Each pixel in the Cloud_Mask SDS is assigned a 10 byte (80 bit) array in the Quality_Assurance SDS noted above. Individual bits or groups of bits are set to denote various cloud conditions and characteristics for that pixel. To get details on the Cloud Mask Quality Assurance bit flags see pages 37-40 of the MODIS Atmosphere QA Plan View PDF.