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Format and Content

Important Note (17 January 2012)

An updated version (v3.10, 12 Jan 2012) 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.

QA Plan

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.

File Format Basics

MOD35_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 9 gridded parameters listed below is stored as a Scientific Data Set (SDS) within the HDF file.

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.

For more detailed information, consult the Cloud Mask Users Guide, written by Kathy Strabela in 2008.

MOD35_L2 Dimension List

  1. Cell_Along_Swath_5km = 406 (typical)
  2. Cell_Across_Swath_5km = 270 (typical)
  3. Cell_Along_Swath_1km = 2030 (typical)
  4. Cell_Across_Swath_1km = 1354 (typical)
  5. Byte_Segment = 6 (array values are 1 to 6 in sequence)
  6. QA_Dimension = 10
  7. Number_of_Instrument_Scans = 203 (typical)
  8. Maximum_Number_of_1km_Frames = 1354 (typical)

MOD35_L2 Scientific Data Set (SDS) List

    Geolocation and Time Parameters

  1. Latitude
      Description: Geodetic Latitude
      Dimensions: (Cell_Along_Swath_5km, Cell_Across_Swath_5km)
      Valid Range: -90 to +90 degrees north
  2. Longitude
      Description: Geodetic Longitude
      Dimensions: (Cell_Along_Swath_5km, Cell_Across_Swath_5km)
      Valid Range: -180 to +180 degrees east
  3. 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

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

    Science Parameters

      Cloud Mask (1km)

  8. Cloud_Mask
      Description: MODIS Cloud Mask and Spectral Test Results
      Dimensions: (Byte_Segment, Cell_Along_Swath_1km, Cell_Across_Swath_1km)
      Valid Range: bit field

      Quality Assurance (1km)

  9. Quality_Assurance
      Description: Quality Assurance for Cloud Mask
      Dimensions: (Cell_Along_Swath_1km, Cell_Across_Swath_1km, QA_Dimension)
      Valid Range: bit field

Cloud_Mask "Quality_Assurance" 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 32-35 of the MODIS Atmosphere QA Plan, available at the top of the page linked above.

Be sure to check this page linked above to make sure you have the latest updated (corrected) version.

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