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NASA Distributed Active Archive Center (DAAC) at NSIDC

ICESat / GLAS Data

Ice, Cloud, and land Elevation / Geoscience Laser Altimeter System

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ICESat/GLAS Surface-Type Mask


The NASA GLAS science team developed a global mask to flag surface types for use in creating standard ICESat/GLAS data products. This mask was originally for science team use only, to ensure proper transition from one surface type to another, with surface types overlapping. NSIDC is now offering this surface-type mask as a standalone product to the public. It should be used strictly as a tool to learn how surface types were flagged for use in creating standard ICESat/GLAS data products. Other publicly available surface-type masks may be more suitable for other applications, depending on the scientific objective. The surface-type mask is available via FTP.

Following is a reduced-resolution image created from the surface-type values.

Surface types

The data are stored as an 10800 by 5400 byte array, where each byte represents the surface type for one 2 arc minute grid cell on the earth. Each 10800 byte record represents a 360 degree latitude band. The first record starts at the north pole and the records follow at 2 arc minute intervals to the south pole. Within a record, the first byte is at 180° W and proceeds east at 2 arc minute intervals to 180° E.

Valid pixel values range from 1 to 15. The surface types are bit-coded, and any combination of surface type is allowed, although not all combinations are actually used. The bit coding is:

bit 0 = land
bit 1 = sea ice (winter maximum extent)
bit 2 = ocean
bit 3 = ice sheet

where bit 0 is the least significant bit. The table below shows the values used in the file.


Color in above image Binary value Decimal value Surface types Percentage of the mask Data in Product
Not used 00000000 0 No type assigned 0
  00000001 1 Land 18.88 GLA05, GLA06, GLA14
Not used 00000010 2 Sea ice 0
Not used 00000011 3 Sea ice + land 0.02
  00000100 4 Ocean 43.12 GLA05, GLA06, GLA15
  00000101 5 Ocean + land 5.12 GLA05, GLA06, GLA14, GLA15
  00000110 6 Ocean + sea ice 17.06 GLA05, GLA06, GLA13, GLA15
  00000111 7 Ocean + sea ice + land 3.69 GLA05, GLA06, GLA13, GLA14, GLA15
Not used 00001000 8 Ice sheet 0
  00001001 9 Ice sheet + land 9.18 GLA05, GLA06, GLA12, GLA14
Not used 00001010 10 Ice sheet + sea ice 0
Not used 00001011 11 Ice sheet + sea ice + land 0.01
Not used 00001100 12 Ice sheet + ocean 0
Not used 00001101 13 Ice sheet + ocean + land 0.02
Not used 00001110 14 Ice sheet + ocean + sea ice 0
  00001111 15 Ice sheet + ocean + sea ice + land 2.90 GLA05, GLA06, GLA12, GLA13, GLA14, GLA15

Page last updated: 03/12/14