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.
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|
|Not used||00000000||0||No type assigned||0|
|Not used||00000010||2||Sea ice||0|
|Not used||00000011||3||Sea ice + land||0.02|
|00000101||5||Ocean + land||5.12|
|00000110||6||Ocean + sea ice||17.06|
|00000111||7||Ocean + sea ice + land||3.69|
|Not used||00001000||8||Ice sheet||0|
|00001001||9||Ice sheet + land||9.18|
|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|