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Summary of bit processing for cloud mask:

Versions 1 and 2 data (1981 through 1993):

cmsk value bit 7 bit 2 bit 1 bit 0
0 0 0 0 0
1 0 0 0 1
2 0 0 1 0
3 0 0 1 1
4 0 1 0 0
5 0 1 0 1
6 0 1 1 0
7 0 1 1 1
128 1 0 0 0

bit 7: 0 = valid data, 1 = missing data
bits 2,1,0: 0 = clear, 1 = cloudy

For example, a data value of 5 in the cloud mask grid means that bits 0 and 2 are on, and bit 1 is off. Bits 0 and 2 refer to the standard CASPR multi-day cloud algorithm and the CASPR multi-day algorithm augmented by time series statistics, respectively. So, a value of 5 means that both of these algorithms detected cloud, while the time series algorithm (bit 1) did not detect cloud. Users can pick and choose which combinations of cloud masks to use, providing some control over how much confidence to have in the presence of clouds. This may also provide some insight into the types of clouds as well, since each algorithm has somewhat different sensitivities.

Version 3 data (1994 through 2000):

cmsk value bit 2 bit 1 bit 0
0 0 0 0
1 0 0 1
2 0 1 0
3 0 1 1
128 1 0 0

bit 2 : 0 = valid data, 1 = missing data
bits 1,0: 0 = clear, 1 = cloudy

For example, a data value of 2 in the cloud mask grid means that bit 1 is on and bits 0 and 2 are off. Bits 0 and 1 refer to the standard CASPR cloud single- and multi-day algorithms, respectively. Thus, a value of 2 means that the single-day algorithm did not detect cloud while the multi-day algorithm did detect cloud. Users can pick and choose which combinations of cloud masks to use, providing some control over how much confidence to have in the presence of clouds.