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MODIS Snow Cover Quality Assurance Fields

Snow Cover PixelQA
Snow Spatial QA

The following discussion refers to the "Snow Cover PixelQA" (MOD10_L2 and MYD10_L2) and "Snow Spatial QA" data fields. The first two bits (0 and 1) provide information about the general quality of the product, and have a common meaning across all MODIS land products:

00 = nominal
01 = abnormal
10 = cloudy
11 = invalid

The remaining six bits are unique to the snow cover products. The local attribute "Key:" describes the setting and meaning of a bit.


Snow Cover PixelQA (MOD10_L2 and MYD10_L2)

Bit 2: Flags the occurrence of dead detectors in MODIS bands used in the snow algorithm. In the case of dead detectors, an averaged data value from MOD02HKM is used and this bit is set to 1, indicating that an averaged data value was used. If the bit is set to 0 a nominal data value was used.

Bit 3: Flags pixels that were viewed at sensor view angles greater than 45 degrees. If the bit is set to 1 then the observation of that pixel was greater than 45 degrees. Range scan angle is then 45 to 55 degrees. If the bit is set to 0 then the observation was within 45 degrees of nadir. Range of scan angles is 0 to 45 degrees.

Bit 4: Flags occurrences of highly uncertain MODIS band 6 radiance calculations from the MOD02HKM algorithm. If the bit is set to 1 then there was great uncertainty in its calculation. If the bit is set to 0 the uncertainty of calculation is much less.

Bit 5: Flags pixels where the cloud mask (MOD35_L2) was not determined. If the cloud mask was not determined, the snow algorithm assumes a clear-view condition. If the bit is set to 1 the cloud mask was not determined and not used. If the bit is set to 0 the cloud mask was used.

Bit 6: Flags the occurrence of noisy MOD02HKM data. If any of the MODIS bands used in the snow algorithm were flagged as noisy in the MOD02HKM data, then that flag setting is transferred to this bit. "Noisy" implies that there is more than a surface signal in the data. If this bit is set to 1 then one or more MODIS bands of input data were reported as noisy. If this bit is set to 0 the input data was nominal.

Bit 7: Flags the occurrence of unusable MOD02HKM data. The MOD02HKM algorithm flags data as unusable for several reasons. If unusable data is encountered the snow algorithm produces a "no decision" result and this bit is set to 1 to indicate that the MOD02HKM data were unusable. If the bit is set to 0 the data is nominal.

Snow Spatial QA

All MODIS snow products except for MOD10_L2 and MYD10_L2 contain a "Snow Spatial QA" field.

Bit 2: Flags the occurrence of observations that lie in the viewing azimuth range of 150 to 210 degrees. If the bit is set to 1 then the observation lies in that range. If the bit is set to 0 then it is outside that range.

Bit 3: Flags the occurrence of observations in a cell that fall below the minimal coverage threshold of 20% considered to be a good observation. If the bit is set to 1 the observation coverage was less than or equal to 20%. If the bit is set to 0 the observation coverage was greater than 20%.

A single byte consists of eight bits, read from right to left. Bit 0 is the "least-significant" bit, and bit 7 is the "most-significant" bit. Bit values are calculated as powers of two. Resulting byte values are calculated by adding the bit values for those bits that are "on."

Spatial QA pixel value Bit 3 Bit 2 Bit 1 Bit 0 Meaning
0 0 0 0 0 Nominal quality
Viewing azimuth out of 150° - 210° range
Observation coverage >20%
1 0 0 0 1 Abnormal quality
Viewing azimuth out of 150° - 210° range
Observation coverage >20%
2 0 0 1 0 Cloudy
Viewing azimuth out of 150° - 210° range
Observation coverage >20%
3 0 0 1 1 Invalid
Viewing azimuth out of 150° - 210° range
Observation coverage >20%
4 0 1 0 0 Nominal quality
Viewing azimuth between 150° - 210°
Observation coverage >20%
5 0 1 0 1 Abnormal quality
Viewing azimuth between 150° - 210°
Observation coverage >20%
6 0 1 1 0 Cloudy
Viewing azimuth between 150° - 210°
Observation coverage >20%
7 0 1 1 1 Invalid
Viewing azimuth between 150° - 210°
Observation coverage >20%
8 1 0 0 0 Nominal quality
Viewing azimuth out of 150° - 210° range
Observation coverage <=20%
9 1 0 0 1 Abnormal quality
Viewing azimuth out of 150° - 210° range
Observation coverage <=20%
10 1 0 1 0 Cloudy
Viewing azimuth out of 150° - 210° range
Observation coverage <=20%
11 1 0 1 1 Invalid
Viewing azimuth out of 150° - 210° range
Observation coverage <=20%
12 1 1 0 0 Nominal quality
Viewing azimuth between 150° - 210°
Observation coverage <=20%
13 1 1 0 1 Abnormal quality
Viewing azimuth between 150° - 210°
Observation coverage <=20%
14 1 1 1 0 Cloudy
Viewing azimuth between 150° - 210°
Observation coverage <=20%
15 1 1 1 1 Invalid
Viewing azimuth between 150° - 210°
Observation coverage <=20%