1

This is about the MCD12Q1.006 product and more specifically code 8 of the QA band.

The description for it is:

Backfilled label: Missing label from stabilization, filled with the pre-stabilized result.

There isn't any more info on this in the User Guide. And frankly I don't really know what this code is telling me. I've also quickly searched the MODIS Land Cover Product ATBD. It's Quality Control section didn't offer any help whatsoever. Finally I also had a quick look at the corresponding publication but didn't find any info on backfilling either.

Does anyone of you have any more info on what has to happen for a pixel to be flagged with this code?

2

The text from the product user guide states the following:

Following supervised classification of smoothed NBAR data, a set of post-processing steps that incorporate prior probability knowledge and adjust specific classes based on ancillary information are applied to the classification results (McIver and Friedl, 2002; Friedl et al., 2002). The final class-conditional probabilities have substantial levels of inter-annual variability caused by residual noise in input time series, missing data, and changes within the training database (Friedl et al., 2010). To reduce interannual variability caused by classifier instability, we developed an approach based on Hidden Markov Models that post-process map results for each year, which dramatically reduces inter-annual variability in the product (Abercrombie and Friedl, 2016). After stabilization, the classifications are condensed into the final set of six legends and associated QA information. Despite improving the stability to the product, we urge users not to use the product to determine post-classification land cover change. The amount of uncertainty in the land cover labels for any one year remains too high to distinguish real change from changes between classes that are spectrally indistinguishable at the coarse 500-m MODIS resolution. For more detailed information about the development and accuracy of the C6 MCD12Q1 product see Sulla-Menashe et al.

(Hightlighting done by me).

Based on the above quotation, it would seem that the "Hidden Markov Models" stabilization referenced in the QA layer can fail for some reason. Pixels with such failures are then labelled with code 8 in the QA, and the resulting value in MCD12Q1 is then the pre-stabilization value. I have been unable to find further details on what can cause such failures in the stabilization.

However, I'd personally interpret the code 8 as an indication of a more uncertain classification, since the interannual noise reduction failed. Although it could be interesting to evaluate the correlation between code 8 and actual land cover changes.

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  • Yes that's very likely the step which generates code 8. I guess I'll check out if Abercrombie and Friedl 2016 have any more info on what can cause it. Thanks! – JonasV Apr 26 at 7:50

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