3. Methods
We proposed an algorithm called “View Zenith Angle (VZA) Stratified COLD” (hereafter referred to as VZA-COLD), which is built on the Landsat-based COntinuous monitoring of Land Disturbance (COLD) algorithm (Zhu et al., 2020). It includes three major innovations that are: (i) cloud/snow buffer; (ii) change detection based on observations from four stratifications of VZA; and (iii) consistent dark pixel (DNB radiance < 1.0 \(nW*m^{-2}*\text{sr}^{-1}\)) removal. The workflow of the VZA-COLD algorithm is illustrated in Fig. 2. The first component involved removing the remaining cloud and snow observations based on the exclusion of cloud and snow edge pixels that are partially influenced by clouds or snow. In the second component, DNB observations were stratified into four groups based on the VZA intervals to mitigate the variance caused by the compounded impacts from the different viewing geometry and surface condition, and thus, reduce the overall time-series variation. For each VZA interval, NTL changes were monitored based on the change detection framework similar to the COLD algorithm to obtain the individual sets of estimated time series models and detect a unified set of breakpoints among all the VZA intervals continuously. The third component involved filtering the consistent dark pixels with a minimum detectable NTL radiance threshold. The VZA-COLD algorithm was calibrated based on the calibration samples, in which the changes detected within the period of ± six months of the calibration sample change intervals were determined as the right call. Metrics of omission rates, commission rates, and F1 score of NTL change were used to evaluate algorithm performance. The final map accuracy was evaluated based on independent validation samples following the “good practice” protocols (Olofsson et al., 2014).