Fig. 8. DNB time series, high-resolution image, and annual NTL change maps of the selected consistent dark pixel with pepper-and-salt noise. It is located in the vegetation areas of Mifflin, PA, US. (a) The DNB time series of a selected pixel with a commission error before the consistent dark pixel removal was applied. The red, green, blue, and grey colors indicate the different VZA intervals, lines represent the estimated models, small dots are the DNB observations, and the large magenta dot is the detected change (red cross in Fig. 8c) which would be excluded by the dark pixel removal (red cross in Fig. 8d). (b) The high-resolution Google Earth image in October 2015 of the selected barren pixel. The red rectangle represents the location and size of the selected pixel. (c) The accumulated annual NTL change maps before the dark pixel removal. (d) The accumulated annual NTL change maps after the dark pixel removal. The dark background in Fig. 8c and Fig. 8d is the Esri ArcMap Dark Gray Canvas base map.
3.5. Accuracy assessment
A total of six VIIRS tiles, including h10v04, h13v11, h18v04, h17v08, h29v05, and h32v12, were selected as the validation tiles to cover all six non-polar continents (Fig. 1). The stratified random sampling strategy (Olofsson et al., 2014) was applied for selecting the validation samples. The annual change maps from 2013 and 2021 of all the validation tiles were used for the stratification, and a total number of 1,088 samples, of which 931 from the stable stratum and 157 from the change stratum were selected. Each reference sample represents not only a location on the ground but also a place in time. Manual interpretation of the validation samples was conducted based on original NTL imagery, high-resolution images from Google Earth and PlanetScope data, International Space Station (ISS) nighttime photos, and other available socioeconomic data. A reference sample will be labeled as correct if the mapped category (e.g., change or stable) is the same as it is interpreted in the selected calendar year.