Fig. 3. Analysis of optimal buffer size for cloud/snow removal.
(a) Calibration accuracies for the cloud/snow buffer with different
moving window sizes. (b) The cloud/snow mask with 5x5 pixel buffer for a
NASA Lunar-BRDF-corrected Black Marble product output (VNP46A2) from
tile h10v04 on day-of-year 45 in 2015.
3.2. Viewing zenith angle stratification
The DNB observations were grouped into four VZA intervals, 0-20°,
20°-40°, 40°-60°, and 0-60°, to mitigate the angular effect. Some pixels
have uniform DNB values among different VZA ranges, while others have
uneven DNB magnitudes and large variations across the VZA (e.g., Fig 4).
This VZA-related DNB radiance disparity is mainly caused by the complex
and case-by-case joint angular effects of their viewing angle and local
geometry (Li et al., 2019a; Tan et al., 2022; Wang et al., 2021). For
low building and open area pixels with streetlights fully shielded, such
as rural settlements without adjacent occlusions (Fig. 4a), almost no
differences in the DNB value were observed among different VZA. While
significant VZA interval related disparities were observed in areas with
multiple story buildings and rural settlements surrounded by dense tree
canopy (Fig. 4b-c). In downtown areas with skyscrapers, near-nadir
observations have the largest overall radiance and variations (Fig. 4b).
In the dense residential areas, however, the off-nadir DNB radiances are
dramatically larger than the ones with lower VZA values (Fig 4c). This
inconsistency of the DNB radiances consequently led to inequalities in
the sensitivity of monitoring NTL changes, therefore disparities in the
occurrences, magnitude, and timing of the changes across the VZA. Fig. 5
shows a sample with disparate magnitudes and occurrences of DNB radiance
change among the different VZA ranges. The near-nadir time series has a
larger magnitude of change in early 2016 than the ones with 20°-40° VZA,
while this 2016 change is hardly noticeable from the off-nadir time
series with 40°-60° VZA (Fig 5). Thus, to enhance the temporal
consistency for angular affected NTL emission areas, we divided the VZA
range into three equal ranges, 0-20°, 20°-40°, and 40°-60°, following
the near-nadir (0-20°) and off-nadir (40°-60°) divisions of the NASA’s
Black Marble product. A fourth VZA interval of 0-60° is also included to
balance the trade-off between DNB time series temporal consistency and
temporal density, as time series observations stratified by VZA will
substantially reduce the data temporal density and therefore reduce
their capability in monitoring NTL changes that can be only observed for
a very short time (e.g., loss of electric grid power supply or
large-scale gathering events ).