Consistent nighttime light time series in 1992–2020 in Northern Africa by combining DMSP-OLS and NPP-VIIRS data

Figures & data

Figure 1. The study area: the northern Equatorial Africa and Sahel region (shown by Google Earth Image). The Sahel region, defined by precipitation gradient, is shown by the red dashed area.

Table 1. Names of DMSP-OLS annual stable imagery product available for the satellite flying years.

Year\Sat.	F10	F12	F14	F15	F16	F18
1992	F101992
1993	F101993
1994	F101994	F121994
1995		F121995
1996		F121996
1997		F121997	F141997
1998		F121998	F141998
1999		F121999	F141999
2000			F142000	F152000
2001			F142001	F152001
2002			F142002	F152002
2003			F142003	F152003
2004				F152004	F162004
2005				F152005	F162005
2006				F152006	F162006
2007				F152007	F162007
2008					F162008
2009					F162009
2010						F182010
2011						F182011
2012						F182012
2013						F182013

Table 2. Specifications of DMSP-OLS VNIR band and the NPP-VIIRS Day/Night band (DNB).

Parameters	DMSP-OLS (VNIR)	NPP-VIIRS (DNB)
Builder	U.S. Air Force	NASA and NOAA
Orbit	Polar orbit satellite	Polar orbit satellite
Swath width	3000 km	3000 km
Nighttime overpass time	roughly around 19:30	1:30
Quantization	6 bit	14 bit
Spectral coverage	0.4–1.1 μm	0.5–0.9 μm
Saturation	Yes	No
On-board calibration	No	Yes
Data pixel value	DN	Radiance (W∙cm^−2∙sr^−1)
Spatial resolution (m)	2700 m (at nadir)	742 m (across full scan)
Dynamic range	10^−9 to 10^–6 W∙cm^−2∙sr^−1	10^−9 to 10^–2 W∙cm^−2∙sr^−1
Minimum detectable radiance	0.5 × 10^−9 W∙cm^−2∙sr^−1	0.2 × 10^−9 W∙cm^−2∙sr^−1

Figure 2. Workflow to generate the harmonized annual nighttime light radiance data for northern Equatorial Africa and Sahel from 1992 to 2020.

Figure 3. DN value time series of three reference pixel examples.

Figure 4. The second-order inter-annual-sensor calibrations (red lines) and scatter plots (black points) of the DN values in each yearly image vs. the DN values in the reference image (DN 2010) for the 7286 reference pixels.

Table 3. Annual values of the $q_1$, $q_2$ and $q_3$ coefficients of Equation 1(1) $D{N}_{cali}=q1\times D{N}^2+q2\times DN+q3$(1) to calibrate the annual OLS DN values to the reference 2010 DN values.

Year.	${\mathbf{q}}_1$	${\mathbf{q}}_2$	${\mathbf{q}}_3$	${\mathbf{R}}^2$
1992	0.001174	0.899175	2.180987	0.955079
1993	0.000764	0.931865	1.729595	0.952568
1994	−0.00437	1.24356	2.198144	0.964932
1995	−0.00177	1.101893	1.090882	0.9683
1996	0.00078	0.936427	1.005481	0.969964
1997	−0.00292	1.183886	0.634996	0.967126
1998	−0.00184	1.112663	0.690939	0.953975
1999	−0.00126	1.078789	0.573832	0.949541
2000	−0.00134	1.082741	0.458452	0.950093
2001	0.000608	0.956139	0.87435	0.942309
2002	−0.00097	1.059713	0.4325	0.944406
2003	−0.00393	1.245241	0.91821	0.939056
2004	−0.00157	1.090647	1.071164	0.934097
2005	−0.00522	1.325527	0.966511	0.935803
2006	−0.00397	1.234824	1.550502	0.940669
2007	−0.00337	1.210531	1.833878	0.943828
2008	−0.00033	0.997291	2.016405	0.950033
2009	−0.00245	1.131488	2.285291	0.958491
2010*	*	*	*	1
2011	0.000214	0.971559	1.335998	0.957656
2012	0.002113	0.844545	1.775839	0.958846
2013	0.002837	0.794146	2.102548	0.953742

Figure 5. Inter-annual-sensor calibration of annual OLS data on stable nighttime lights from 1992 to 2013 in the study area: TNDNs time series of the original OLS annual data for each satellite are shown in different colors, and TNDNs time series of the calibrated -OLS annual data are shown by black lines with circles.

Figure 6. DN time series of pixel example in the original OLS annual data for each satellite are shown in different colors, and DN time series of pixel example in the calibrated -OLS annual data are shown by black lines with circles.

Figure 7. Relationship between 2013 OLS DN and 2103 VIIRS radiance: (a) Median calibration relationship between median VIIRS radiance and OLS DN values; (b) inverse function of Figure 7a; median radiance points (red) and best fit (black line) according to Equation 3(3) $L=\frac{-a_3+\sqrt{4\times a_2\times ln\left(1-\frac{DN}{a_1}\right)+{a_3}^2-4\times a_2\times a_4}}{2a_2}$(3) (a, left) and Equation4(4) $DN=a_1\times \left(1-e^{a_2\times L^2+a_3\times L+a_4}\right)$(4) (b, right).

Figure 8. RMSE between the 2013 actual OLS DN data and the 2013 GLF filtered synthetic OLS DN data; optimal window size of 7 × 7 pixels and optimal standard deviation = 1.51 estimated as the values applying to the minimum RMSE; the color bar denotes the RMSE in radiance units because the calibration Equation 3(3) $L=\frac{-a_3+\sqrt{4\times a_2\times ln\left(1-\frac{DN}{a_1}\right)+{a_3}^2-4\times a_2\times a_4}}{2a_2}$(3) establishes a 1 to 1 relationship between DN and radiance values.

Table 4. The root mean square error (RMSE) and the correlation coefficient between the synthetic OLS DN data and the actual OLS DN data in 2013 before and after applying the Gaussian Low-pass Filter (GLF).

Error indicator	Before GLF	After GLF
RMSE	1.41	1.29
correlation coefficient	0.84	0.86

Figure 9. Histograms of the OLS2013 (blue) and the synthetic-OLS2013 (red) radiance data.

Figure 10. TNLs in 2013 vs. (a) Longitude and (b) Latitude in northern Equatorial Africa and Sahel; OLS2013 (blue line), synthetic-OLS2013 (red line) and VIIRS2013 (green line). (c) and (d) show the low-radiance detail of (a) and (b), respectively.

Figure 11. Local profile of the 2013 nighttime light intensity of a sample image segment in Accra, Ghana: (a) synthetic-OLS2013; (b) OLS2013; and (c) VIIRS2013; (d) nighttime light radiances along the transect-pixels in the three images (a, b and c), respectively. The transect is delineated by the yellow box.

Figure 12. Local profile of the 2013 nighttime light intensity of a sample image segment in Dakar, Senegal: (a) synthetic-OLS2013; (b) OLS2013; and (c) VIIRS2013; (d) nighttime light radiances along the transect-pixels in the three images (a, b and c), respectively. The transect is delineated by the yellow box.

Figure 13. Local profile of the 2013 nighttime light intensity of a sample image segment in the southern coast of Nigeria: (a) synthetic-OLS2013; (b) OLS2013; and (c) VIIRS2013; (d) nighttime light radiances along the transect-pixels in the three images (a, b and c), respectively. The transect is delineated by the yellow box.

Figure 14. Total radiance of nighttime lights in northern Equatorial Africa and Sahel based on the harmonized time series of OLS radiance data on nighttime lights between 1992–2020.

Figure 15. Relationship between 2013 OLS DN and the logarithm of 2103 VIIRS radiance and fitted curve (red line) according to Ma’s method in northern Equatorial Africa and the Sahel.

Figure 16. (a): actual OLS DN vs. NPP-VIIRS DN (generated by Median model) in 2013. (b): actual OLS DN vs. NPP-VIIRS DN(generated by BiDoseRsep model)in 2013. The black dotted lines indicate the 1:1 line (i.e. actual OLS DN = NPP-VIIRS DN). The red lines refer to the fitted linear model.

Data Availability Statement

The nighttime light data that support the findings of this study are available from the Earth Observation Group from Colorado School of Mines at https://eogdata.mines.edu/dmsp/downloadV4composites.html and https://eogdata.mines.edu/download_dnb_composites.html.