Abstract
Obtaining population data is essential to identify the probability of
environmental degradation to occur in a certain area. However, in most part of the world
particularly in the developing world, few reliable data on population distribution are
available with reasonable spatial resolution (e.g. a few to several km.) An attempt was
made to estimate population of small regions using precisely calibrated "radiation data
set" by nocturnal DMSP/OLS imagery, which was newly elaborated by NOAA-NGDC.
Hokkaido Island of Japan was selected as the study area, for (a) this region has a variety
of population density, from very crowded city areas to virtually unpopulated rural zones,
and (b) most of the pixels covering the study area are free from saturation often
observed in previously elaborated "city lights data set" by extremely bright city lights.
The population data of the Hokkaido in the third-order standard grid system of the
Japanese government (45 arc. sec. in longitude and 30 arc. sec. in latitude - about 1 km
by 1 km) after the National Census of Population conducted in 1997 were used for this
analysis. Since the resolution of DMSP/OLS is 2.8 km, data sets were resampled into
the grid system of 90 arc. sec. by 90 arc. sec. (about 2 km by 3 km). Regression models
were developed between population density and (a) observed radiance in the "radiation
data set", or (b) probability of light detection in the "city light data set". The "radiation
data set" was found to represent the population density in a small region much well than
the previously developed "city light data set". It was owing to the broader dynamic
range of the "radiation data set" vis-ŕ-vis "city light data set". The .radiance data set.
could detect nocturnal light emitted by human activities even in low population density
area of 8 residents per km
2 . Further study should be carried out to see if the "radiance
data set" could be useful to estimate population density in the developing world.
1. Population as cause of environmental degradation
Obtaining population data is thus essential to identify the area, where
environmental degradation is likely to occur. However, in the developing world, very
limited amount of reliable data on population distribution are available in particular
with reasonable spatial resolution.
Attempts to estimate population with remote sensing imagery were made even
in rather early days of satellite remote sensing. Ogrosky (1975) showed that population
of some cities in the U.S.A. could be estimated with such parameters as area, number of
links to other cities, urban area of nearest larger neighborhood, and distance to the
nearby city. These parameters could be estimated with remotely sensed imagery. On a
developing country, Welch (1980) suggested that population of cities in China could be
expressed as a function of area of these cities as identified with LANDSAT/MSS
imagery.
2. Global nocturnal light data sets with DMSP/OLS data
The Operational Line-scan System (OLS) of the Defense Meteorological
Satellite Program (DMSP) detects both daytime and nighttime imagery of the Earth in
two bands of VNIR and TIR (thermal infrared). The OLS is able to measure at night
radiance in the VNIR band down to 10 -9 watts/cm 2 /sr/um, which is more than four
orders of magnitude more sensitive than other VNIR band sensors (Elvidge, et. al.,
1997a).
The NOAA/NGDC previously developed a global data set of the city lights
("city lights data set" hereinafter) by compiling hundreds of nocturnal DMSP/OLS
scenes (Elvidge et. al., 1997a). In this "city lights data set", probability (0% to 100%) of
detecting city lights by DMSP/OLS is given for each 30 arc. sec. grid of the world.
The NOAA/NGDC recently developed another global data set using
DMSP/OLS, in which precisely calibrated nocturnal radiation in visible-near-infrared
band is given for each 30 arc. sec. grid. This data set ("radiation data set" hereinafter)
was synthesized with DMSP/OLS data taken when the sensor was especially set to low
gain mode in March 1996, January 1997 and February 1997 (Elvidge, et. al., 1999).
3. DMSP/OLS data to estimate population
The aim of this paper is to figure out the feasibility of estimating population in
small regions with DMSP/OLS data.
The magnitude of nocturnal lights in a given area is supposed to be more or
less proportional to the population of the area. Elvidge et. al. (1997b; 1997c) showed
that population of a country is fairly well correlated with the area lit in "city lights data
set" of the same country as detected by DMSP/OLS on 52 countries of the world, both
in developed and developing regions. Elvidge et. al. (1999) also showed that the
cumulative radiation of 49 United States states, calculated with “radiance data set”,
correlates well with population in these states. Sutton et. al. (1997) applied the "city
lights data set" to estimate the population of counties in the United States. The result of
these studies inspired the authors that population of smaller regions, as compared with
states or counties, could be estimated using DMSP/OLS imagery.
4. Study area and data sets used
Hokkaido, the biggest island of Japan in its northern part, was selected as the
study area, for (a) this region has a variety of population density, from very crowded
city areas to virtually unpopulated rural zones, and (b) most of the pixels covering the
study area are free from saturation in "city lights data set" caused by extremely bright
city lights, as observed in the prefectures around the Tokyo metropolitan area. Hokkaido
has the area of 83,411 km 2 and the population of 5.7 million.
The population data of the Hokkaido in the third-order standard grid system of
the Japanese government (45 arc. sec. in longitude and 30 arc. sec. in latitude - about 1
km by 1 km) after the National Census of Population conducted in 1997 were obtained
from the Japanese government.
The resolution of DMSP/OLS is 2.8 km (Elvidge, et. al., 1997a). All the data
sets were accordingly resampled into the grid system of 90 arc. sec. by 90 arc. sec.
(about 2 km by 3 km). Figure 1a (left) shows the study area by "city lights data set". The
population density of the study area is shown as figure 1b (right).
Figure 1 : Hokkaido by "city lights data set" (left) and by population density (right)
5. Relations between data sets
The relation between population density and DMSP/OLS "radiation data set"
for pixels with population is shown in figures 2. The “radiation data set” apparently
represents the population density much well than the “city light data set” (Nakayama,
1998). It was thanks to the broader dynamic range of the “radiation data set” vis-ŕ-vis
“city light data set”.
Figure 2: Relation between population density and DMSP/OLS "radiation data set"
