Running Title: Nightsat Potential for Global Mapping of Development Via A Nightsat Mission

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Running Title: Nightsat

Potential for Global Mapping of Development Via A Nightsat Mission

CHRISTOPHER D. ELVIDGE 1

JEFFREY SAFRAN 2

BENJAMIN TUTTLE 2

PAUL SUTTON 3

PIERANTONIO CINZANO 4

DONALD R. PETTIT 5

JOHN ARVESEN 6

CHRISTOPHER SMALL 7

1 Earth Observation Group, NOAA-NESDIS National Geophysical Data Center, Boulder,

Colorado 80305 USA (tel. 1-303-497-6121, fax 1-303-497-6513, chris.elvidge@noaa.gov).

2 Cooperative Institute for Research in the Environmental Sciences, University of Colorado,

Boulder, Colorado USA.

3 Department of Geography, University of Denver, Denver, Colorado, USA.

4 Istituto di Scienza e Tecnologia dell’Inquinamento Luminoso (ISTIL), Thiene, Italy

5 NASA Johnson Spaceflight Center, Houston, Texas, USA

6 Cirrus Digital Systems, Tiburon, California, USA

7 Lamont Doherty Earth Observatory of Columbia University, Palisades, New York, USA

Submitted for publication in the GeoJournal Special Issue "Where Are We?" on November 2,

2006.

Manuscript

Click here to download Manuscript: Nightsat_GeoJ_20070530Anon.doc

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Potential for Global Mapping of Development Via A Nightsat Mission

ABSTRACT

Nightsat is a concept for a satellite system capable of global observation of the

location, form and density of lighted infrastucture and development within human

settlements. Nightsat’s repeat cycle should be sufficient to produce an annual

cloud-free composite of surface lighting to enable detection of growth rates.

Airborne and satellite imagery have been used to define the range of spatial,

spectral, and detection limit options for a future Nightsat mission. Our conclusion

is that Nightsat should collect data from a near-synchronous orbit in the early

evening with 50- to 100-m spatial resolution and have detection limits of 2.5E-8

Watts cm-2sr-1μm-1 or better. Multispectral low-light imaging data would be

better than panchromatic data by providing valuable information on the type or

character of lighting, a potentially stronger predictor of variables such as ambient

population density and economic activity.

Keywords: Urban, exurban, nighttime lights, global mapping.

1. Introduction

Artificial lighting is a unique indicator of human activity that can be measured from space. We

propose a Nightsat mission, which could be used to map the extent and character of development

more accurately and completely than most currently available tools. Improved global mapping of

human settlements and their annual growth rates would address a variety of science and policy

issues in the 21st century — an era in which human population numbers are expected grow

substantially from the current 6+ billion mark.

The density of infrastructure — or ‘urbanness’ — can be viewed as a continuum ranging from

wilderness at one extreme to central business districts at the other extreme (Weeks, 2004).

Human beings tend to cluster in spatially limited settlements with more than 50% of global

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population living in dense settlements occupying less than 3% of the world’s land area (Small

and Cohen, 2004). Today more than half of the population lives in urban areas (United Nations,

2006). Urban populations are growing rapidly in the developing countries of Latin America,

Asia, and Africa. In Europe, North America, and Japan the population percent in urban

populations are already near 80%. Sprawl on the urban fringe and exurban development are

widespread. But structural change permeates urban areas through continuous redevelopment

through the replacement of aging infrastucture. Over time urban areas are in a constant state of

redevelopment and flux that reflect both growing urban populations and the evolution of

technologies.

A recent report written by the Space Studies Board of the National Research Council had this to

say regarding the observation of human impacts (Space Studies Board, National Research

Council, 2007):

“Human influences on the Earth are apparent on all spatial and temporal scales. Thus, an effective

Earth information system requires an enhanced focus on observing and understanding the impact of

humans, the influence and evolution of the built environment, and the study of demographic and

economic issues. For instance, space-derived information on urban areas can provide a platform for

fruitful interdisciplinary collaboration among Earth scientists, social scientists (e.g., urban planners,

demographers, and economic geographers), and other users in the applications community. Data on

the geographic “footprint” of urban settlements, identification of intra- urban land-use classes, and

changes in these characteristics over time are required to facilitate the study of urban population

dynamics and composition, and thereby to improve the representation of human- modified landscapes

in physical and ecological process models. Because of the rapid growth in urban areas, particularly in

the developing world where there are few alternative sources of information on urban extent and land

cover, these observations are needed to understand a growing source of anthropogenic forces on

regional weather and climate, air and water quality, and ecosystems, and to apply this understanding

to protect society and manage natural resources.”

The potential value for satellite remote sensing of artificial lighting stems in part from the

difficulty in global mapping of human settlements in a repeatable, timely manner from traditional

sources. Although development features can be extracted from high spatial resolution (~1-m)

satellite imagery, the production of annual maps of human settlements on a global basis from

these data sources is not feasible (at this time) from either a collection or analysis perspective.

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Moderate resolution Landsat-style systems are capable of global collections on an annual basis

and such data have been used effectively for mapping urban areas and tracking growth in local

settings. However, recent comparative analyses of Landsat data from diverse urban areas

worldwide indicate that the diversity of construction materials and construction types precludes

the existence of any unique spectral signature for urban areas as a thematic class (Small 2005).

In contrast, the remote sensing of artificial lighting provides an accurate, economical, and

straightforward way to map the global distribution and density of developed areas. The

widespread use of such lighting is a relatively recent phenomenon, tracing its roots back to the

electric light bulb commercialized by Thomas Edison in the early 1880s. Artificial lighting has

emerged as one of the hallmarks of modern development and provides a unique attribute for

identifying the presence of development or human activity that can be sensed remotely. While

there are some cultural variations in the quantity and quality of lighting in various countries,

there is a remarkable level of similarity in lighting technology and lighting levels around the

world.

The only satellite sensor currently collecting global low-light imaging data suitable for mapping

urban lighting is the U.S. Air Force Defense Meteorological Satellite Program (DMSP)

Operational Linescan System (OLS). Results to date obtained from DMSP-OLS nighttime lights

data indicate that this data source falls short of the science community’s requirements for urban

studies. The objective of this paper is to review the science community’s requirements for global

satellite observations of human settlements and to define a Nightsat mission concept to address

an number of these requirements.

2. The DMSP-OLS

Beginning in the early 1970s the U.S. Air Force Defense Meteorological Satellite Program

(DMSP) has operated polar orbiting platforms carrying cloud imaging sensors with low light

imaging capability. These sensors have two broad spectral bands: visible–near infrared (VNIR)

and thermal infrared (TIR). The program began with a sensor known as the SAP (Sensor

Aerospace Vehicle Electronics Package) flown from 1970 to 1976. The current generation of

OLS sensors began flying in 1976 and are expected to continue flying until at least 2012. At

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night the visible band is intensified with a photomultiplier tube to permit detection of clouds

illuminated by moonlight. The OLS detects radiances down to the 5E-10 Watts cm-2sr-1 range,

which makes it possible to detect artificial sky brightness surrounding large cities and gas flares.

A digital archive for the DMSP-OLS data was established in mid-1992 at the NOAA National

Geophysical Data Center. In 1994 NGDC began developing algorithms for producing annual

global cloud-free composites of nighttime lights using OLS data (Elvidge et al. 1997a, 1999,

2001). These annual products have been used for a variety of applications, including:

• Spatial modeling of population density (Dobson et al. 2000, Sutton 1997 and 2003,

Sutton et al., 1997, 2001, 2003,) and economic activity (Elvidge et al. 1997b, Doll et al.

2000, Ebener et al. 2005, Sutton et al. 2007).

• Quantifying both spatial and size distributions of urban land use for comparative analyses

of the global scaling properties of settlement size distributions (Small et. al., 2005) and

discrimination of urban and rural population distributions (Balk et al, 2005, GRUMP,

2006, Small, 2004).

• Estimation of the density of infrastructure (Elvidge et al. 2004) for use in hydrologic

modeling, flood prediction, the assessment of losses in agricultural land (Imhoff et al.

1997), terrestrial carbon dynamics (Milesi et al. 2003 and 2005, Imhoff et al. 2004).

• Modeling of artificial sky brightness and its effect on the visibility of astronomical

features (Cinzano et al. 2000, 2001a, 2001b, 2004).

• Spatial modeling of anthropogenic emissions to the atmosphere (Saxon et al., 1997 and

Toenges-Schuller et al. 2006).

From these and other studies the shortcomings of the OLS data for urban analyses have been

defined: 1) coarse spatial resolution (2.7 km ground sample distance), 2) lack of on-board

calibration, 3) lack of systematic recording of in-flight gain changes, 4) limited dynamic range,

5) six-bit quantitization, 6) signal saturation in urban centers resulting from standard operation at

the high gain setting, 7) lack of a thermal band suitable for fire detection, 8) limited data

recording and download capabilities (most OLS data are averaged on-board to enable download

of global coverage), 9) lack of a well characterized Point Spread Function (PSF), 10) lack of a

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well characterized Field-of-View (FOV), and 11) lack of multiple spectral bands for

discriminating lighting types.

The follow-on for the OLS is the Visible/Infrared Imager/Radiometer Suite (VIIRS) which will

fly on the National Polar-orbiting Operational Environmental Satellite System (NPOESS). The

first VIIRS is currently being built and represents an improved, but still imperfect, instrument to

measure nocturnal lighting (Lee et al. 2004). The NPOESS VIIRS instrument will provide low-

light imaging data with improved spatial resolution (0.742 km), wider dynamic range, higher

quantitization, on-board calibration, and simultaneous observation with a broader suite of bands

for improved cloud and fire discrimination over the OLS. The VIIRS is not, however, designed

with the objective of sensing nighttime lights. Rather, it has the objective of nighttime visible

band imaging of moonlit clouds — the same mission objective of the OLS low-light imaging.

While the VIIRS will acquire improved nighttime lighting data, it is not optimal for this

application. In particular, the VIIRS low-light imaging spatial resolution will be too coarse to

permit the observation of key nighttime lighting features within human settlements and the

spectral band to be used for the low-light imaging is not tailored for nighttime lighting.

3. A Remote Sensing Product Suite for Human Settlements

Below is a listing of a basic product suite for human settlements that could potentially be derived

from satellite observations of lights. Because of the rapid changes ongoing in human settlements

worldwide, these key global datasets would ideally be updated annually with a product latency of

not more than one year for measurement of the rates of change and annual growth increments.

• Products depicting the geographic “footprints” of human settlements of all sizes. This

includes the outline of the developed areas and estimates of the constructed area.

• Location and extent of sparse development in rural areas.

• Identification of intra-urban classes, such as residential areas, heavily lit commercial and

industrial areas, and open lands with little or no development present.

• Vectors for streets and roads based on alignments of lights.

Our assessment is that a low light imaging system optimized for deriving the listed human

settlement products would generate data suitable for a wide range of urban, environmental and

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socio-economic applications. It is clear that having radiance calibrated data is a crucial

requirement for the quantitative applications and change detection using nighttime lights data.

4. Data

To explore the remote sensing of nighttime lights we have examined high spatial resolution

airborne and moderate resolution satellite data collected at night.

4.1 Airborne Imagery

NASA acquired Cirrus Digital Camera System (DCS) data at night over Las Vegas, Nevada and

Los Angeles, California on September 27, 2004 at 13.7 km above sea level. Digital photography

was acquired using an 80 mm lens and a 1/60th second exposure. The Cirrus DCS is designed as

a color camera. For the nighttime collections the infrared blocking filter was removed and the

signal from each of the three bands were aggregated in post flight processing to form

panchromatic imagery. The collections span desert areas devoid of lighting, cross a wide range

of development types, and encounter the world’s brightest light, emitted from the Luxor Hotel

and Casino in Las Vegas. The Luxor lighting installation is composed of thirty-nine 7000-Watt

xenon lamps, pointing straight up into the sky. The lighting installation intensity reaches 40

billion candelas. The spatial resolution of the Cirrus camera data was 1.5 m at 16-bit

quantitization. The Cirrus camera was subsequently calibrated using an integrating sphere at the

NASA Ames Research Center.

As seen in Figure 1, it is possible to detect many individual light sources in the Cirrus imagery.

Note that at this spatial resolution nighttime lights provide a very different view of development

when compared to high resolution daytime imagery (Figure 1). To analyze the spatial resolution

limits that should be considered for a Nightsat, the high spatial resolution imagery was

aggregated to 25, 50, 100, 200 and 742 meter resolution (Figure 2). The 742 meter aggregation

simulates the low light imagery that will be collected by the VIIRS instrument.

4.2 Space Imagery

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We investigated the feature content of moderate-resolution color imagery of lights at night from

space with digital photography acquired from the International Space Station. Astronauts have

long marveled at the shear beauty of cities at night. However, the features have been difficult to

photograph due to mismatches between the optimal exposure times and the velocity of the

spacecraft. During Expedition 6 (November 23, 2002 through May 3, 2003) to the International

Space Station, a mechanized but manually driven image motion compensation mount was built

from existing hardware and color images were acquired of cities at night all over the world

through an optical-quality nadir-viewing window. Images were acquired with a Kodak-Nikon

760 digital camera with 58mm, f1.2 nocto aspheric lens. The ground sample distance (GSD) of

the imagery was generally near 60 meters. Figures 3 and 4 show color images of Washington,

DC and Abu Zaby, United Arab Emirates.

5. Discussion

The data in Figure 2 can be used to establish the spatial resolution range for a future Nightsat

sensor. Compared to the 1.5 meter resolution source data there was very little degradation in

spatial detail in the 25 meter resolution aggregation. Lighting features associated with large

buildings and roads are still discernable at resolutions of 50 to 100 meters. When aggregated to

200 meters the primary urban forms are clear. At 742 meter resolution many of the urban form

features have been lost. Our assessment is that a Nightsat should acquire low-light imaging data

in the 50 to 100 meter range.

In examining the Cirrus camera imagery we found that the dimmest detectable lighting had

radiances in the range of 5E-5 Watts cm-2 sr-1μm-1. These areas were generally lit terrain closely

surrounding shielded lights. The signal-to-noise ratio (SNR) for this lighting was about nine.

Lighting from individual poorly shielded 100-Watt incandescent bulbs present on the exteriors of

recently constructed homes produced measured radiances in the range of 1E-4 Watts cm-2 sr-1μm-1

The Cirrus camera saturated at radiances above 3.83E-3 Watts cm-2 sr-1μm-1. The upward

pointing light on the Luxor casino in Las Vegas produced a 37-m diameter circular zone of

saturated pixels in the Cirrus imagery. Other small areas of saturated data were found on large

casinos near the Luxor. Since the Luxor beacon is the brightest light on earth, this effectively

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defines the saturation radiance for a Nightsat at approximately 2.5E-2 Watts cm-2 sr-1μm-1, a level

suitable for daylight imaging of the earth.

The Cirrus camera was unable to detect terrain lit by shielded lights – a key requirement for a

Nightsat. It is estimated that detection of this type of lighting and sparse lighting in rural

environments would require detection limits in the 2.5E-8 Watts cm-2 sr-1μm-1 range for 50 meter

resolution imagery.

The 60 meter resolution ISS imagery shown in Figure 3 confirms that Nightsat imagery collected

at 50 to 100 meter resolution would provide a substantial level of detail on urban structures and

urban forms. The camera used on the ISS acquired true color imagery and it is possible to

discriminate orange, green an white lights in the images. For a Nightsat it would be possible to

tailor the spectral bands more specifically artificial lighting. Using the photopic (0.51 to 0.61

μm) and scotopic (0.45 to 0.55 μm) bands, standards recognized in the lighting engineering

community would make sense. A third low-light imaging band could be placed in the 0.6 to 0.9

μm range.

It is important to recognize some of the caveats related to the distinction between lighted area

and population density. Comparisons of the OLS lights with higher resolution imagery shown

here indicates that stable lights detected from space are related primarily to outdoor lighting

rather than leakage of indoor lighting. At fine spatial scales, the lights are actually a proxy for

the presence of lighted infrastructure rather than population density. While the presence of

lighted infrastructure usually indicates the presence of population at global scales, the spatial

scales differ such that many lighted areas are actually not inhabited at spatial scales approaching

the dimension of the area lit by an individual light (e.g. parking lots). At coarser spatial scales

larger lighted complexes may be sparsely populated (e.g. oil production facilities). At still

coarser scales, approaching the dimensions of small cities, scattering of light in the atmosphere

results in an overglow that extends beyond the actual built area (Imhoff et al, 1997, Small et al,

2005). The extent to which area and intensity of lighting corresponds to population density, or

even infrastructure extent, depends on a variety of historical, p olitical and socioeconomic

factors. At global scales there is some correspondence between lighted area and population

density but there is considerable variance in the relationship (Sutton et al, 1997, 2001, 2003). At

the finer spatial scales advocated for Nightsat, the most brightly lighted areas would not

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necessary correspond to the most densely populated regions but the data could be aggregated to

spatial scales where lighted infrastructure would generally be a reasonable proxy for population

density above some threshold level.

6. Conclusion

Nighttime lights provide a useful proxy for development and have great potential for recording

humanity’s presence on the earth’s surface and for measuring important variables such as annual

growth rates for development. Current and planned systems for global low-light imaging from

space lack the spatial resolution to meet primary objective in the urban and socio-economic

sciences. Therefore, we propose a Nightsat mission capable of acquiring sufficient data in a

year to form a global map the spatial distribution, size and brightness of artificial lighting.

Moderate resolution low-light imaging sensor data would be an important complement to the

mapping capacity of moderate resolution daytime imaging sensors such as Aster and Landsat

because it would provide an unambiguous indication of the presence of development and growth

in development.

To be effective in delineating primary nighttime lighting patterns, Nightsat low-light

imaging data must be acquired in the range of 50- to 100-meter spatial resolution and achieve

minimal detectable radiances in the range of 2.5E-8 Watts cm-2 sr-1μm-1 (or better) with a SNR of

10. While panchromatic low-light imaging data would be useful, multispectral low-lighting

imaging data acquired with three to five spectral bands would enable more quantitative

applications and enable the detection of lighting type conversions. Cloud and fire screening of

the low light imaging data would be accomplished using simultaneously acquired thermal band

data. The thermal band data could come from VIIRS if Nightsat were flown on an NPOESS

satellite. The system would use a combination of methods to produce radiance-calibrated data.

Geolocation accuracy would be ~50 m, comparable to that of Landsat. The system objective

would be to collect a sufficient quantity of imagery to construct annual global cloud-free

composites of nighttime lights. A near-sun-synchronous polar orbit, with an early evening

overpass, would provide temporal consistency important for change detection.

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The weight of scientific evidence increasingly points to human activity as the primary

driver for environmental and biological change on the planet. While other satellite missions

focus on observing the changes in the environment, the Nightsat mission will focus on mapping

the spatial distribution and intensity of an indicator of human activity – nocturnal lighting. The

Nightsat sensor requirements have been set to cover a wide range in brightness levels and

lighting types, providing detection of sparse development in rural areas and detailed mapping of

forms present in urban areas. The ability to track the growth in lighting globally, in a consistent

manner on an annual basis would enable a synoptic understanding of the human footprint on

natural systems and socio-economic processes.

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Figure 1a. Nighttime lights observed for a section of Las Vegas, Nevada at 1.5 meter resolution.

The image has been co-registered to the Ikonos image shown in Figure 1b.

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Figure 1b. True color Ikonos imagery for a section of Las Vegas, Nevada.

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Figure 2. Simulated nighttime lights data of Las Vegas, Nevada at 25, 50, 100, 200 and 742

meter resolution. Source imagery was acquired at 1.5-m resolution from NASA’s ER-2 flown at

13.7 km above the earth with a Cirrus DCS digital camera. Note that major buildings and many

streets and roads can be discerned at 25 to 50 meter resolution. Some of this detail is lost at 100

meter resolution. At 200 meter resolution it is still possible to map the urban form. At 742

meter resolution (simulation of VIIRS low-light imagery) much of the detail in the urban form

has been lost.

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Figure 3: Washington, DC and Abu Zaby (United Arab Emirates) imaged at night from the

International Space Station (ISS). The resolution of the imagery is approximately 60 meters.

Note that orange, green and white lighting can be discerned. It is likely that the orange lights are

from sodium vapor lamps. The linear features are roads lit by string of streetlights. The ISS

cities at night digital photography demonstrates the feasibility of collecting moderate resolution

multispectral low-light imaging data globally from a satellite platform.