APCBEE Procedia 00 (2014) 000–000
ICBEE 2014: September 15-16, Paris, France
Satellite Imaging in the Analyses of Light Pollution
Jacek Dziurdzikowski
a,, Artur Janowski
b, Marek Przyborski
band Jakub Szulwic
ca Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Gagarina Str. 11, 87-100 Torun, Poland
b Faculty of Geodesy and Land Management, University of Warmia and Mazury in Olsztyn, Oczapowskiego Str. 1, 10-719 Olsztyn, Poland
c Faculty of Civil and Environmental Engineering, Gdansk University of Technology, Narutowicza Str. 11/12, 80-233 Gdansk, Poland
Abstract
The study includes the presentation and the practical implementation of the theoretical model of the propagation of light pollution in the atmosphere, on the basis of measurements of night radiance of city lights by satellite systems dedicated to the night observations. The authors emphasize the importance of light pollution in the modern world and give an example of its numerical analyses.
The article presented the results of analyses for the selected area of Poland, obtained from satellite imaging systems.
Reference was made to the methods proposed by Garstang and Cinzano. The studies assumption was that the resulting preparation will present only the portion of the surrounding without the light analysis outside the portion of the input satellite scene. Despite such approach, the results of the experiment indicate the ability to perform analysis and assessment of light pollution also using an uncomplicated software and standard personal computers.
© 2014 Published by Elsevier B.V. Selection and/or peer review under responsibility of Asia-Pacific Chemical, Biological & Environmental Engineering Society
Keywords: LIGHT POLLUTION, NIGHT SATELLITE IMAGING, THE DETECTION OF LIGHT POLLUTION
1. Introduction
Modern satellite imaging systems in the most part record the electromagnetic radiation reflected from the Earth or emit a beam of microwave radiation and record its reflection from objects located on Earth. Some systems recording the satellite imaging are equipped with a more sensitive sensors and they record the light
Corresponding author. Tel.: +48 58 347 28 69; fax: +48 58 347 20 37 E-mail address: geodesy@geo.edu.pl.
emitted from the surface of Earth.
Night imaging provides accurate, cost-effective and straightforward method of the detection of populated areas and any other symptoms of human activity, as well as they allow to track their changes over time.
Although there are some differences in the quantity and quality of lighting in different countries and they in general depend on the level of country development, this may constitute an additional starting point for further researches. As a result of analyses and researches it was possible to confirm the direct relation between the light amount in the specific area and the welfare [1-10]. The regions where per capita income is high emit more light than regions with low incomes. There are also some differences depending on the country’s involvement in energy saving, but essentially the economic development results in light pollution of the space and limits the maintenance of the natural brightness of the night sky. Therefore, measurements of artificial light actually bring opportunities in studying socio-economic characteristics such as population density, gross domestic product, poverty, and probably many others.
2. The Problem of Light Pollution
Night satellite imaging cause a good perspective for the study over the problem of light pollution [11-16].
The effects of this type of pollution are in the course of time more and more visible. Along with the expansion of urban areas, they cause more and more real consequences: apart from the seemingly irrelevant matters, such as a limited ability to conduct astronomical observations and the need to construct observatories far enough away from human habitats, also other, much more important, even for health, ecology, economics or even safety and other scientific researches (especially with the usage of visual and optical techniques). The developing element of light pollution is also the use of lasers (in measurements, engineering and everyday use) [17-22]. Although the negative impact of light pollution on astronomical observations is not the most important, but it is given here as the first, because of historical reasons. The astronomy was the first, where this pollution has become cumbersome and astronomers were the first who started to pay public’s attention to this problem.
Satellite systems having the ability to capture the faint light emitted from urban areas belong to the niche ones: there are three operating systems. Until the turn of 2011/2012, the primary night imaging sensor, i.e.
having the opportunity to register a weak signal from the artificial lighting was Operational Linescan System (OLS), installed on the satellites belonging to the program of U.S. Air Force Defense Meteorological Satellite Program (DMSP). Currently, imaging is also realized by Suomi National Polar-orbiting Partnership (Suomi- NPP) which is the meteorological satellite, owned by NOAA and the concept of Nightsat is being implemented. In addition to many other detecting systems, Suomi-NPP has an innovative VIIRS instrument (Visible Infrared Imaging Radiometer Suite) on board. Observations related to the analysis of light focused especially on the data, provided by the National Geophysical Data Center (NGDC) Earth Observation Group (EOG) service - http://ngdc.noaa.gov/eog/. The group started working with DMSP data in 1994 and has produced a time series of annual cloud-free composites of DMSP night-time lights. EOG's current focus is on night-time VIIRS data.
The "light pollution" term is used in relation to the occurrence of light in the environment, not originating from nature, as a factor disturbing the natural state and balance of lighting. Here we assume, that this light comes from artificial sources and it is generated by human.
The most basic, materially affecting result of bad light management is a waste of electricity. The essential cause are improperly designed lighting devices and negligent management of their activity and location. Lots of energy is not used and it is directly radiated into space as a form of light recorded by the satellite systems.
Light pollution is also not neutral for human health and it contributes to the disorder of natural diurnal cycles, which may cause more frequent headaches, fatigue, stress, feeling of anxiety, insomnia. Medical test
results indicate that the artificial night lighting is the factor which causes breast cancer by reducing the amount of melatonin hormone which is produced at night [17]. It has a huge impact on the imbalance in the environment of flora and fauna and it may interfere the animals spatial orientation, change the interdependencies between the different species and affect the psychology, foraging, reproduction, communication and other relevant animal behaviour [22,23]. Finally, light pollution robs us the opportunity (often neglected) to observe the starry sky. Man has always been accompanied by a view of the surrounding universe and it has prompted people to reflect, it fascinated and provoked to think. It had enormous influence on the development of science, art, religion, philosophy and literature – it always shaped our culture.
Nowadays, the impact of artificial light almost completely deprived the majority of the population, living in cities, the possibility to see this stunning view. It should be aware that the greater part of the younger generation had never had an opportunity to see the Milky Way.
Signalling the problem of light pollution in the environment of engineers involved in planning the urban space development and deciding on the technologies related to power engineering, construction, environment engineering seems to be one of the ways to reduce this more and more common pollution. Therefore, using the remote sensing methods, the authors indicate simple methods of pollution assessment, on the basis of available night satellite imaging.
3. Methods
The method which is presented here, it was carried out using the data available in NGDC EOG service, originating from OLS instrument. In case of this work, it has been used the technique of propagation modelling on the basis of theoretical solutions proposed by Garstang [24-27], and later – slightly improved by Cinzano and other authors [13-16]. In addition to testing the general level of light pollution on wide territories, Garstang'a concept allows for a detailed modelling of the sky brightness for each point in the sky at a particular location. This aspect can be helpful in case of the exact simulation of the observation conditions associated with disturbing light at potential locations of observatories or other places dedicated to night observations, such as the so-called dark sky parks [28], and what is also important, in case of forecasting changes of these conditions over time along with the increase of urbanization.
The result of performed calculations and executed program is the output matrix with the size of the input matrix, written to an image format allowing to maintain the resulting floating point data (in this case – TIFF format). The obtained values show the flow of energy per area unit of the telescope, collected from unitary solid angle, expressed in International System of Units (SI, French: Le Système international d'unités):
[
].
(1)4. Results and Discussions
The calculation process is an iterative process with a very large number of steps. It needs a long time to be carried out - even counted in the hundreds or thousands of hours depending on the CPU capacity, size of the input matrix and a fixed level of accuracy (condition of distance between the source and the observer, atmosphere range taken into account, the size of step when calculating the propagation function). In the presented examples, it was decided to present demonstrative results based on the sample, simplified conditions. Relatively small portion of the Radiance Calibrated composition around the south-western Poland was subject to processing with the use of Interactive Data Language (IDL). In order to shorten the operation time in the calculation, only the atmospheric layer with a thickness of 0.5 km at an altitude of 40 km above sea level was taken into account, 0.1 km was assumed as a step in bringing the propagation function and furthermore, the distance on which the propagation of light pollution occurs was minimized from 200 km to
80 km. Below, there is a direct result of the calculation made by the algorithm in the form of visualization, along with the input image.
a b
Fig. 1. Visual representations of data: a) input data and b) output data in linear mapping.
a b
Fig. 2. Fragment of Radiance Calibrated composition from Poland area: a) indicates visualization in linear mapping, while b) in Gauss mapping, dark area in the a) picture is not actually as dark as it seems, however, the peak pixel values in the centres of bigger cities cause masking of weaker lights. The white frame indicates the area used for calculations.
It is necessary to take into consideration, that the above resultant image (Fig. 1 b) is only some demonstrative method of interpreting the information saved in the matrix. In the case of graphical representation of one- dimensional matrix with data, IDL automatically performs 8-bit grayscale mapping. This means that the whole range of any type values of data is linearly scaled to a "byte" type. For this reason, the displayed images are in fact a representation of bits matrix, where the highest value from the previous set is assigned with a new value of 255.
Because the image in shades of grey is not the best form for the presentation and analysis of these results, it would be the most accurately to present them in numerical form. However, in order to present the results in graphical form in a comprehensible manner, it is the best to determine the conditions of mapping in the form of so-called array of colours. It is a relationship between the displayed intensity (separately for each of the three primary colours or equal for all at the same time when mapping in a grayscale) and the actual data value.
Suitable determination of colour array allows to emphasize the desirable characteristics, for example: faintly visible of especially large (in comparison to other) values due to occasional occurrence (Fig. 2). It also allows for a colourful interpretation of data, by setting colours to the respective scopes of presented values (Fig. 3).
Another way to visualize the data which greatly facilitates their interpretation is to create a three-dimensional surface (Fig. 4).
a b
Fig. 3. Graphical presentation of obtained results in mapping which is a) linear and b) which has a colour array modified in such a way that one colour corresponds to the scope of value with the difference of about 10-4 .
Fig. 4. Graphical presentation of obtained results in the form of so-called surface.
It should also be noted that the presented preparation, as a portion of the environment, does not include light sources located outside the portion of the input image. Therefore, the values of pollution in areas near the edges are understated. Furthermore, because of the difficulties in determining value of the second dispersion factor for angle of Ψ=90o, it is assumed that the calculated values of sky brightness will not include the contribution of the area on which the observer stands (this can be considered as a concept in which it is necessary (for the purpose of observation) to select places around which there is no source of light and the brightness of the sky comes from the surrounding areas).
The presented results of the test analysis are intended to serve as an example of a simple implementation of the presented model and the use of the night composition in practice of the night, which may serve as a starting point for further work related to the analysis of light pollution. Although the theoretical aspect of the problem of light pollution has been developed very carefully, its realization is still far from perfection. In order to make the obtained results closer to the truth, it is first of all necessary to take into account the actual terrain model, a more detailed model of the distribution of aerosols in the air, taking also into account their diversified condensation depending on the region, and to link the shape of emission function with important factors that determine it. You can increase the accuracy of the results analysis after performing the calibration on the basis of sky brightness measurements, carried out from Earth. The way in which these measurements should be carried out, would have to take into account the natural brightness of the sky, depending on solar activity, in order to avoid overestimation of the artificial brightness level. In addition, they should be carried out during the well-known atmospheric conditions to appropriately bind the results with the parameters used in the model.
5. Conclusions
The study, in addition to presenting the example application, was also aimed at raising awareness of the unique (among other remote sensing imaging) data source concerning the artificial lighting as a symptom of human activity on Earth. The range of information possible to obtain on this basis is extremely broad and far beyond the study on the light pollution. List of topics for studies in which the night imaging can be used as a robust tool for the analysis is very long and it contains the subjects in the field of economics [2-8,10], power engineering [5,12,29,30], the environment protection [3,31] or even crisis management in case of natural disasters, catastrophes or wars [32-34] and economy [4,5,9,29,34]. Even despite many imperfections of the DMSP system relating to the described use, being the result of designing sensors for different applications, the data so far available have repeatedly shown their high functionality. With the launch of the Suomi NPP satellite, a series of difficulties with which they authors of studies had to face when using the night remote sensing will be a significantly reduced. In the course of time, this technology has the potential for further development, particularly thanks to the realization of NIGHTSAT concept in which, the night imaging is the fundamental task.
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