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Error Simulations of Uncorrected NDVI and DCVI During Remote Sensing Measurements from UAS

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Error Simulations of Uncorrected NDVI and DCVI During Remote Sensing Measurements from UAS MISCELLANEA GEOGRAPHICA – REGIONAL STUDIES ON DEVELOPMENT Vol. 18 • No. 2 • 2014 • pp. 35-45 • ISSN: 2084-6118 • DOI: 10.2478/mgrsd-2014-0017 Error simulations of uncorrected NDVI and DCVI during remote sensing measurements from UAS Abstract Remote sensing from unmanned aerial systems (UAS) has been gaining Michał T. Chiliński1 popularity in the last few years. In the field of vegetation mapping, digital Marek Ostrowski2 cameras converted to calculate vegetation index (DCVI) are one of the most popular sensors. This paper presents simulations using a radiative transfer model (libRadtran) of DCVI and NDVI results in an environment of 1Department of Geoinformation and Remote Sensing possible UAS flight scenarios. The analysis of the results is focused on the Faculty of Geography and Regional Studies comparison of atmosphere influence on both indices. The results revealed University of Warsaw, uncertainties in uncorrected DCVI measurements up to 25% at the altitude Institute of Geophysics of 5 km, 5% at 1 km and around 1% at 0.15 km, which suggests that DCVI University of Warsaw can be widely used on small UAS operating below 0.2 km. e-mail: [email protected] Keywords 2Laboratory of Image-based Information and Modelling Remote sensing • vegetation index • digital camera • UAS • atmospheric Faculty of Biology correction University of Warsaw © University of Warsaw – Faculty of Geography and Regional Studies e-mail: [email protected] Received: 14 November 2013 Accepted: 8 April 2014 Introduction The last decade has been a period of rapid increase in the the infrared and red channels, the infrared and blue channels are application of the lightweight unmanned aerial system (UAS) in used. Digital cameras usually have a colour filter array (Bayer areas such as science, farming and emergency management filter) which splits the CCD/CMOS matrix into a mosaic of blue, (Hunt et al. 2005; Lelong et al. 2008; Nebiker et al. 2008; Rango et al. 2009; green and red pixels, which usually works in the following ranges: Hardin & Hardin 2010; Xiang & Tian 2011; Zahng & Kovacs 2012). Today, 400–525, 500–625, 600–700 [nm] with overlapping regions. After UAS are cheap, very mobile, and easy to pilot, enabling extremely the removal of the IR-filter, red pixels register 600–825 [nm] – quick preparation of measurements and low operational costs. both red and IR channels. To receive two separated channels, From this perspective, it is the best method for measurements colour filters which block light between ~450–700 nm are used with a lightweight sensor for low altitudes. UAS are a solution (for example: SCHOTT BG3, Rosco #2007 VS BLUE). Pictures for aerial imagery in restricted areas of developed countries, taken with such filters have the IR channel registered on red where full-size research planes are prohibited; they also make pixels, the blue channel on blue pixels and the green pixels are it possible to conduct measurements from the sky in developing dark (as a separation between channels). During a conference countries, which do not have sufficient resources for a fleet of where methods of calculating the vegetation index from digital full-size planes. cameras were presented, many scientists (XX Polish Conference Due to the smaller size of UAS in comparison to regular on Photointerpretation and Remote Sensing, Warsaw, 2010; XXI research planes, they can carry a smaller payload, which means Polish Conference on Photointerpretation and Remote Sensing, that many aerial sensors cannot be used in UAS. The costs of Toruń, 2012) disagreed on applying the name of NDVI to both development and production of remote sensing devices are indices. Therefore, for the purpose of this paper the vegetation significant, making end-user prices higher than the costs of flying index calculated from digital camera pictures with the infrared platforms. In such a situation, a common practice is using simpler and blue channel is called the Digital Camera Vegetation Index sensors, i.e. home-made ones, made from mass production units. (DCVI). In Figure 1 examples of DCVI pictures are presented. An example of a sensor made from a standard consumer The modifications to digital cameras required for acquiring product is a digital camera, without an IR filter (hot mirror), DCVI mosaics are easy and inexpensive. Moreover, they can equipped with a colour filter, with the ability to register the near usually be made by scientists themselves, although certain infrared spectrum range, which can be used for the calculation of a companies offer ready-modified devices. The availability of vegetation condition index. The vegetation index calculated from these cameras is increasing and their popularity is growing the data obtained from a digital camera is a differential normalized constantly. The increase in DCVI usage, however, does not index, calculated in the same way as the Normalized Differential entail the development of special software for processing the Vegetation Index (NDVI) (Rouse et al. 1973); however, instead of pictures and the RAW data for the purpose of calculating the 35 MISCELLANEA GEOGRAPHICA – REGIONAL STUDIES ON DEVELOPMENT Vol. 18 • No. 2 • 2014 • pp. 35-45 • ISSN: 2084-6118 • DOI: 10.2478/mgrsd-2014-0017 Figure 1. Examples of DCVI pictures taken from UAS (A: tundra) and full-sized plane (B: agricultural area). Photos by M. Chiliński vegetation indices. Hence, their users have to prepare their invention NDVI has been very intensively used for monitoring the own programming solutions, such as script examples from the condition and cover of vegetation from many types of satellite camera modifiers, or simple codes extracted from the Internet and airborne sensors (Tucker et al. 1986; Running 1990; Myneni et al. (Fisher et al. 2012). The pictures from UAS are primarily used for 1997; Ichii et al. 2001; Burgheimer et al. 2006). Figure 2a presents the orthophotomosaics, and thus there is software on the market reflectance of a fresh standardized leaf and how it changes at 4, dedicated to mosaicking and orthorectification of images from 7 and 24 hours after picking. In the same figure, the MSS bands non-photogrametric cameras. However, there is still a significant used to calculate NDVI are presented. DCVI uses exactly the problem with the use of remote sensing software packages for same formula, but with the blue channel instead of the red one, performing radiometric corrections and removing the influence of which is depicted in Figure 2b. the atmosphere on measurements. This leads to a situation in There are three main types of interaction between radiation which most DCVI mosaics are calculated without any atmospheric and matter: absorption, emission and scattering (diffraction, corrections. refraction, reflection), all of which lead to a change in propagation This paper describes the results of simulations with a radiative direction. One effect of absorption is the weakening intensity of transfer model, presents the possible impact of the atmosphere radiation passing through matter. In this process, energy can on DCVI mosaics, shows how varied that influence is for different be absorbed only in strictly defined quanta, depending on the altitudes, and focuses on the geometry of sun-target-sensor. The frequency of the radiation (Formula 2). main goal is to compare the sensitivity of DCVI and NDVI for the interaction of radiation with the atmosphere, and to determine the (2) parameter areas where the use of DCVI without any corrections can give reliable results. Different molecules, owing to their composition, absorb NDVI is one of the simplest and most popular remote sensing different energy photons, which are usually represented by the methods used for the assessment of vegetation condition. It is characteristic absorption spectra distinct for every substance. based on the difference between the red and infrared radiances Different characteristic regions are found in UV, visible, infrared, reflected by the vegetation. The amount of radiation in both TIR or microwaves. The composition of the Earth’s atmosphere channels depends on the biophysical characteristics of the results in the presented absorption spectra (Figure 3). It can be vegetation. To ensure the data was independent of the sun zenith noted that the clear atmosphere is nearly 100% transparent in the angle, the calculated index was normalized by dividing it by the 400–700 nm range, known as visible light. sum of radiance in both of the used channels (Formula 1). The process of scattering leads to a change in radiation propagation direction. There are two types of scattering: (1) (1) elastic, where the frequency of scattered radiation is the same as before the scattering; and (2) nonelastic, during which a change NDVI has values from -1 to 1, where values around -1 of frequency occurs. In the case of remote sensing, elastic correspond to water, values around 0 correspond to bare soil, scattering is dominant. The characteristics of scattering depend urban areas and rocks, and positive values above 0.3 represent on a unit-less parameter, i.e. the ratio of the scattering particle vegetation. Lower values (0.3–0.5) are common for grasslands size and the wavelength of the resulting radiation (Formula 3). and lower vegetation. Higher values (up to 1) represent intensive green areas, such as temperate and tropical rain forests. The (3) biological foundations of NDVI are based on the very high reflectance of vegetation in the near infrared (750–950 nm), and Rayleigh scattering occurs in a situation where x<<1. The strong absorption in the red (650–700 nm). The reflectance in insensitivity of the scattered radiation in the Rayleigh process is the infrared for vegetation is five times greater than in the visible inversely proportional to the fourth power of the wavelength, which spectra, where the highest reflectance is observed in the green. leads to a substantially greater scattering for shorter wavelengths When a plant becomes dry, its cell structure changes, which leads (mainly the UV and blue light) (Formula 4).
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