Measurement of visual parameters of landscape using projections of photographs in GIS
Graphical abstract
Introduction
It is often necessary to measure the visual perception of existing elements and structures in the landscape, or of possible transformations of the landscape whose effects must be assessed. Ever since the concept of “valuable landscape” was enshrined in international agreements such as the European Landscape Convention (Council of Europe, 2000), procedures are required for the “objective” assessment of a landscape's aesthetic attributes (Uuemaa, Mander & Marja, 2013), associated with its “information functions” according to the framework proposed by De Groot (2006). However, the available “landscape metrics” for studying spatial patterns (for example using FRAGSTATS type processes) have proved insufficient when making assessments oriented towards the visual aspects of landscape (Uuemaa et al., 2013). These difficulties have led some authors to state that “the quantitative evaluation of the quality of a landscape remains a challenge” (Li & Mander, 2009, p. 40).
Various recent researchers have been working with some of these perception-related parameters, such as field of vision (Dupont et al., 2015, Palmer, 2015), visual angle (Foley and Matlin, 2009, Greater London Authority, 2012, Mairie de Paris, 2013) or the visual surface area occupied by an element of the landscape in the image. This last parameter is also known as visual magnitude and is defined as “the number of square minutes that a unit of landscape or a structure occupies in the field of vision” (Iverson, 1985, p. 16). For a particular viewpoint, these kinds of parameters can be estimated using a Geographic Information System (GIS). Significant progress has been made in the calculation of visual magnitudes using procedures related with the calculation of viewsheds (Grêt-Regamey et al., 2007, Chamberlain and Meitner, 2013), but at times visibility calculations are not totally accurate due to the effect of perspective (Sang et al., 2008, Nutsford et al., 2015) and may require adjustments to fit better with the observations in the field (Bishop & Hulse, 1994). These problems are frequently related to the difficulties inherent in correctly modelling the landscape effect of vegetation (Tomko, Trautwein & Purves, 2009).
One way to address these inaccuracies is to use photographs for measuring visual parameters. This additional input is of particular interest when dealing with sloping areas, where the effect of the slope makes the visual perception of the landscape very different from the topology that appears on the maps (although some GIS calculations, such as visual magnitude, take this into account). Photographs have been used for decades in the analysis, assessment, planning and design of the landscape (Dunn, 1976, Shuttleworth, 1980, Countryside Commission, 1988, Li and Mander, 2009, Bishop and Miller, 2007) and in a wide range of applications such as for example the assessment of the visual impact of forestry operations (Marc, 2008) or new buildings in rural areas (Hernández, García & Ayuga, 2004), the analysis of urban views (Greater London Authority, 2012), the study of visual quality (Arriaza, Canas-Ortega, Canas-Madueno & Ruiz-Aviles, 2004) or the assessment of the evolution of landscape over time (Puschmann & Dramstad, 2003). Many researchers have used simulations which display or evaluate the effect of specific changes (Sheppard, 1989, Lange and Bishop, 2005, Grêt-Regamey et al., 2007). Photographs can provide the GIS with additional territorial information that does not appear or is not updated in the land use categories (Ode, Tveit & Fry, 2010), and for evaluating the visual effect of certain specific elements that are not included in Digital Terrain Models.
However, there are no standardized procedures in the literature for measuring visual parameters in photographs, although some scholars have combined landscape metrics with visualizations in order to address perception issues (Ode, Fry, Tveit, Messager & Miller, 2009). This is firstly due to the wide variety of techniques used. Authors in this field normally detail the type of camera and the focal length they use (Downes & Lange, 2014), and the field of vision (Palmer, 2015); on some occasions they precisely define fixed positions, fields of vision and focal lengths as a reference so that images covering exactly the same field can be obtained in the future (Puschmann & Dramstad, 2003). There is also wide disparity in the way the photograph is processed. In some cases, the dimensions and the pixel size of the final image are considered and the apparent size of the photographed objects is calculated (Tomko et al., 2009). In other cases, the visual magnitude of certain elements is measured manually by superimposing a grid on the photographs (Schroeder, 1988, Grimmond et al., 2001), by counting the pixels (García-Moruno, Montero-Parejo, Hernández-Blanco & López-Casares, 2010) or using electronic planimeters (Marc, 2008). However, in all these cases the procedures are generally not comparable as they depend on the technical parameters of each photograph.
In this article we propose a method for the objective analysis of a panorama obtained from a certain viewpoint, using a quick and inexpensive procedure, which establishes a common system of references between the panoramic photographs and the cartographic projection in the same study area. For this purpose, we used various free software applications for creating panoramas and cartographic projections. These enabled us to identify particular structures and elements simultaneously in both representations of the view and to measure the different visual parameters (such as visual angles and visual magnitudes) with great accuracy and precision. We also applied these procedures in the measurement of a new visual parameter called apparent density.
In this way we provide a systematic method with objective results that are comparable for different viewpoints. This method can be applied to a wide variety of processes of visual analysis of the landscape and its scenic attributes.
Section snippets
Materials and methods
The measurement procedures can be applied both to single photographs and to compositions of various pictures in panoramic format. In this section we will be describing the general procedure to be applied in the latter case (Fig. 1), providing additional instructions for the single photograph case when necessary. For each stage, we will present the general procedure for application of the method with any combination of panorama-creation and GIS software, and then mention some specific details
Measurement of visual angles and magnitudes
Fig. 7 shows the results of the application of the methods to a panoramic view taken from a viewpoint near the village of Ardales. The panoramic image is made up of six photographs of 10 megapixels each, taken with a handheld Olympus E-510 camera and a Zuiko 14–42 mm lens. Within this panoramic view we have cropped an area of 140° × 60°. Various vertical control lines were defined in each photograph using the Hugin programme interface, so as to ensure that the yaw, pitch and roll parameters were
Discussion
The method we are presenting allows us to analyse the image obtained from a particular viewpoint in an objective way, regardless of the camera equipment used, by means of spherical projections and the establishment of a common system of reference between the ground plan and the panoramic photographs. The method enables the measurement of different visual parameters and introduces a new magnitude parameter: apparent density. The method can be applied systematically and inexpensively to various
Conclusions
Although the measurement of visual parameters of the landscape from photographs offers a wide array of possibilities for the objective evaluation of its scenic attributes, as of today there are no systematic and homogeneous procedures for performing this task. For this reason we propose a method for the objective analysis of the image and its components, by means of spherical projection of the photographs and the establishment of a common system of reference for the photographs and the
Acknowledgments
This research was possible thanks to funding provided by the Spanish University Teacher Training Programme (Programa de Formación de Profesorado Universitario – FPU, Ministry of Education) (FPU-AP-2009-4864), and by the University of Málaga (Programa de fortalecimiento de las capacidades de I + D + I en las Universidades 2014–2015). We would like to thank three anonymous reviewers for their inspiring comments on earlier versions of this paper, and Nigel Walkington for his assistance with the
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