Incorporating vegetation into visual exposure modelling in urban environments

Abstract

Visual exposure modelling establishes the extent to which a nominated feature may be seen from a specified location. The advent of high-resolution light detection and ranging (LiDAR)-sourced elevation models has enabled visual exposure modelling to be applied in urban regions, for example, to calculate the field of view occupied by a landmark building when observed from a nearby street. Currently, visual exposure models access a single surface elevation model to establish the lines of sight (LoSs) between the observer and the landmark feature. This is a cause for concern in vegetated areas where trees are represented as solid protrusions in the surface model totally blocking the LoSs. Additionally, the observer's elevation, as read from the surface model, would be incorrectly set to the tree top height in those regions. The research presented here overcomes these issues by introducing a new visual exposure model, which accesses a bare earth terrain model, to establish the observer's true elevation even when passing through vegetated regions, a surface model for the city profile and an additional vegetation map. Where there is a difference between terrain and surface elevations, the vegetation map is consulted. In vegetated areas the LoS is permitted to continue its journey, either passing under the canopy with clear views or partially through it depending on foliage density, otherwise the LoS is terminated. This approach enables landmark visual exposure to be modelled more realistically, with consideration given to urban trees. The model's improvements are demonstrated through a number of real-world trials and compared to current visual exposure methods.

Modelling landscape experience using “experions”

“…Other methods include the use of distance bands (Higuchi & Terry, 1983). Slope and aspect also effect visibility, as a surface that is facing the observer is more visible than a surface that is side on (Bartie, Reitsma, Kingham, & Mills, 2011). Viewshed analysis tends to provide a binary result showing whether a pixel can be viewed or not.…”

Personalising the viewshed: Visibility analysis from the human perspective

“…While the VVI overcomes a number of the shortcomings of standard viewshed analysis, limitations common to all visibility analyses remain and are also worth discussing in the context of the VVI. Accurately incorporating vegetation proves to be the most difficult challenge due to its semi-transparent and often non-uniform distribution across terrain (Bartie et al, 2011;Kumsap et al 2013;Wheatley & Gillings, 2000). Partial solutions have been developed (Bartie et al, 2011;Llobera, 2007;Murgoitio et al, 2013;Tomko et al, 2009), however are not based on the viewshed approach and therefore are not applicable to the VVI methodology.…”

“…Accurately incorporating vegetation proves to be the most difficult challenge due to its semi-transparent and often non-uniform distribution across terrain (Bartie et al, 2011;Kumsap et al 2013;Wheatley & Gillings, 2000). Partial solutions have been developed (Bartie et al, 2011;Llobera, 2007;Murgoitio et al, 2013;Tomko et al, 2009), however are not based on the viewshed approach and therefore are not applicable to the VVI methodology. While not an ideal solution, the most common approach is to add a raster vegetation layer to digital terrain models, where new elevation values represent ground height plus the average height of the dominant vegetation type (Wheatley & Gillings, 2000).…”

“…The vast majority of visibility analysis is conducted in either the 2nd dimension such as isovists (for urban and architectural studies) or in 2.5 dimensions with viewshed analysis (Bartie et al 2011). While the viewshed can be an extremely useful tool, especially in large scale terrain analysis for which it was designed, it takes a God's eye view approach and fails to portray the vertical dimension of terrain, a characteristic of visibility which is particularly important from the human perspective.…”

Airborne LiDAR and Terrestrial Laser Scanning Derived Vegetation Obstruction Factors for Visibility Models

“…), and the Beer‐Lambert Law of Attenuation (Bartie et al. , Llobera ). Despite these and other studies, no framework has been presented which incorporates actual three‐dimensional forest vegetation data into short‐range visibility models and that has been tested with systematic and objective accuracy assessment methods.…”