Visual Impact & Zones of Theoretical Visibility/Intrusion (ZTV, ZVI)
Many companies involved in the process of consenting a development will have the capability and the experience to compile and assess visual impact diagrams (ZTV, ZVI) and reports that have the professional qualifications required to attain planning consent approval.
Such in depth assessments, although necessary, can be both lengthy and costly.
At the initial optioneering and site selection/scoring phase of the project you may not need such in-depth analysis and reporting, particularly when the majority of the optional sites are to be rejected.
By keeping the development modelling simple, phase relative, and therefore cost effective, it is possible to gather enough relevant data to compare site locations which can be used to progress early decision making during the site selection phase of the project.
Data can be gathered and presented in several ways and analysis is generally either HEIGHT based, WIDTH based, or both.
HEIGHT based ZTV analysis is used when trying to determine how much height of the development is shown, for example, can you see the whole height of the development or just the top 25%. This usually involves analysis at a single point position but readings taken at multiple point heights i.e. 25%, 50%, 75% & 100%.
WIDTH based ZTV analysis is used when trying to how much of a “spread” you can see over the width of a site or area and could be used to determine how many wind turbines or overhead line towers along a route you can see. The ZTV analysis in this mode would require multiple points (showing the height) representing each wind turbine or OHL tower.
ZTV, ZVI HEIGHT Based Analysis (Single Point)
ZTV, ZVI analysis works by taking a 3D terrain model of the area where the development is to be located and strategically inserting a TARGET POINT (shown in red) to represent the developments centre position in the terrain. A height is then given to the TARGET POINT to represent the height of the development. (e.g. highest point).
The analysis program then scans the 3D terrain model and analyses from where, in the terrain, you can see the TOP of the TARGET POINT and from where, in the terrain, you can’t.
Areas in the terrain from where the TARGET POINT is visible are given a value of ‘1’ and areas from where the TARGET POINT is hidden are given a value of ‘0’. By attaching a coloured shade to the areas which are value ‘1’ (visible) and leaving the value ‘0’ (hidden) areas transparent you can create a layer that can be overlaid on an O.S. map of the area thus providing evidence of the Zones of Theoretical Visibility (ZTV).
Ideally you would want local populated areas to be located in the clear area of the diagram indicating that the development is adequately screened from view.
ZTV, ZVI HEIGHT Based Analysis (Multiple Points)
IMPORTANT NOTE: By using just a single TARGET POINT for analysis it can be argued that theoretically only the top of the development could be visible. Even the top 100mm of the development being shown will analysed by the program as a ‘1’ value (visible). To address this issue it is possible to insert multiple points (at the exact same location) in the terrain model to analyse visibility at various heights of the development and give a more accurate assessment of how much of the development can be seen.
In the example below we have a proposed development showing a platform at level 63m and a building which is 24m high (to peak).
4 separate TARGET POINTS are ALL positioned at the proposed buildings centre. As shown, each point has been allocated a colour and a height reflecting how much of the building can be seen.
The points are colour coded for identification and represent the following
Target Point 1 (red) = height 24m (This is the total height of the building roof peak, or, highest point of development is visible)
Target Point 2 (yellow) = height 18m (The top of this point sits at 6m below roof peak = Top 25% – 49% is visible)
Target Point 3 (green) = height 12m (The top of this point sits at 12m below roof peak = Top 50% – 74% is visible)
Target Point 4 (blue) = height 6m (The top of this point sits at 18m below roof peak = Top 75% – 100% is visible)
By reducing the height of the points by increments (eg 25% 50% and 75%) and capturing the analysed outputs, you can create individual areas to reflect each TARGET POINT’s visibility.
The areas can then be overlaid on top of each other, producing a composite coloured diagram where each colour can define where 100%, 75%, 50% or only 25% of the development is visible. As expected, areas of visibility are reduced as the target height is lowered.
The composite ZTV diagrams can be produced with SOLID colour shading to show clarity between areas, or, it can be produced with a more subtle TRANSPARENT colour shading which allows detail from the background location map to be viewed.
These diagrams can give early indication of areas where potential screening could be an issue (i.e. designs and costs) and can be taken into consideration when scoring each site for suitability, something that is often overlooked….?.
ZTV, ZVI WIDTH Based Analysis (Single Point)
Multiple points with set heights can be used to simulate Windfarm Turbines/Overhead Lines or points within a building development simulating building height. The site analysis program will generate coloured areas representing how many of the points (Turbines/OHL Towers) can be seen from the surrounding landscape. Alternatively, there is a graphical output which will show, in grid squares, the numerical amount of points in view from that particular grid square. This is useful if you want a quantified output.
In both cases above it is possible to develop the analysis to introduce simplified screening models (solid blocks) to represent earth bunding or existing/proposed forestry. This will add more, but not expensive, detail that will give a clearer picture of potential visual impact, again, allowing better assessment when scoring site options
Overhead Lines (OHL’s) can also use this type of presentation to show visual impact of overhead line routes where multiple points along a route can represent tower heights. The resultant diagrams would clearly show coloured areas on the route where visibility was an issue and mitigation (alternate route section…?) is required.
As with Height Based, Single Point, the analysis program then scans the 3D terrain model and analyses from where, in the terrain, you can see the TOP of the TARGET POINTS and from where, in the terrain, you can’t.
Where WIDTH based differs from HEIGHT based analysis is the resultant output data.
In height based analysis the output is either ‘1’ or ‘0’ for visible or hidden. In width based analysis, because you are analysing multiple points (for each turbine/tower) the program will record HOW MANY of the points (turbines/towers) can be seen from the analysis area (i.e. ‘0’ for hidden, ‘1’ for 1 tower, ‘2’ for 2 towers and so on).
As in height based analysis, areas in the terrain from where the TARGET POINTS are visible are given a value of ‘1’ (and above) and areas from where the TARGET POINTS are hidden are given a value of ‘0’. By attaching a coloured shade to the areas which are value ‘1’ and above, (visible) and leaving the value ‘0’ (hidden) areas transparent you can create a layer that can be overlaid on an O.S. map of the area thus providing evidence of the Zones of Theoretical Visibility (ZTV).
Ideally you would want local populated areas to be located in the clear area of the diagram indicating that the development is adequately screened from view.
ZTV, ZVI WIDTH Based Analysis (Multiple Points)
Multiple points with set heights can be used to simulate Windfarm Turbines/Overhead Lines or points within a building development simulating building height. The site analysis program will generate coloured areas representing how many of the points (Turbines/OHL Towers) can be seen from the surrounding landscape. Alternatively, there is a graphical output which will show, in grid squares, the numerical amount of points in view from that particular grid square. This is useful if you want a quantified output.
In both cases above it is possible to develop the analysis to introduce simplified screening models (solid blocks) to represent earth bunding or existing/proposed forestry. This will add more, but not expensive, detail that will give a clearer picture of potential visual impact, again, allowing better assessment when scoring site options
Overhead Lines (OHL’s) can also use this type of presentation to show visual impact of overhead line routes where multiple points along a route can represent tower heights. The resultant diagrams would clearly show coloured areas on the route where visibility was an issue and screening mitigation (alternate route section…?) is required.
The points are colour coded for identification and represent the following
Target Point 1 (red) = height 26m (This is the total height of the tower, or, highest point of development is visible)
Target Point 2 (yellow) = height 19.5m (The top of this point sits at 6.5m below top of tower = Top 25% – 49% is visible)
Target Point 3 (green) = height 13m (The top of this point sits at 13m below top of tower = Top 50% – 74% is visible)
Target Point 4 (blue) = height 6.5m (The top of this point sits at 19.5m below top of tower = Top 75% – 100% is visible)
By reducing the height of the points by increments (eg 25% 50% and 75%) and capturing the analysed outputs, you can create individual areas to reflect each TARGET POINT’s visibility.
The areas can then be overlaid on top of each other, producing a composite coloured diagram where each colour can define where 100%, 75%, 50% or only 25% of the development is visible. As expected, areas of visibility are reduced as the target height is lowered.
Note: As towers are high they are always going to be visible and visibility of lower sections will not be as relevant but lower target points CAN highlight differences between a high, pylon type, tower route and a lower wood pole route.
The composite diagrams can be produced with SOLID colour shading to show clarity between areas, or, it can be produced with a more subtle TRANSPARENT colour shading which allows detail from the background location map to be viewed.
These diagrams can give early indication of areas where potential screening could be an issue (i.e. designs and costs) and can be taken into consideration when scoring each site for suitability, something that is often overlooked….?.