Zones of Theoretical Visibility (ZTV)

A Zone of Theoretical Visibility (ZTV) also known as a Zone of Visual Influence, is a computer-generated tool to identify the likely (or theoretical) extent of visibility of a development. The elevation (or a set of elevations) of the development is tested against a 3D terrain model.

The terrain model is usually, but not always, a bare-earth model; that is, it does not feature buildings, vegetation or other boundaries which may have a significant effect on the visibility of a development. This is the principal reason why it is known as a Zone of Theoretical Visibility. Neither does the ZTV, of itself, take acccount of the effects of distance in reducing the significance of a development. It might be considered, therefore, that it is of limited use, but it does have particular benefits which, if used carefully, aid the landscape and visual impact assessment process:-

• In open terrain with few intervening features, ZTV provides a comprehensive representation of visibility. For example, a ZTV of a windfarm in an upland area will generally provide an accurate representation. However, most lowland rural areas have significant tree and hedgerow cover; whilst in urban areas, visibility is usually determined by intervening building masses.
• ZTV provides a useful guide to site survey in advance of LVIA, by showing areas from which visibilty may occur. It focuses the visual assessment process on those areas which will be affected and avoids those which won't.
• It is possible to add visual obstructions within the terrain model, which modify the ZTV extent. If you refer to the Leicester Park and Ride Project, right, you will see that obstructions, such as the houses immediately east of the site and commercial units immedaitely north, affect the ZTV extent. Whilst it is not possible to illustrate the effects of partial screening (eg by winter-time vegetation) this is a significant refinement over a bare-earth ZTV.

ZTV maps tend to present an extreme view of intervisibility - in terms of pure line-of-sight, many developments can be seen across a wide area. However, it is important to recognise that their actual visual significance is dependent on a number of factors, such as scale, setting and the nature of the viewing experience. Thus, the benefits and limitations of ZTV should be understood by all parties - it is simply a useful tool, amongst others, like visualisation, to illustrate and explain the effects of a development.

Project examples:

Lounge Disposal Point

Lounge Disposal Point

This study provided a Planning Inquiry with ZTVs for a low point (A, yellow) and a high point (B, green) within a contested restoration site. This formed part of the evidence as to the extent of the local area which would be affected by developments within the site area. (For more information see Lounge Disposal Point under Landscape Assessment)

Leicester Park and Ride

Leicester Park and Ride Project (without terrain)

As mentioned in the main text, it is possible to insert visual obstructions into the ZTV, and you can see the effect here: the ZTV is limited by building lines immediately east of the site and by the urban boundaries to the north, south and east. Other obstructions included commercial building units and woodland areas, for which heights were estimated to inform the ZTV calculation. The pink area can 'see' the target only when it is 10m high. The yellow area can 'see' the target at either 7m or 10m height.

Leicester Park and Ride

Leicester Park and Ride Project (with terrain)

In this variation of the previous ZTV, terrain shading and sun-shadow is added, to provide the viewer with greater understanding of the nature of the local topography. The previous image was considered to illustrate the ZTV more effectively, but this image shows the potential of experimenting with graphic techniques.

Quarry ZTV

Quarry ZTV with boundaries

Due to the notable screening effect of built areas around this sand and gravel quarry proposal, our initial ZTV included boundaries formed by buildings (upper two images). However, local vegetation also provided significant screening benefit, so we added boundaries formed by local tree belts, and produced a far more realistic impression of the visual envelope of the proposal (lower two images).
To some extent, this could also be said to illustrate the winter/summer difference in screening, although even in winter, tree belts provide a useful screening function.

Small Wind Turbine ZTV

Small Wind Turbine ZTV

As well as the normal plan view ZTV, we produced a perspective view of the ZTV for this small wind turbine in southern Scotland. Given the hilly terrain, the perspective view helped to illustrate why the ZTV was mostly limited to the south-west quadrant of the study area.

iztv