Visual Envelope Mapping (VEM) and Zone of Theoretical Visibility (ZTV)

VEM and ZTV Services in Landscape Surveying

When it comes to carrying out a landscape survey, the circumstances of the development site, the ramifications of the project’s plans and the type of assessment can mean other methods are needed. For example, it could be difficult to visualise real-world visibility from key parts of the site, opportunities for site visits or photography could be limited, local communities and stakeholders might be concerned about sightlines, there may be uncertainty around the visual impact of the structure, or complex terrain or large development goals could make it difficult to judge the impact in a straightforward manner.

Each of these cases involves a development’s visual footprint potentially influencing planning decisions. In order to ensure compliance and reduce the likelihood of planning rejection, it is vital to accurately define the parts of the landscape that could be visually affected by the planning project. Visual envelope mapping (VEM) and the zone of theoretical visibility (ZTV) will be able to do this, working together to provide computer-generated insights into a development’s visibility using data from a digital terrain model (DTM) and other mapping sources to contribute to the landscape assessment.

What is Visual Envelope Mapping?

Visual envelope mapping (VEM) is a technique in landscape surveying that gauges where a new development will be visible from within the natural and built environment surrounding it. A study area will be chosen based on wherever the proposed development can be seen from, such as footpaths, hills, towns, villages and roads. Every possible real-world element that could affect the visibility of the completed development will be considered, including buildings, trees, vegetation and other physical obstacles.

Through site visits and the use of computer-generated digital modelling technology with a digital terrain model (DTM), a landscape consultant can create an accurate picture of how the new structure will appear from chosen viewpoints, resulting in a more realistic representation of how the development will be seen in the real world. Images produced from VEM can then help architects, developers and other individuals involved in the development understand its visual effects, as well as assist local planning authorities who may otherwise refuse planning consent without such evidence.

Visual Envelope Mapping Limitations

Although a VEM is a valuable visualisation tool, it has certain limitations. For example, the accuracy of the results depends on the quality and resolution of the DTM, as well as how effectively real-world land cover is integrated, such as buildings and vegetation. It may also fail to fully account for transient visual obstructions like seasonal foliage, temporary structures or recent changes to the landscape that occur between the assessment and the completion of the proposed development.

Additionally, a VEM typically relies on assumptions – such as average viewer height and generalised tree heights or canopy density – which may not reflect the actual on-site conditions. In cases where bare earth data is used without high-quality land cover input, the visibility estimations can be less accurate, potentially overestimating or underestimating how visible the development will actually be.

Visual Envelope Mapping Methodology

The following steps are used in visual envelope mapping:

1. Definition of the study area based on the location and scale of the development
2. Creation of the high-resolution DTM
3. Identification of relevant landscape features
4. Simulation of sightlines from the development location using visibility software
5. Refinement of the visibility zone through integration of land cover data with obstacles included
6. Production of maps or viewsheds to illustrate the visual reach from specific viewpoints

What is the Zone of Theoretical Visibility?

The zone of theoretical visibility (ZTV) is the name given to areas where part or all of a proposed development could be seen, assuming no visual obstructions. Based entirely on topography, it focuses on landforms like hills and valleys while ignoring buildings, trees and other real-world features. A ZTV is theoretical and doesn’t usually reflect actual conditions on the site, instead acting as a kind of visibility heatmap showing where the development could be seen, provided nothing blocks the view.

With visual obstructions excluded, it uses a bare earth model to produce an estimated depiction of visibility from multiple viewpoints, helping to outline the maximum possible visibility of the structure. In simple terms, it gives architects, developers and others involved in the project a ‘worst-case scenario’ based on landform alone. The imagery from a ZTV can be a valuable tool for identifying the development’s zone of visual influence (ZVI) across a wider study area, especially during applicable landscape assessments.

Zone of Theoretical Visibility Limitations

Despite its usefulness, the ZTV has specific limitations. By design, it omits all surface-level obstructions – including buildings, vegetation and human-made structures – and instead relies solely on topography. As a result, the imagery can overestimate the visible area, producing a worst-case scenario that may appear unrealistically inflated compared to real-world visibility.

Because of this, ZTVs are rarely used as standalone tools and are almost always paired with site-based or model-enhanced methods. Another key limitation is that while a ZTV shows where visibility occurs, it does not indicate how much of a structure is visible, which part of it is seen, or how noticeable it is from each location, making it a helpful yet simplified tool.

Zone of Theoretical Visibility Methodology

The stages below are used for the zone of theoretical visibility:

1. Input of the development’s location and dimensions into GIS software
2. Utilisation of a digital terrain model (DTM) to simulate terrain without buildings or vegetation
3. Confirmation of target viewer height of around 1.6 to 2 meters, simulating average human eye level
4. Generation of line-of-sight analysis from the development location using computer software
5. Production of a ZTV map to show areas where visibility could theoretically occur
6. Comparison between the results and real-world data to gauge any need for refinement

Connection Between VEM and ZTV

More often than not, visual envelope mapping and the zone of theoretical visibility will be used together. It is common to do this because, between them, they provide two essential perspectives that will help with evaluating the visual impact of a development.

While the ZTV helps with defining the theoretical extent of visibility over a bare earth model, VEM refines the model to reflect real-world obstructions and environmental context.

The ZTV defines the outer boundary of the study area, and VEN outlines where visibility is realistically possible.

Opportunities for Overlap

All of the surveys below either use, reference or are supported by both assessments:

A VEM could also potentially feature in a façade analysis, and both a VEM and ZTV are optional additions in a green belt impact assessment.

Refer to the Experts at Arbtech for Advice

Based on thousands of five-star reviews and a seemingly endless list of new and returning clients, Arbtech is the best option when it comes to choosing a provider of surveys to support planning. Our knowledge ranges from the execution of various landscape surveys to the use of highly-specialised survey equipment and technology, including a digital terrain model to better understand the development site’s zone of theoretical visibility (ZTV) or zone of theoretical influence (ZVI) and undertake visual envelope mapping (VEM).

In landscape and visual impact assessments, townscape and visual impact assessments and any other survey services that need the input of ZTV and VEM, our landscape consultants are more than capable of assisting you to enhance your development plans. All zones of theoretical visibility and visual envelope maps will then be detailed within a report, giving you more insight into how your project will affect the appearance of the landscape and providing your local planning authority with evidence that will contribute to a successful planning application.

If you think you might need to access the information that only comes as the result of a VEM/ZTV, speak to our team, and we can work out which landscape assessment would be the most fitting based on your needs, requirements and the circumstances of your development. You can do this by filling out an online quote form, emailing or calling. Shortly after, you will receive a free quote for the suitable assessment, and if you’re happy to proceed, let us know, and we can choose a date for starting the survey process and getting your project moving forward.