Zone of theoretical visibility: setting a study area

A zone of theoretical visibility is a map showing every point in the surrounding area from which some part of a proposed development could in principle be seen, calculated from a digital terrain model. It defines the study area for a landscape and visual assessment and guides viewpoint selection. It also overstates visibility, usually by a wide margin, because the standard version models only the shape of the ground.

Under GLVIA3 practice the zone is a tool for scoping, not a finding. Treating it as a map of actual visibility is the mistake that produces arguments at inquiry.

What the zone shows and what it hides

The calculation is a line-of-sight test. The software takes a terrain surface, places a target at the height of the proposed development, places an observer at eye height across a grid of points, and marks each grid cell as visible or not visible. The output is a shaded area, often with bands for how much of the scheme is theoretically visible.

What it captures well is landform. Ridgelines, valleys, escarpments and the way a hill blocks a view are modelled properly, and that is useful for a wind farm, a reservoir or a tall building where topography dominates.

What it misses is everything that stands on the ground. Buildings, walls, hedgerows, woodland blocks, individual trees, embankments, highway cuttings and boundary planting all screen views and none of them appear in a bare earth calculation. On a lowland site in a wooded or built-up area, the mapped zone can be ten or twenty times the area from which the scheme is actually visible.

So the honest reading of a bare earth zone is a worst case. It tells you where to look, not what will be seen.

Bare earth against screened modelling

There are two terrain products in general use and the difference between them decides what the zone means.

The surface model has a trap. Vegetation captured by lidar reflects the canopy on the day of the flight, so a hedgerow that has since been grubbed up or a plantation that has been felled will still screen the view in the model. Deciduous canopy also screens far less in winter than the summer capture suggests.

Which is why a defensible method often runs a bare earth zone to set the study area, then a screened model for the viewpoint discussion, with the screening assumptions listed and the capture date stated. Say which model produced which figure. An assessment that presents a single zone with no statement of the model used invites the question.

Choosing the radius for a study area

The radius comes from the scale and nature of the development rather than from a rule. In broad practice, the taller and more extensive the scheme, the wider the study area, and the study area is then clipped to the point where the development ceases to be a perceptible element in the view.

Practical anchors that hold up in discussion:

• For a single dwelling or a small infill scheme, a radius of 1km to 2km is usually ample and the zone is often barely worth generating.
• For a mid-scale scheme such as a 100-dwelling edge-of-settlement site or a solar farm, 3km to 5km is a common starting point.
• For tall structures, the radius extends with height, and wind energy practice has long worked at 15km to 35km for the largest turbines.
• Where a National Landscape, National Park or World Heritage Site sits just beyond the chosen radius, extend to include it rather than defend the cut-off.

Clip the zone to the receptors that matter as well as to distance. Settlements, public rights of way, recreational routes, designated landscapes and heritage assets with a visual element to their setting are the reasons a study area exists. A zone covering open farmland with no public access generates no effects to assess.

One honest point of variation: there is no agreed table of radii by development type, so the figure is a judgement the assessor has to justify. Expect the authority to ask why.

Using the zone to select viewpoints

Viewpoint selection is where the zone earns its keep. The method that stands up:

1. Generate the bare earth zone and overlay receptors: settlements, rights of way, designated landscapes, heritage assets, roads and recreational facilities.
2. Identify candidate viewpoints where the zone intersects receptors, spread across distance bands and directions.
3. Visit every candidate on the ground and record whether the scheme would actually be visible, with photographs and notes.
4. Discard candidates that are screened in reality and record why, because the discard list is part of the audit trail.
5. Add viewpoints the zone did not predict, such as a gap in a hedge or a first-floor window line, where field work finds them.
6. Submit the proposed list to the authority for agreement before the photography programme starts.

Step four is the one most often skipped, and it is the one an objector will attack. If you cannot show why a viewpoint inside the zone was dropped, the inference is that it was dropped because it was inconvenient.

Field verification also catches the opposite error. Bare earth modelling sometimes misses visibility across a valley where an intervening hedge has gone, and finding that on site is better than having it found for you.

Verified views and photomontage

Once the viewpoints are agreed, the zone has done its job and the work moves to representation. The images that carry weight record the camera position as a surveyed coordinate, the camera height, the lens focal length and sensor size, the field of view, the date and time, and the intended printed size and viewing distance.

A zone of theoretical visibility is not a substitute for those images and cannot show magnitude of change. It cannot tell you whether a scheme will be a minor incident on a distant skyline or the dominant element in a view, because it has no information about scale, materials, light or the composition of the scene. That judgement belongs in the Landscape and Visual Impact Assessment itself, supported by the verified views, and in a townscape context in a Townscape and Visual Impact Assessment.

Agreeing the method with the authority

Put the method in writing before the work starts, ideally at pre-application stage or through an EIA scoping request where the scheme is EIA development. The points worth agreeing:

• The terrain model used, its source and its date.
• The observer eye height and the target heights taken from the scheme.
• The study area radius and the reason for it.
• Whether a screened model will be run and what screening is included.
• The viewpoint list, including those offered and discarded.
• The representation level sought for each viewpoint.

An agreed method is worth more than a thorough unagreed one. Where the scheme is EIA development, the Environmental Statement must be prepared by competent experts under the Town and Country Planning (Environmental Impact Assessment) Regulations 2017, so the landscape chapter and its visualisations have to be in qualified hands. A PlanScope draft is a starting point for that expert rather than a substitute for them.

How objectors use the mapping

From the other side, the zone is the quickest way to test whether an assessment is complete. Overlay the submitted viewpoints on the zone and look for gaps: a village inside the zone with no viewpoint, a long-distance trail crossed by the zone and unrepresented, a listed church tower with a visual setting and no image.

Then ask three questions in a representation. Which terrain model generated this zone, and on what date. Which candidate viewpoints were discarded, and on what evidence. And does the zone extend far enough to reach the nearest designated landscape.

Those questions are answerable by a good assessment in a paragraph each. Where they are not answered, the authority has grounds to ask for more before determining. The report index shows which landscape and heritage documents normally travel together on a visually sensitive application.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. This is general guidance rather than legal advice. Check the position with your local planning authority or a qualified professional before relying on it.