ZTV (Zone of Theoretical Visibility) analysis identifies the areas from which a selected target can be seen within a defined study area. The analysis considers the terrain and buildings that may obstruct visibility between an observer location and the target. Results show the extent of visibility across the study area and, when using height bands, the elevations from which different parts of the target are visible.

Selecting targets

ZTV analysis supports two approaches to selecting targets.

Select individual targets

Select individual imported models and created massing. Multiple targets can be selected together, including a combination of imported models and massing. The selected geometry is used to calculate visibility. The way the target geometry is represented means that subdividing otherwise identical geometry should not change the result.

Draw target areas

Draw one or more areas on the ground to select the geometry within them as targets. This can include VU.CITY buildings, imported models and created massing. Buildings and models within the selected areas can obstruct visibility of one another, so the results account for the surrounding target geometry.

Configuring the ZTV

The analysis settings determine the area being assessed and the observer’s position.

Radius

The radius defines the circular study area around the target. The maximum radius currently available in the web application is 1,500 m.

Resolution

The resolution setting ranges from 1 to 10 and represents cell area in square metres. The setting affects the level of detail used when sampling the study area. Higher resolution settings take longer to process.

Observer height

Observer height defines the height above ground from which visibility is assessed. The default is 1.6 m, and the height can be adjusted to suit the analysis.

How ZTV calculates visibility

The ZTV calculation uses a voxel representation of the target geometry rather than tracing directly against the original model triangles. The study area is represented by a grid of cells, with visibility rays originating from the centres of sampled observer cells and travelling towards the target.

This approach makes the calculation less sensitive to the triangle density of target models. Subdividing otherwise identical geometry should not change the result, although models with higher triangle counts can use more memory.

The analysis samples the study area rather than assessing every possible observer position. Small visibility differences may therefore not be captured. The minimum observer cell size is 1 m × 1 m, so results should be understood as a sampled representation of visibility rather than a continuous assessment of every point.

The new ZTV calculation is designed to handle complex models more efficiently than Legacy. It is less affected by high triangle counts and can produce more accurate results in some cases, particularly where target models are poorly prepared. Early testing has shown broadly similar results between the two applications, with the new calculation completing some analyses significantly faster.

Understanding the results

ZTV results show where the selected target is visible across the analysed area. The results include the overall visibility extent, visibility by target height and the number of visible hits. Each provides a different way to interpret the outcome of the analysis.

Visibility extent

Visibility extent describes the proportion of the analysed area from which at least part of the selected target is visible. The result reflects visibility across the full study area, including locations where the target cannot be seen.

Height bands

Height bands show the elevations from which the lowest visible point of the target can be seen.

A result labelled Visible from [elevation] up identifies the lowest visible target point for that band. It does not mean that every part of the target above that elevation is visible from the same locations.

The height-band percentages are calculated against the full analysed area. The visible bands together correspond to the visibility extent, while the not-visible percentage represents the remainder of the study area.

Displayed percentages may not add up exactly to 100%. Very small values can be displayed as <1%, so the figures shown in the interface may not reflect the underlying values precisely.

Visible hits

Visible hits represent hits recorded among the cells that intersect any part of the target. This figure should be interpreted in the context of the visibility results rather than as a separate measure of the proportion of the study area from which the target is visible.

Understanding the legend and colours

The legend represents the analysis results on the ground. How the target itself is coloured depends on the selected display mode.

Height bands

The colours applied to the target correspond to the height-band colours displayed on the ground and in the legend.

Cell AOD

In Cell AOD mode, the colours represent the elevations of the ground cells. The target is not coloured because the cell elevation values do not describe the target’s height.

Colours may appear different from the legend depending on the display style. Cell-based results use an unlit shader and should appear close to the legend colours. Projected raster results are affected by terrain shading, while styles such as Cartoon and Blueprint can also alter their appearance. These differences do not necessarily indicate a problem with the analysis.

Geometry and analysis boundaries

The analysis only considers geometry within the selected study area. Geometry outside the area is excluded from the calculation.

City tiles that are unavailable to the user are also excluded. The analysis can still run, but missing geometry will not contribute to the results, and the application does not currently display a warning when this occurs.

If geometry is visible in the scene but unexpectedly absent from the analysis results, this may indicate an issue that requires investigation.

Tips for reliable results

• Avoid selecting unnecessarily large target areas, as this can significantly increase processing time.
• Processing time depends on both the target geometry and the extent of potential visibility. A tall building surrounded by low-rise buildings may be visible across a large area and take longer to analyse than the same building surrounded by taller buildings that obstruct more views.
• Higher resolution settings take longer to process.
• Remember that the analysis uses a sampled grid. Small visibility differences may not be captured, particularly where the geometry or visibility changes over short distances.
• Check that the relevant city tiles are available if expected geometry appears to be missing from the results.