Visibility

Visibility answers questions about lines of sight and about light: what can be seen from a given point, how much sky a place is open to, how long the sun reaches it, and where the shadows fall at a given hour.

These tools support siting studies, wind turbine and antenna visibility assessments, solar potential, urban microclimate work and archaeology.

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Viewshed

The classic. Give it your elevation model and a point layer of observation stations, and it returns which cells are visible from them.

Parameter	What to set
Input	Your elevation raster.
Stations	A point layer holding the observer positions.
Height	The observer’s height above the ground, in the vertical unit of your DEM. The default of 2 metres represents a standing person. Raise it for a tower or a turbine hub.
Output	A raster counting, for each cell, how many stations can see it.

Because the output is a count, one station gives a plain visible or not visible map, while several stations give a map of how exposed each place is, which is usually the more useful answer.

Two things to remember. The analysis sees only what is in your elevation model, so a bare earth DEM will happily look straight through a forest and a row of buildings. And the earth’s curvature and atmospheric refraction are not accounted for, which matters beyond a few tens of kilometres.

Sky View Factor

The proportion of the sky dome visible from each cell, between 0 and 1. Open plains approach 1, deep valleys and narrow streets drop well below.

It is the standard measure behind urban heat island studies, since a place that sees little sky both receives less sun and radiates less heat away at night. It is also what gives archaeological relief visualisations their readability, better than a hillshade in flat terrain.

Openness

Yokoyama’s measure, in two flavours. Positive openness is high on exposed ridges and peaks, negative openness is high in enclosed hollows. Together they make an excellent pair for landform interpretation, and a strong visualisation of archaeological micro relief.

Time In Daylight and Shadow Image

Time In Daylight gives, for each cell, the proportion of the day it is lit rather than in shadow, for a date and a location you specify. Directly useful for solar potential and for agriculture in mountain valleys.

Shadow Image gives the shadow intensity at a single date and time, and Shadow Animation produces an interactive HTML viewer showing shadows moving through the day.

All the tools

Tool	What it does
Viewshed	Counts, per cell, how many observation stations can see it.
Visibility Index	A general visibility index built from sampled viewsheds across the raster.
Sky View Factor	The proportion of the sky dome visible from each cell.
Openness	Positive and negative topographic openness, marking exposed and enclosed places.
Horizon Angle	The maximum angle to the horizon along a chosen azimuth.
Average Horizon Distance	Mean distance to the horizon, averaged over azimuths.
Horizon Area	The area of the horizon polygon, in hectares.
Skyline Analysis	Horizon trace and HTML report for one or more observation points.
Time In Daylight	The proportion of daytime each cell is lit, for a given date and place.
Shadow Image	Shadow intensity at a given date and time.
Shadow Animation	An interactive HTML viewer and animated GIF of shadows through a day.

Notes

Which surface model. For visibility between people, use a surface model that includes buildings and trees. For solar potential on roofs, the same. For a landform study, use bare earth. The choice of input changes the answer more than any parameter here.

Cost. Viewshed and the horizon tools trace rays from every cell, so they are among the slower tools in the toolbox. Clip your DEM to a sensible radius around your stations first.

HTML outputs. Skyline Analysis and Shadow Animation produce reports, which are downloaded rather than added to the map.