Line of Sight vs Viewshed: Visibility Analysis

How to perform visibility analysis in GIS

Line Of Site Viewshed Visibility Analysis
Line of site vs viewshed - How to perform visibility analysis?

Last Updated: Sep 13, 2017

Line of Sight vs Viewshed

When I was 5 years old, I used to play hide-and-seek with my friends. Just like any kid, I’d always try to find the best hiding spots. I used to wonder:

If I hide in this spot, what is visible from the observer’s point of view all around them (viewsheds)?

Or if the observer looks in a straight line, what is obstructed or not (line of sight)?

In a game of cat and mouse, viewsheds and line of sight would be your secret weapon of choice. This is why the military often use these tools with very accurate digital elevation models.

Line of Sight: Obstructed and Non-Obstructed Along a Line

The line of sight analysis uses a polyline as input. From an observer’s point of view, it determines where they have a clear view. The tool cuts the line up into obstructed and non-obstructed sections.

Because line of sight determines what is visible and hidden along a line, the input is a polyline. Also, it’s important how you draw the line. For example, the order you draw vertices determines the start and end point of the visibility analysis.

Lastly, you will need an elevation surface. Line of sight uses elevation to determine what is visible or not. For example, here is a polyline from an observer’s point of view:

Line of sight
Line of sight from observer point

As shown below, the line of sight tool breaks the line up into obstructed and unobstructed sections.

Line of sight
Line of sight showing visible (green) and non-visible (red) along line.

Viewshed Analysis: Visible and Non-Visible from Observer

On the other hand, a viewsheds analysis determines locations visible to an observer in all directions. In this type of analysis, it shows visible and non-visible in all of the surrounding areas.

Instead of using a straight line as an input, viewshed analysis uses a single point.

Likewise, we try to understand visibility from an observer’s point of view. But instead it’s a raster output indicating visibility. For example, you simply place an observer point on the map.

Viewshed observer point
A point representing an observer point of view

In the result below, we see the viewshed in green from a point.

Viewshed
Viewshed analysis displaying visible areas from observer point.

This extent is how far the observer can see, usually in raster format. In other words, if you look in 360 degrees, this is what is visible to the observer.

Visibility Analysis Applications

Although line of sight and viewshed are both visibility analysis, they produce different results.

Whether it’s line of sight or a viewshed, both enable you to understand distance with sight. While line of sight carves out a straight line as obstructed and non-obstructed, viewshed dictates all visible areas from an observer’s point of view.

Amongst other GIS applications, we often use visibility analysis in real estate, construction and military applications.

Imagine using these types of analysis in a game of hide and seek. You’d be unstoppable. Where else do you see viewshed and line of sight in the world today?

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