CUBIZEN: Global Mapper, CityEngine & Creative Tech

Today’s topic is Viewshed Frequency Analysis (also known as Viewshed Frequency). As the name suggests, it refers to how frequently a certain area is visible from multiple viewpoints. While standard Viewshed Analysis identifies the area visible from a specific viewpoint, Viewshed Frequency Analysis determines how many viewpoints can observe each part of the project site. Essentially, it overlays multiple viewshed layers and counts the Viewshed frequency for each location.

If an area has high Viewshed frequency, it means any topographical changes or new buildings in that spot will be noticed from more locations. In simpler terms, it is a tool to find areas where development is likely to have a significant landscape impact.

In development projects, a landscape plan considering these factors should ideally be established before the architectural or civil engineering plans. In reality, however, the process is often reversed: designs are made to maximize profitability, and the landscape plan is then used to justify them. In many cases, landscape planning becomes a post-justification step rather than a design constraint.

Let’s get started.

1. Preparing the Data

First, please open the 009-Viewshed Analysis.zip file in Global Mapper. This file is in Global Mapper's package format (.gmp), which bundles multiple data layers into a single file for convenience. (Verified and safe file for Global Mapper.)

Once opened, you should see four layers in the Control Center: DEM, Satellite Image, Boundary (Project Site), and Viewpoints.

2. Performing Viewshed Analysis for Each Viewpoint

I have already pre-set viewpoints 1 through 8 for this exercise. We will now perform a viewshed analysis for each of these points toward the project site.

Select a viewpoint point feature, right-click, and navigate to: Analysis/Measurement -> Calculate Viewsheds at Selected Point(s).

Important Technical Considerations Before Analysis

Before proceeding with the bulk calculation, you must ensure that all selected viewpoints share the exact same elevation attributes. If your viewpoint vector layer contains mismatched height values (e.g., some points snapped to the ground while others are absolute heights), Global Mapper will produce inconsistent viewshed boundaries, which will heavily distort the final frequency overlay results.

Pro Tip: Selecting the Optimal Grid Resolution

When configuring the viewshed setup for multiple points simultaneously, the Sample Spacing (Resolution) becomes the most critical factor affecting your processing speed. While a tighter 1-meter grid yields beautiful, high-resolution results for urban visual assessments, computing this for 8 or more viewpoints at once can easily freeze your system or exhaust available RAM. For initial project drafts, starting with a 5-meter or 10-meter spacing is highly recommended to streamline the calculation workflow.

Setting the Viewing Angle

Many landscape assessment reports use a full 360-degree viewshed analysis, but I personally prefer focusing only on the project site. While it requires setting the viewing range for each point individually, it makes it easier to understand the relative positioning between the viewpoint and the target area.

Adjust the settings as shown below, specifically the parts circled in red. As a reminder, Start Angle is where the analysis begins (0° is North, 90° East, 180° South, 270° West), and Swept Angle is the horizontal field of view. Set the Swept Angle just wide enough to cover the site.

After performing this for all 8 viewpoints, you will get a result similar to the one below. (Note: The colors of the visible areas may vary.)

Counting Overlapping Viewshed Layers

Now that we have 8 overlapping viewshed results, we need to calculate the number of overlapping visible layers for each raster cell. Go to the menu bar and select: Terrain Analysis -> Count Overlapping Raster/Terrain/Viewshed Layers...

A window will appear asking you to select the layers to count. Select only the Viewshed Analysis layers and click OK.

Global Mapper will rasterize the viewshed layers and generate a count grid.

Once finished, a new layer titled 'Count of Layers' is created, and the Viewshed frequency is visualized as a categorized raster layer.

3. Refining the Result (Layer Management & Cropping)

Managing Layer Viewshed

Since Global Mapper places new layers at the top, we need to move the 'Count of Layers' below the boundary or adjust the order so the project site boundary is visible.

The screen might look cluttered with all individual viewshed layers turned on. Disable the individual viewshed layers to see the frequency result clearly.

Cropping the Result to the Project Boundary

Next, we don't need to see the frequency outside the project site. We will use the Layer Cropping feature. Select the project site polygon in the main window. Then, right-click the 'Count of Layers' in the Control Center and select Options.

In the Elevation Options window, go to the Cropping tab and select 'Crop to Currently Selected Polygon(s)'. This ensures only the area within the boundary is displayed.

Now, only the Viewshed frequency within the boundary is visible. You might notice the legend on the left uses "m" (meters). This is because Global Mapper treats the overlap count as elevation data. For example, "4m" simply means that 4 layers are overlapping in that area.

4. Interpreting the Analysis

By zooming into the boundary (use the magnifying glass icon in the Control Center with the boundary layer selected), we can see the results clearly.

Looking at the map, the Viewshed Frequency is relatively high on the western and parts of the eastern side of the site. This tells us that any development in these specific areas will be visible from a larger number of viewpoints. To minimize landscape impact, architectural layouts or civil designs should take these high-frequency zones into account.

5. Statistical Analysis (Area by Frequency)

Using Raster Reclassification

Finally, let's calculate the exact area for each frequency level. Since the result is a raster image, we can classify it by "color" (count) to find the area. Go to: Raster Analysis -> Raster Reclassification...

In the Reclassification window, '0' represents areas not visible from any viewpoint, and '5' (or higher) represents areas visible from that many viewpoints. Although some frequency classes may appear visually dominant, a detailed histogram will show the precise distribution.

That concludes our quick look at Viewshed Frequency Analysis. 

Viewshed Frequency Analysis is particularly useful during the early planning stage because it identifies areas that are visually exposed to the greatest number of observers. By combining multiple viewsheds into a single frequency surface, planners can make more informed decisions about building placement, grading, and landscape mitigation measures.

That's all for today!