Today was supposed to be the day for picking sweet persimmons, but since some are still ripening, I've decided to wait until next week. Thanks to that, I’ve found some time to write another post.
Today’s topic is Visual Frequency Analysis (also known as Viewshed Frequency). As the name suggests, it refers to the frequency with which a certain area is visible. While standard Viewshed Analysis identifies the area visible from a specific viewpoint, Visual Frequency Analysis determines how many different viewpoints can see the project site. Essentially, it is the process of overlapping multiple viewshed results and counting them.
If an area has high visual 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. It feels a bit backward, doesn't it?
Let’s get started.
1. Preparing the Data
First, please open the '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. (This is a safe proprietary data 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).
Many reports perform a full 360-degree 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.)
3. Calculating Visual Frequency (Overlapping Count)
Now that we have 8 overlapping viewshed results, we need to count how many layers overlap at each pixel. 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.
The software will then proceed to rasterize the data and build a grid.
Once finished, a new layer titled 'Count of Layers' is created, and the visual frequency is displayed in the main window, categorized by color.
4. Refining the Result (Layer Management & Cropping)
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. Turn off the individual viewshed analysis layers to see the frequency result clearly.
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 visual 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.
5. 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 visual 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.
6. Statistical Analysis (Area by Frequency)
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. While green might look wider than blue at first glance, a detailed histogram will show the precise distribution.
That concludes our quick look at Visual Frequency Analysis.
That's all for today!
No comments:
Post a Comment