Viewshed

In this article:

• Calculate Viewshed
• Viewshed Calculation - Resolution Variation with Distance and Field of View
• Viewshed Display – Distribution of Query Points

Calculate Viewshed

The 3D viewshed calculates and marks all areas on the terrain and on 3D models and objects that are visible from a selected point on the terrain. 

To create a viewshed:

1. On the side toolbar > > Tools tab > Viewshed.
2. In the 3D Window, click to define your viewpoint for the 3D viewshed, and then click again to define your desired endpoint (defining the distance of sight). The viewshed appears on the terrain as a sector of a circle, where the center point is the viewpoint and the arc is the endpoint (marking the end of the distance of sight analysis). Within the sector, areas that are visible from the viewpoint are colored green, whereas areas that are not visible from the viewpoint are colored red.
3. In the viewshed property sheet that is displayed, set its properties:
   

   Description	The description or name of the 3D viewshed (read-only). This text appears in the sidebar’s Analysis tab as the name of the object.
   Altitude	The altitude above the ground for the viewer location.
   Field of View	Drag the slider to set the angle limits, in degrees, of the viewshed.
   Distance	The length of the viewshed analysis from the viewer in meters.
   Direction	The horizontal angle of the camera when viewing the viewer location.
   Visibility Distance	Sets the distance from the camera at which the viewshed disappears.

4. The initial viewshed is for preview only. Once you have adjusted the properties as required, if you want to generate a precise viewshed, click Calculate.
5. Set some additional properties:
   

   Spacing	Distance between each of the rays that is sampled. The lower the value, the more accurate the viewshed but the longer it takes to calculate.
   Set Viewer Altitude	To set an altitude above the ground for the viewer location, toggle on this option, and then set the Viewer Altitude.
   Calculate	Select what you want calculated: Visible and Not Visible Visible Not Visible
   Target Altitude	Sets the required altitude for the target.

6. Click Calculate. The 3D viewshed is saved and is available from the Project Tree tab section of the side toolbar’s Layers tab.

TerraExplorer Fusion Plus extends the preview viewshed capabilities of Fusion Viewer, offering the capability to precisely calculate and query viewsheds. The tool is designed to strike a balance between resource optimization and accuracy, ensuring that visibility analyses are both highly exact and computationally efficient.

Viewshed Calculation - Resolution Variation with Distance and Field of View

The resolution of the calculated viewshed is directly affected by both the distance from the selected observed viewpoint and field of view.

Resolution Variation with Distance:

Resolution is highest near the viewpoint and gradually decreases with increasing distance, as follows:

• 1 km: 0.375 m/px
• 2 km: 0.75 m/px
• 4 km: 1.5 m/px
• 8 km: 3 m/px

Resolution Variation with Field of View Within 1 km of the Viewpoint:

Within 1 km of the viewpoint, resolution is also influenced by field of view. As the field of view increases, encompassing a wider viewshed area, resolution decreases as follows:

• 30° arc: 0.5 m/px
• 60° arc: 1 m/px
• 120° arc: 2 m/px

Additionally, within 1 km, query points will also be displayed on the walls of the model.

Viewshed Display – Distribution of Query Points

For distances of sight up to 1 km, query points are evenly distributed every 2 meters. Beyond 1 km distance, a grid-based approach is used due to resource considerations and the complexity of long-range visibility assessments. This approach optimizes resource usage while providing meaningful visibility information. The area is divided into grid cells, and within each cell, query points are evaluated for visibility (green for visible, red for hidden):

• If all points in a cell are green (visible) or all are red (hidden), the system selects the center point of the cell for display.
• If a cell contains both green and red points, the system selects the topmost green point (most visible) and the bottommost red point (least visible) for display.

Query Grid                                                        Display Grid