Sometimes, the output image from the Viewshed module contains non-continuous pixels (see Figure 1). This is the result of surface curves as explained in the following example.
Starting with a symmetric surface (Figure 2), the center, upper left corner, and lower right corner of the image have the same height, 0. Heights increase linearly with the increase of distance from those three points. Also, there is one view point at the center at a height of 350 meters. We will specify a search distance of 4000 meters. The output image from the Viewshed module with the above parameters is shown in Figure 1. It is evident that there are two non-contiguous parts. To illustrate further, we draw a line from the search area boundary to the center (Figure 3), and generate the profile of the surface transect along this line (Figure 4). The black line in Figure 5 represents the view line from the viewer point to the boundary of the search area. We can analyze where on the surface this can be seen. Within 66 pixels around the center, all the area can be seen. When we zoom into the near boundary area (Figure 6), we can see there are two pixels that cannot be seen, which is consistent with the output image (Figure 7).
For Figure 1 to Figure 7, please see attachments.