What is a Viewshed?
The term ‘viewshed’ was coined in the 1960s by analogy to watershed. Just as a watershed is the area of terrain that can be reached by flowing water, a viewshed is the area of terrain that can be seen from a specific observation point. Just as you may see an island in the middle of a lake, you may observe islands of obstructed views in the middle of a viewshed. For example, a hill rising from an otherwise flat plain will obstruct views of land behind it.
What is a Horizon Profile?
The horizon profile is the set of points on a map that surround an observer and mark the limit of visibility in all directions. It’s a simplified representation of the most distant points of the viewshed.
How Viewshed and Horizon Profile Help You Plan a Shot
The Viewshed and Horizon Profile tools provide an overview of what can be seen from a given point – a fundamental consideration in landscape and nightscape photography. Use the tools to assess quickly whether a potential shooting location affords views of important landmarks, such as a mountain summit or valley.
Both tools are extremely quick to use and to update. That means it’s perfectly feasible to assess multiple locations in just a couple of minutes, helping you to identify the most promising potential camera positions for your shoot.
360° Sightlines
The Viewshed tool is like having a 360° sightline visualisation. While it’s perfectly fine to assess a point-to-point sightline using the Geodetics tool with the secondary map pin, the power user approach is to use the Viewshed tool initially. This is particularly beneficial when scouting locations where you have some discretion over which elements might be included in the scene. For example, you’re photographing in Colorado and you need some of the Front Range summits visible in your shot, but you don’t necessarily need a specific peak – Viewshed will show you which subset will be visible at a glance.
Viewshed vs Horizon Profile
Consider this photo, showing the Rockies on an early March day. The image was taken from around 90 miles away, roughly due east, on Highway 36. The highest point in the photo is Mount Blue Sky (formerly Mount Evans) to the left of centre, just above the green road sign:
Here’s the viewshed from this location:
When Viewshed is combined with the Field-of-View overlay, it’s particularly useful for seeing what might be included in a shot:
Compare the original photograph carefully with the combined field-of-view and viewshed image, and you can readily identify and correlate areas shown in each. You can see this shot plan in TPE by clicking this link: Mount Blue Sky from Highway 36.
Next, here’s the corresponding Horizon Profile for the same camera location (red pin):
The horizon profile is easier to read and makes the limit of visibility crystal clear, but overall it conveys less information than the full viewshed. However, a further advantage is that the horizon profile data can be downloaded as GeoJSON or KML to be used in other tools, such as Google Earth.
Furthermore, if your primary concern is knowing that a celestial target will clear the topographic horizon, the horizon profile is ideal. The horizon profile can be used with TPE’s celestial calendar to determine annual or monthly visibility from a given location. In this example, we used the horizon profile from the Grandstand in Death Valley, a dark sky area, to determine when the Great Orion Nebula would set behind the mountains to the west:
Click here to see this shot plan in TPE.
Summary
- Use the Viewshed tool to understand what is visible from a given potential camera location
- Use the Horizon Profile tool to understand the limits of visibility in all directions
- Apply a Horizon Profile when you are planning nightscapes or deepscapes and have limited visibility due to terrain
