This article covers the sightline analysis tool available to PRO subscribers in Photo Ephemeris Web.
- A detailed tutorial on the practical use of this tool is available
- For technical implementation details, please read the technical note
The sightline analysis tool is an extension to the geodetics tool in Photo Ephemeris. Geodetics allows you to perform point-to-point measurements, such as from camera to subject, on the map. It measures, distance, bearing, elevation change and apparent altitude (that is, the elevation angle corrected for curvature and refraction).
The sightline tool does this for a series of points between a start and end position, showing what can be seen from the observer's location (the primary map pin, red) in the direction of view (to the secondary pin, grey).
The tool can be used to answer the following types of questions:
- Can I see the summit of a mountain from this location?
- How far above the ground must I be before I can see a distant feature, such as a lake?
- Can I see the top of a building from here?
- Is the mid-ground visible from my shooting location?
- Is the Sun lower than this ridge at this time of day?
- Is the Moon aligned with the top of the building?
- How large will the Moon appear in relation to my subject?
In the example, below, geodetics is enabled using the geodetics button on the right hand side of the map and the sightline analysis is displayed (highlighted in pink):
- Elevation profile from primary (red) to secondary (grey) pin is shown (
- Elevation is shown in the y-axis (vertical)
- Distance from the primary pin is shown along the x-axis (horizontal)
- Visible portions of the sightline are denoted by the solid dark grey line
- Obstructed areas are shown with a dotted thin grey line
- Hovering the mouse over the chart shows details of the underlying point on the profile: distance from the primary pin, elevation above sea level (absolute, not relative), and apparent altitude angle, and an icon showing if the point is visible or not
- When the mouse is over the chart, the corresponding position on the map is shown by a 'target' icon (highlighted pink above)
Elevated Viewpoints and Subjects
If the viewer (e.g. the camera) is elevated above the ground, and an elevation offset has been entered for the primary pin (red), the sightline analysis accounts for the effect of such elevation. Use cases where this might apply include:
- Shooting from an upper floor in a building or skyscraper
- Drone photography, where the drone is positioned at a known height above the ground
An elevation offset can also be applied to the grey pin, for example:
- The height of a building
- The elevation of a flying subject (e.g. hot air balloon)
The following graphic shows the effect of elevation offsets on the displayed sightline:
Camera at +5ft, Subject at 0ft (not visible)
Observe that the most of the sightline is shown as not visible.
Camera at +250ft, Subject at 0ft (visible)
The majority of the sightline is now shown as visible
Camera at +5ft, Subject at +1,000ft (visible)
The majority of the sightline is not visible, but the elevated subject is visible
Camera at +250ft, Subject at +1,000ft (visible)
The majority of the sightline is visible and the elevated subject is visible