# Curved Horizon from 46,000 ft through a HUD, CurvApp overlay

Can we see a curved horizon from an airplane at 46,000 ft altitude? Does the horizon raise to eye level? What does the globe model predict and what can we see in reality?

The following image is a screenshot from a video filmed from the cockpit of a Bombardier Global Express 600 flying at an altitude of 46,000 ft. The camera used was an iPhone 7 with a 35 mm equivalent focal length of 29 mm (field of view about 73°).

Note: the curvature of the horizon is not due to lens distortion. You can check this by following the link to the raw uncut video, where the camera was paning up and down to show that the curvature does not change in different parts of the frame, as would be the case on distorting lenses.

Overlay of iPhone image with Curvature App graphic

The green symbols and numbers are data from the Head-up display. It shows essential informations to the pilot when he looks through the front window. The horizontal bar with the 2 circles is part of the Attitude indicator and shows an artificial horizon at eye level. You can move the head around, this line will remain at the same position with respect to the real horizon. The real horizon of the earth appears way below eye level at this altitude and is clearly curved.

The overlayed yellow grid, the long horizontal green line and the information at the right is created with the Curvature App. I entered an observer altitude of 14,020 m = 46,000 ft, and a focal length of 29 mm (iPhone 7) and selected the aspect ratio of 16:9.

The calculated graphics of the Curvature App matches the iPhone image perfectly, both the horizon drop of 3.8° as well as the curvature of the horizon.

Reality matches the prediction of the Curvature App, based on the globe model, perfectly.

Load image in the Curvature App
Note: you can change the transparency of the image in the App by clicking in the image. More left → more transparent.

The Horizon is not at Eye Level. Flying West over water with clear skies
Raw uncut video showing both the drop and curve from 46,000 ft altitude on a clear distinct horizon as seen through a HUD. Filmed by Wolfie6020.

Sensor on second iPhone 7 Plus camera is smaller than main one, calculations show; https://9to5mac.com
Information about the iPhone 7 camera, e.g. focal length of 29 mm corresponding to a field of view of about 73°.

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