• Basics
• View
• Target 1
• Target 2
• Refraction⇒
• Std
• 0
• Units
• Save/Restore
• Reset All

• Angles
• No Refraction
• Refraction

Note: Values marked with a * are not dependent on Refraction in reality. The marked values show the apparent values if refraction is not zero. So to display the real values, set Refraction = 0. This is true for all Horizon Data as well.

Basics Panel

Obsever Height: Height of the observer above sea level.

Target Distance: Distance from observer to target along the surface.

Target Size: Height of the target from sea level to the top of the target.

Refraction: Refraction Coefficient k. See Panel Refraction for more parameters. If you click on Std then standard refraction is calculated according to the observer height and standard atmospheric conditions. For show k is calculated see Refraction-Coefficient k.

Zoom, Diagonal FOV: Zoom factor f (focal length in 35 mm equivalent units) or diagonal Field of View (FOV) in degrees can be used to magnify the image. This two parameters are linked by the following equations (see Angle of view):

The Diagonal FOV is measured diagonally not horizontally. If we assume a 3:2 aspect ratio we can use a conversion factor of c = 1.20185 to convert between diagonal and horizontal FOV. So if you want a horizontal FOV of 60° in the App, you have to enter

Horizontal FOV = c · 60° = 1.20185 · 60° = 72.111°.

Nearest Target Data Panel

In this panel some calculated object data is displayed. If multiple objects are selected, the data for the nearest object is displayed.

Visible, Hidden: how much of the object size is hidden behind the horizon and how much is visible.

Angular Size, Visible Angle, Hidden Angle: like above but in angular size. The angular size is arctan( size / distance ) in degrees.

Drop: is the amount the surface at the target has dropped from the tangent plane at the surface of the observer. This amount depends on the surface distance between observer and target. This distance is dependent on the Target Distance and the Side Pos of the target via Pythagoras.

Refraction Angle: How much of the object appears lifted due to refraction expressed as an angle. See Refraction-Angle ρ how this angle is calculated.

Lift Absolute: absolute amount of apparent lift of the object with respect to eye level due to refraction.

Relative to Horizon: amount of apparent lift of the object with respect to the horizon due to refraction. The horizon appears lifted with respect to eye level by refraction too. If an object lies behind the horizon, its lift relative to the horizon is smaller than the absolute lift of the object with respect to eye level.

Target Top Angle, Target Top Angle FE: Angle α between target top and eye level for globe and flat earth (FE) respectively. The angle is positive if the target top is above eye level. Some theodolites measure a so called zenith angle ζ. The zenith angle is the angle between the vertical up and the target top. The correlation between this angles is α = 90° − ζ.

Tilt: is the angle between the observer, the center of the earth and the nearest target.

Sagitta (Bulge): is the maximal amount the surface appears to bulge up from the direct line through the earth from the surface at the observer and the surface at the target. This distance is dependent on the Target Distance and the Side Pos of the target via Pythagoras. Note, because the surface bends down in every direction on the globe, the "bulge" is always lower than the plane tangential at the surface of observer.

(Refr.) LoS Clearing: is the smallest distance (Clearing) between the line of sight (LoS) and the surface of the earth, both geometrically and apparent due to refraction. For derivation see  Derivation of Line of Sight Clearing.pdf 

Horizon Data Panel

Dip Angle: is the angle between the horizon line and the eye level line as measured at the observer.

Refraction Angle: How much the horizon appears lifted due to refraction expressed as an angle. This angle is the difference between the Dip Angle without refraction minus the Dip Angle with refraction.

Dist on Surf: is the distance of the horizon line from the base of the observer along the surface.

Grid Spacing: is the distance between the grid lines of the globe model.

Dist from Eye: is the line of sight distance of the horizon line from the observer.

Dist on Eye-Lvl: is the distance of the horizon measured on the eye level plane.

Drop: is the drop of the horizon line as measured down from the tangential plane with origin at the surface of the obsever.

Drop from Eye-Lvl: is the drop of the horizon line as measured down from the tangential plane with origin at the observer height. Drop from Eye−Lvl = Drop + Height.

Left-Right Width specifies the horizontal distance between the 2 points where the curved horizon meets the border of the frame. The line between the 2 points passes through the earth. The distance of the line between this 2 points is smaller than the distance to the center of the horizon Dist from Eye.

Frame Width: Is the width of the frame as measured with a ruler at the center of the horizon. This amount depends on the distance of the horizon and the FOV or focal length. Compare with Left-Right Width.

Left-Right Drop (Angle) are the apparent drop height and angle respectively from the horizon tangent to the line between the 2 points where the curved horizon meets the border of the frame. The drop height is calculated from the drop angle using the distance to the center of the horizon Dist from Eye.

For a derivation of how Left-Right values are calculated see Calculating left-right Horizon Drop.

Radius Earth: is the radius of the earth used for all calculations.

Caution: This value can be changed and results in calculations that do not match reality! Don't change this value unless you know exactly what you are doing!

Apparent Radius: Due to refraction the earth appears this much bigger as Radius Earth.

Exact Equation for Curvature Drop

The Curvature Drop is how much the earth curves down from horizontal at a certain distance. This drop can be geometrically and is often approximated with 8 inches per miles squared. The apparent drop takes the refracted radius of the earth into account:

Approximation Equation for Curvature Drop

The famous approximation drop = 8 inches per miles squared is derived from the following equation by using imperial units:

Note: The Advanced Earth Curvature Calculator always uses the exact equations, not the approximations.

Exact Equation for the Hidden Height

How much of an object is hidden behind the curvature of the earth, the so called hidden height h_h, depends on the distance of the object from the observer and from the height of the observers eye above the surface h_O. The distance can be expressed as the line of sight d to the object, tangent to the horizon, or as the arc length s along the surface of the earth between observer and target.

Note: To calculate the hidden height you must not use the famous equation 8 inches per miles squared! This equation is an approximation to calculate the drop of the earth surface from a tangent line on the surface at the observer. It calculates not the hidden part of an object.

Depending on whether you know the line of sight distance d or the distance along the surface s the following equations calculate the hidden height exactly:

The same equations can be used to calculate the hidden height with and without refraction. Without refraction you set k = 0, which results in R' = R. Because at standard refraction the earth looks less curved, you can use a bigger radius for the earth than it is in reality. For standard refraction 7/6 · R use k = 0.143. At sea level standard refaction is slightly more k = 0.170.

There are multiple slightly different values for standard refraction in use. Near the ground the bigger value for k more accurate. For higher altitudes the smaller value is more accurate. If you press the button Std the App calculates refraction depending on the observer altitude.

The hidden height equations are only valid if the object lies behind the horizon. That is if the distance to the horizon d_H or s_H is less than the distance to the target d or s.

Approximation Equation for the Hidden Height

If the observer height h_O is much smaller than the radius of the earth R, the Exact Equation for the Hidden Height can be simplified by the following approximation:

Note: for observer height much less than the radius of the earth, the line of sight distance d and the surface distance s are identical for all practical purposes. So the equation above holds for both cases.

Note: The Advanced Earth Curvature Calculator always uses the exact equations, not the approximations.

Exact Equation for the Distance to the Horizon

The exact distances to the horizon can be calculated with the following equations:

Approximation Equation for the Distance to the Horizon

The distance to the horizon can also be approximated by the following equation:

Note: The Advanced Earth Curvature Calculator always uses the exact equations, not the approximations.

For all practical purposes while the observer is within the troposphere, so that the observer height h_O is much less than the radius of the earth R, you can use the approximation equations. In this case for the distance between observer and target you can use the line of sight or the distance along the surface. They are practically identical.

Angle to Target Top

The angle α to the target top is measured from the eye level line of the observer to the top of the nearest target. If the target top is below eye level, the angle is negative, else positive.

Note: this equation is robust and gives positive and negative angles α correctly without the need of handling multiple cases.

 Derivation of Angle to Target Top Equation

Apparent Horizon Radius

How much the horizon drops at each end of the image depends on the focal length (zoom) of the camera. The curve of the horizon is barely noticable if the Field of View (FOV) is narrow (high zoom value). Due to perspective distortions, if we are not thousands of kilometers away from earth, the apparent curve of the horizon is only approximately a circular arc. This gets obvious on very wide angle lenses beyond a FOV of 70°. But at the center of each image the apparent radius of the horizon is exactly the refracted radius of the earth (about 7/6 R for standard refraction). The reason is because we are looking tangential to a great circle passing through the horizon point from left to right.

Why does the curvature appear different from different altitudes? Because as we increase the observer altitude the horizon distance gets bigger. Due to perspective, everything at the horizon appears smaller. This includes the radius of the earth at the horizon, which is the apparent radius mentioned here. So although the curvature of the horizon is always the refracted R, it appears different due to perspective and the distance to the horizon.