Horizon Manual Correction

A typical example where the vertical is directly accessible on the subject, the horizontal can not be determined directly. 

The reference element must of course be perfectly vertical in reality. If it is also the case in the image, it means that the horizon does not need to be corrected.

The most classic cases where vertical elements are not lacking.The shorter the vertical reference, the more accurate the placement of the marks needs to be.

This procedure works for both versions

There is no magnifying in PhotoLab. This absence is compensated by zooming (Ctrl + scroll wheel) on the selected area. it’s just as fast and accurate.

On this illustration nothing seems plumb, it is necessary to rectify the horizon before passing to the correction of perspective. The problem is to find a “true” vertical, which is not the case with the walls of the ditch at the bottom.
The vertical tool is displayed on the axis, and it aligns almost exactly with the front of the massive, truly vertical entrance gate. A fine adjustment gives a correction angle of 0°! The image, perfectly horizontal, is ready for perspective correction.

A body of water whose opposite bank is located far away.This photo is the typical subject for adjusting the manual horizon recovery. The bank is located at a distance compatible with the sea horizon.
The automatic correction does not work because of the absence of vertical lines and the “horizon” line of the bank is disturbed by the waves and boats. The eye succeeds without problem to draw an exact line. The reference line (opposite bank) does not occupy the entire image, but its length is sufficient for perfect correction.

It is sometimes desirable to correct a troublesome “pseudo horizon”, a distant line leaning in reality, but which gains to be rectified.

In reality, the skyline is exactly at eye level, straight in front of you. In photography, where the sensor is often inclined, the horizon is rarely on the line of sight (median height of the image).

Two essential rules for identifying the skyline:
– Rule 1: It is located at the photographer’s eye level (in the same horizontal plane)              
– Rule 2: It is defined by the convergence to infinity of coplanar lines*  of landscape elements
   * lines located on the same horizontal or vertical plane  

The rules of the skyline; the convergence of lines is verified. Here the horizon line is already known but the line convergence would allow finding the horizon line if this one was not visible.

This building fits this criterion well.
This is a slightly low angle shot performed shot, the skyline is approximately located in the middle of the platform height but can not be defined exactly. A significant horizontal facade alignment is used to correct the horizon.

First clue according to rule 1:
A person is close to the front on the right. The skyline is probably at his eye level since the photo was taken with the camera at eye level.
The horizon line would thus be at the height of the first window bar.

Observation:
On the other hand, the two people in the center of the image are clearly below. Either the soil goes down here or the reference is not good.

Second clue according to rule 2:
The virtual lines of the recent paving (probably a regular plan) converge a little below the first bar.

Verification:
All first window bars are not on the same line: the skyline is not located exactly on these bars, but a little below.
 the bottom of the windows as a reference, we realize that they are all located on the same straight line: It is the skyline !

This neo-Renaissance building is, at first glance, not very conducive to this exercise. But it has a very useful element in many similar cases; a staircase.
When the steps are horizontal, one of these steps will be close to (or on) the skyline, it is the first one whose upper face is no longer visible. 

The solution is to take this step as the reference, then, from this point, to find the elements located on the same plane. All those above  and below this step are not aligned.

A 15th century castle. With its round towers and a strong low angle shot, finding the skyline seems complicated. In fact, on the first tower, just look for the line where the stone beds pass from a concave line to a convex line: it is the horizon line! 

All the horizontal elements, whatever their orientation, located on the skyline are on the same plane and therefore appear merged on the same line. On the contrary, they will appear to follow divergent lines or elliptical arcs if they are on different planes than the horizon line.
This applies to all forms (parallelepiped, cylinder, sphere, cone …). 

The two towers of the entrance do not have their stone beds at the same level, which is why the skyline is in the middle of a bed on one side and on a joint of the other.