2.  
to establish lateral position and cross-track velocity, most pilots require to be able to see not less than a three-light segment of the centreline of the approach lights, or runway centreline, or runway edge lights ;

3.  
to maintain a lateral level, most pilots require to be able to see a lateral element of the ground pattern, i.e. an approach lighting cross-bar, the landing threshold, or a barrette of the touchdown zone lighting ;

4.  
to make an accurate adjustment to the flight path in the vertical plane, such as a flare, using purely visual cues, most pilots require to be able to see a point on the ground which has a low or zero rate of apparent movement relative to the aircraft.

Minimum visual segments at DH have been established for each category. Typical values are 60m for CAT III and 90m for CAT II automatic landing and 225m for CAT II with manual landing.
Note 1:  the visual segment is the runway segment that a pilot can see from his position
Correlation between visual segment: SVR ; RVR
A formula can be used to compute the SVR required for the pilot to acquire visually the specified visual segment, at the DH for a particular operation.

(purely geometrical relation)
v : visual segment (m)

h : pilot's eye height above ground level (m)

w:  Cockpit cut-off angle (°) =  Down vision angle (°) - pitch angle (°) Figure 2.4

"... nature of the meteorological phenomena..."
With regard to fog structure, data gathered in the United Kingdom over a twenty-year period have shown that in deep stable fog there is a 90% probability that the SVR from eye heights greater than 15ft above the ground will be less than the RVR. There is some evidence in pilots' reports that other low visibility conditions (heavy rain, blowing snow, dust, etc.) could produce a relationship similar to that observed in fog.
So, to convert required SVR into required RVR, the model established in the UK for deep stable fog can be used. Refer to the following graph providing SVR/RVR ratio as a function of eye height. On 90% of occasions, the SVR is expected to be this proportion of RVR or more.
Figure 2.5

Example of establishment of required RVR
For CAT II operations with auto pilot down to DH=100ft and manual control below, the required visual segment is 90m. The required SVR for such operation is 220.7m (using the example as shown in Figure 2.4).
Eye height = DH + 20ft =120ft = 36.6m
Cut-off angle = 20°
Pitch = 4°,
w = 16°,
Visual segment = 90m
Using above formula we find SVR =220.7 m

At 120ft above ground level, the SVR/RVR is expected to be 0.68 or more, so we obtain a required RVR equal 324.6m.
RVR = SVR x 1/0.68 = 324.6 m (for SVR/RVR=0.68)
The same method may also be used to evaluate the visual segment for a given RVR.
Other more or less sophisticated methods, have been used in the past. But withrecent experience, it has been found that with the improvement in the performance of visual aids, and the increased use of automatic equipment in the new larger aircraft, most of the variables cancel each other out and a simple tabulation can be constructed which is applicable to a wide range of aircraft.
　
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