Corten, A Survey Navigation System 1 3 
¡ertainty. Its 15 scales 
i a logarithmic scale. 
us flight elements any 
jrformance. 
together with the air- 
output. 
le’s turn speed V will 
ft 8 can be introduced 
id spacing M as output 
jether with the elapsed 
us R produces S-turn 
i flight line spacing A 
roduced during elapsed 
light line spacing A as 
e spacing at all points, 
l h v the side lap v x is 
ng 
;o scale 
e 
ut to obtain the desired 
x as output. 
3.6 Line azimuth and heading. 
3.6.1 Three data sources for course. 
In flying survey lines, the maintenance of a constant heading is important but can 
never be the predominant factor for preserving flight line parallelism. Any attempt to 
base survey navigation on a heading reference only is condemned to fail: minor changes 
in crosswind component, in magnetic variation over longer distance, autopilot’s heading 
reference hunt, etc. make it impossible to keep the mean deviation in track substantially 
below one degree. Also, in many cases it is unimportant whether the flight line directions 
are correct in absolute sense. Any relative but constant heading reference may be of 
great use; a gyromagnetic compass, solar compass, or inertial heading can be applied 
to advantage. 
In addition to heading reference, two more data sources are applied: relative bearing 
A 
of the line-of-sight of distant A or — points, and gyro-nadir or gyro-oblique check 
A 
on A or I points. 
The line-of-sight used has a certain non-linearity caused by tilt angles and imperfect 
corrections for terrain height during the production of the oblique-angle A points; the 
heading reference serves here as a means of adjustment. 
When passing over the A points as checked by a gyro-nadir sight, or when sighting 
A 
the ^ points with a gyro-oblique angle, the track will show certain deviations. Some 
navigators use these discrepancies as a basis for the corrections to be applied. We do 
not agree with that method because nadir points should be considered as check on the 
result obtained whereas the line-of-sight is not only a much more sensitive means of cor 
rection but also allows for correction before the points are overflown. Also, it is logical 
to use the error-generating flight element — i.e. improper heading — as the element to 
be corrected rather than the error result — i.e. the across-line nadir displacement. 
3.6.2 Input — Output. 
Line azimuth 
by heading 
1 st line : 
adjoining lines: 
TH or RH + D : 
♦ ^ i 
MH+VAR =TH, or RH 
t t t 
INPUT: MH VAR or RH D 
Heading ref., rel. or absolute 
TC °(H) 
as basic reference T CO( R b) 
as adjustment 
as adjustment TT(N) as check 
" reference " " " check 
TCo (H> 
RB=TCo( RB ) 
* 
D.adj.RBto D n 
f l-O-S 
^ ^ =T1 (N) 
D A dr A 
line of-sight 
N Aorf 
gyro nadirand oblique 
As input for the first true course determining possibility (i.e. the heading reference) 
serve either an absolute or a relative heading. If magnetic heading is to be used as ab 
solute reference, it should be corrected for variation and deviation to the highest degree 
of accuracy. This is not always possible and magnetic heading can then be used as relative 
reference; it should be constant except for the crossing of the isogonic lines’ pattern. 
Absolute or relative bearings can also be obtained by solar compass and by inertial 
reference. 
Drift angles must be measured with high accuracy and be added to the true heading 
or relative heading to obtain the true course to be made good for the survey line. 
The most important quality of the subject photographed 
which determines exposures and development is its maximum 
and minimum brightness. We have as yet no instrument to 
measure these. In consequence usual procedures are based 
extensively on guess.