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.