8 
Photogrammetria, XIX, No. 6 
3.2 Flight line s-pacing by oblique sighting. 
3.2.1 Side sighting angles 
For a given altitude, any flight line spacing A can be expressed by its corresponding 
oblique sighting angle /?. It is appropriate to have the line spacing determined either by 
the angles to the centre of the next line track (distance A) or by the angles to the centre 
/ A\ 
of their common sidelap I distance — I according to 
A 
Z r . tan /? = A or — • 
For both applications, the terrain elevations h x must be known, preferably as a frac 
tion of the reference flight height h 1 /Z r = w T v 
They can be measured by oblique radar or be estimated visually; they are introduced 
together with their corresponding corrections d/? according to 
A 
(1 — w r l ) Z r . tan (/5 r + d/?) = A or = 
3.2.2 Reference elevation h r . 
Basically, any terrain elevation can be taken as reference over which heights and 
altitudes are measured, and many criteria are used to decide upon such reference elevation. 
It is logical, however, to take a critical survey specification as a criterion and as such the 
minimum side lap can be chosen. Considering 10% S side lap as bare minimum and allowing 
5% S for random flight track oscillations, we may consider 15% 5 side lap as a minimum, 
generally occuring at the highest points in the common overlap. This value, together with 
the camera which has been chosen, determines the width/height ratio A/Z; if reference 
scale and reference height are given, the flight line spacing A is now determined. 
Choice of reference elevation in such a way that the side lap obtained has a constant 
value at that reference elevation, will simplify the navigational procedure because all 
computations and all navigational equipment constructions can be based on this fixed side 
lap value which is constant now for all types of terrain. A useful value of such a choice 
is v — 20% S; it allows for 5% variation due to slight relief (e.g. for WA photography 
the relief tolerance w is approximately 6% Z r ). Any further appreciable relief must be 
specially compensated by correcting the corresponding sighting angle /? with a value A 
which can be computed for any given width/height ratio. 
3.2.3 Input — Output 
Line spacing by 
obliquesighting 
INPUT 
Input data for obtaining line spacing by oblique sighting are: 
x = relative terrain heights of the highest points in the common overlap expressed as 
fraction of Z r 
Z r — flight height over reference elevation 
fl r = precomputed oblique angle to the centre of next flight line, or the centre of the 
common side lap, determined by the width/height ratio to be made good. 
A fl = correction to oblique angle, to cope with relative height variation w r r 
Output: 
Flight line spacing A made good. 
( 1 -w.) Z r tan(/?+A/?)=A 
w r1 Z r A r A A 
3.3 
3.3. 
met! 
nece; 
inac( 
sand 
such 
navij 
but 
and 
tanc( 
data 
line 
cont; 
3.3. 
X 
A freq 
vert 
head 
VAR 
GS 
D 
A 
DIST 
TT 
3.4 
appro 
impor 
point 
turns 
flight