6 
Photogrammetria, XIX, No. 6 
3.2 Flight line spacing constancy is established by oblique sighting angle, correcting for 
clearance variations. 
3.3 Alternatively, line spacing can be established by means of doppler-plus-computer; also 
in that case, sighting angle points must be applied, to correct for the heading reference 
errors and terrain height variations. 
3.4 Approaches are established by means of precomputed turns using true air speed, angle 
of bank, rate of turn, drift, and time; heading is used for final adjustment. The wind dis 
placements are taken into account, intermissions or S-turns are introduced whenever neces 
sary. 
3.5 Lateral overlap is obtained by controlling the line spacing and by adapting it to local 
terrain heights. 
3.6 Flight line azimuth and heading is established by a combination of visual and instru 
mental methods. First line’s course is obtained by means of a heading reference (mostly 
magnetic) and drift; line-of-sight is used for adjustment. 
Adjoining flight line’s courses are established by parallelism using oblique angles and 
drift angle; heading reference is used for adjustment and to avoid bending by error 
addition. 
3.7 Flight line limitations, area delineation and rectangularity are established by means 
of deduced reckoning course-and-groundspeed timing either using the wind star vector or 
angular groundspeed plus clearance and drift, or by means of doppler drift and speed 
timing with a heading reference fed into a doppler computer. 
3.1 Absolute altitude and scale. 
3.1.1 Barometric altitude and indicated altitude to fly Z Q . 
Accurate determination and realization of the indicated altitude to fly a certain abso 
lute altitude, is one of the basic components of survey navigation. 
Altimetry is based on the standard barometric height formula 
18400 log — (1 + atj (l + 0.377 —) (1 + B cos $) (l + — 
where p in mm Hg 
a = coefficient of thermal expansion 
i = barometric mean temp, of air column (in °C) 
e m = mean water vapour pressure of air column 
p m = mean air pressure of air column 
1 + B cos <P = correction factor for gravity change per geographic latitude 
r = earth radius 
Z 0 — flight altitude, absolute altitude over msl (in m) 
For the purposes of aerial survey, this can be simplified to an expression for height 
differences in layers 
Z 2 — Z 1 = 221.15 T m i 2 (log p 1 — log p 2 ) 
where Z = altitude (in ft) 
T m = barometric mean absolute temperature (in °K) 
p = air pressure (in mb) 
In this relation p 0 at Z () is measured by the altimeter and is reproduced in the form 
of indicated altitude (7 ALT); this altimeter indication is based on the corresponding 
pressure altitude in ICAO atm. and must therefore be corrected during flight to real 
atmosphere.