62 SURVEY NAVIGATION, CORTEN
The quality of this navigation is one of the determinants of the economy of photogram-
metric mapping. Proper survey navigation shall result in the smallest possible number
of photographs. Requirements to be fulfilled are: straight flight lines, constant overlap
as small as feasible, constant side lap as small as feasible, constant flying height.
Notwithstanding the fact that the above-mentioned principles are generally recog-
nized, there is still too much survey photography showing poor results. In many cases this
is caused by the fact that rather arbitrary trial and error methods are applied even by
experienced survey navigators. Optimum results can be obtained only if the survey is
carried out by an extremely competent crew applying sound navigational principles.
Various possibilities do exist, some of them described in literature. The best method
inasfar as is known by the author is the method designed by the Groupe des Escadrilles
Photographiques of the Institut Géographique National, France. This method is standard
procedure applied by the crews of 12 B-17 and 8 HD 34 survey airplanes; their results
are excellent and constant over many years.
Principle of the LG.N. visual navigation for constant side lap.
For the positioning of the flight lines no DR procedures are used because these
depend on compass heading references which cannot be sufficiently accurate for aerial
survey; the same applies to drift, to integration of air speed over the time and to other
data. For this reason, the method is based on the use of visual landmarks, applying them
as if they were beacons in line.
Y Ai .
Mean datum plane Z, is deter-
dB mined according to
d
fr aua P
Zo TT Zmax 1:5;
where Z,,, — highest point of the
area
q — mean side lap
S — photo scale.
Flying height is then deter-
: mined; it is established by means
Ri R2 Altitude Zo . d ? WE . x J
of the barometric altimeter and
7 OF kept constant within twenty me-
A 7 ters by means of statoscope. The
Fig. 3 flying height differences are sta-
"1g. 9.
g.9 toscope recorded.
Flight line parallelity is obtained in flight line R, by visually picking out land marks
a(l—q) :
on the next flight axis E, ,, in such a way that tan f f where a = photo size.
A. pelorus visor can be used but a gyro stabilized drift sight is more appropriate.
In mountainous terrain, if the elevations differ from mean datum plane, by a value
; : s.sin 25 ;
dZ,, corrections df must be applied to angle f i.e. df — j .dZ, (figure 3).
Experienced navigators estimate the terrain elevations with sufficient accuracy to
determine the new flight line's axis within 196 tolerance of side lap. This method is called
“Ja methode en parallele” 1).
1) In comparison with the I.G.N. methods, it may be noted that the U.S.S.R. navigation
uses a solar compass as heading reference in standard procedure; figure 4.
