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SURVEY NAVIGATION, CORTEN 79 
Performance. 
The measurements take only a few minutes per photograph, they can be carried out 
in each single photograph or stereoscopically. If the horizon as such is not clearly visible, 
they are carried out at a cloud line or at an inversion layer. The relative values of the 
inclinations of consecutive photographs, if stereoscopic mensuration is applied, are ob- 
tained with not more than m, , = + 2¢...3¢ mean error. 
In order to obtain the absolute values of the camera’s inclinations, it is necessary to 
have ground control, preferably at the beginning and at the end of each flight line. An 
alternative to the use of ground control data is the use of the average value of spirit 
level indications. 
The objection against the horizon method that, in many cases, a usable horizon image 
'annot be obtained, is denied by those having experience with this method. 
The remarkable fact that this simple and 
accurate method has hardly found any ap- 
plication outside of Finland can, perhaps, be 
explained by the expectations that we had of 
the future of the vertical gyro in photogram- 
metry. It is now known that it is basically 
impossible to have gyroscopic stabilization 
with this same accuracy (paragraph 4.4); 
the only vertical reference systems capa- 
ble of performing with standard errors of 
Qe... 3c or 1...1,0" of arc are: the horizon 
camera method, the inertial vertical system 
and the solar periscope. 
A modern horizon camera, recording 
four horizon images synchronized with the 
main camera, is constructed by Wild and is 
now under trial (figure 21). 
As the exact recording and the photo- 
grammetric use of the vertical deviation 
is the alternative to completely automatic 
-- and consequently more complicated — 
inertial vertical guidance, the results of these 
trials are awaited with interest. 
  
Fig. 21. Wild horizon camera coupled 
with RCS. 
4.4. Gyroscopic vertical recording and servo stabilization. 
In a moving aircraft the existence of many acceleration forces prevent the establish- 
ment of “the vertical”, i.e. the perpendicular to the geoid at sea level. This fact challenged 
the photogrammetrist to apply the properties of the gyroscope for establishing a vertical 
reference in flight; inasmuch as it is physically impossible to etablish “the” vertical, 
such a reference should at least be a stable one. 
Research and development have been concentrated on three major aims: 
1. to establish a stable reference direction for recording the deviation of each aerial photo- 
graph’s axis from a vertical position; 
2. to establish a stable reference which controls and commands the camera mounting to 
an exactly vertical optical avis; 
3. to establish a stabilized camera platform in order to prevent image deterioration (un- 
sharpness) due to angular movements of the camera at the moment of exposure. 
Principles of the gyro reference. 
A gyro tends to remain fixed in orientation relative to inertial space (the stars). If