78 SURVEY NAVIGATION, CORTEN
4. Navigation and orientation methods determining the angular orientation.
4.1. Fore and aft photography.
Principle.
Oblique photographs can be taken synchronously with the main vertical photograph.
On these photographs straight lines in flight direction can be determined and transferred
onto the main vertical photographs by means of a “Straight line plotter". When bridging
the strip, this flight line linearity is used to control the azimuth and to keep the error pro-
pagation of y bending within close limits.
Performanee.
Exact information about this method and its inherent accuracy is not yet available.
It is felt that more statistical material is needed for proper evaluation of its merits and
performance.
4.2. Solar periscope.
Santoni’s solar periscope is an instrument for use with aerial triangulation. It deter-
mines the inclination of the camera’s optical axis at the exposure station by measuring
two angles: one direction to the sun and the other dx azimuth. The position of the aerial
‘solar
€
camera with respect to the sun’s direction is synchro-recorded by means of a
‘solar computer” which — after introduction of
the main photograph’s dx — delivers as output the values of ¢ and «o to be used in the
€ ,
camera"; its image is measured in the
bridging procedure.
Performance.
Jomparative experiments of O.E.E.P.E. have shown that the method can perform
with remarkable accuracy; mean errors of 3¢ were obtained. For further information
we refer to the literature concerned.
4.3. Horizon photography.
By means of photography of the horizon synchronized with the vertical photography,
the inclination of the aerial camera’s axis with respect to the vertical can be recorded.
This method is known and applied since 1930, and improved to a remarkable degree
of accuracy, but its use in photogrammetric routine has, until now, been limited to Fin-
land. The method can be carried out with a special photogrammetric camera photogra-
phing the horizons — via prisms — on the main vertical camera film, or it can be applied
with separate synchronous photography by means of additional horizon recording
cameras.
Principle of the method.
One or more photographs of the horizon are taken in exact synchronization with the
main vertical exposure. Any inclination of the camera system will become apparent in
the horizon photography in two components: one will show up as an inclination of the
horizon image, the other one as an elevation of the horizon image. Both angular inclina-
tion components (a and f) of the vertical camera can be measured for each exposure
with respect to the foregoing exposure's and to the following exposure's inclination data;
the resulting values are inclination differences between consecutive photographs. The
two camera inclination components a and f can be related to the orientation elements
y and oc if the camera's orientation differences dx about the vertical axis and drift
angle à are known.
