and F.L.J.H. Corten, Economy in aerial survey by means of the ITC PHOTNAV 
system, (Commission IV). 
The system is based on digital processing and integration of data which 
are sensed with high accuracy during flight. 
Its basic sensors are a doppler, a precision compass, and an attitude gyra. 
Their data are processed in a digital computer which controls the auto-pilot, 
and the interface-computer. The latter provides the exposure command, 
controls the aerial camera, and the numerical recorder. The co-ordinates of 
each exposure station are also recorded along the margin of each aerial 
negative, in longitude and latitude. In addition, they are printed in a recorder 
which also records any other event synchronised with the aerial exposure, e.g. 
magnetometer, infrared scan, sniffler, radar or laser terrain height, etc. 
The basic function of the system is thus twofold: 
1. to guide the aircraft automatically, towards pre-programmed exposure 
stations, i.e. to produce ideal navigation in parallel, gap-free lines in a 
perfectly symmetrical array of pin-pointed photographs; 
2. to record the co-ordinates of each photograph's nadir point, and of 
each other event sensed and recorded during flight, i.e. to produce the 
actual position data of each photograph and each event. 
An example of the navigational function is illustrated in figures 1 and 2, 
depicting the ideal block of principal points and wing points, optimised in a 
completely symmetrical pattern. D 
This symmetrical pattern can either have an ideal internal accuracy, in a 
mutual array, irrespective of its position on earth, or — as an additional 
advantage — it can have exposure stations pre-programmed in the longitude 
and latitude co-ordinates desired for each photograph. 
The authors have named the first type of symmetrical block photography" 
a "symphot block", and the second a *pin-pointed symphot block". 
1 The first author has introduced a new term “symmetrical block photography" 
denoting an array in which all principal points are aligned in two mutually 
perpendicular directions. 
The resulting block may be called — in some cases - a SYMPHOT BLOCK or 
— in other cases — a PIN-POINTED SYMPHOT BLOCK.