THE PHOTOGRAMMETRIC SURVEY OF THE MAP 1/200. 000 
OF SAHARIAN REGIONS BY AIRBORNE CONTROL 
  
  
  
(Summary of the paper to be presented to Commission IV) 
In the regions difficult of access, desert or very uneasy to run over, one of the main 
problems of small scale mapping is the determination of the ground control, necessary for 
plotting, the control points being, for purposes of economy, in as few number as possible 
and distributed in a manner as arbitrary as possible. Of course, spatial aerotriangulation 
affords a mean of solving the problem, but it requires the use of plotting-machines of first 
order, and computation and compensation works which need much time and many people. 
The new method consists in determining the vertical control for each pair from the 
registerings of an airborne instrument, the Air Profile Recorder. After the pictures have 
been taken, the profiles are registered in a second flight, at much lower altitude along lines 
contained in the lateral overlapping zones between adjacent strips, and along some straight 
lines at right angle with the direction of the photographic flight. 
The registered profiles give differences of heights, with reference to an isobaric 
surface, between the points of one profile. By means of transverse profils, those differences 
ofheightsare first transformed into altitudes, differing of the true ones by a constant 
value. This constant value is then computed from a few points of which the altitude is de- 
termined by ground operations. 
As for horizontal control, it is obtained by way of a photographic slotted templet 
triangulation, with central point, based on astronomic points. 
Such a method, although it requires a flight time approximately double, offers five 
important advantages 
- quickness and cheapness of field work , since vertical control is very scarce and 
may be distributed in a rather arbitrary manner; 
- no necessity for the use of very expensive plotting machines as the case is with 
spatial aerotriangulation ; 
simplicity and rapidity of computation and compensation procedures ; 
lessening of necessary delays, since plotting work can be started very soon after 
the field work is achieved ; 
- very good consistency of heights derived, in bare and gently moved ground. 
By this method, it was possible to achieve the ground work for an area of about 
56.000 square miles in 1959 and an area of 80.000 square miles in 1960, with a very small 
number of field operators ; the paper presents a number of results, which justify fully the 
use of the method for mapping at scales of 1/100.000 or 1/200.000 in regions without very 
strong relief. 
The plotting of the map made it possible to experiment plotting machines of lower 
accuracy (second order or third order), and a short statement is given about the interest 
of using such plotting machines, for mapping at such scales.