axis (tip) is a modification of the x parallaxes proportional to x. The engineering 
problem to be solved in the most practical and economical manner is to superimpose 
the two compensating parallaxes without dead travel to the x parallaxes to be 
plotted (accuracy 0,01 mm!). The basic solution to this problem contains for each 
direction a ruler with support which is parallel to y and x when in zero position 
or neutral, and which can be revolved by an angle a or f with reference thereto 
according to the order of the desired correction. The scanning of the picture pair 
in the x and y directions will cause an additional double x motion of the right- 
hand picture holder with the concomitant movement of the 5, spindle. 
In Fig. 7, the rulers 8 and 9 control the tip and tilt of the model; 10 and 11 
are the graduated scales on which the values of inclination are set and 12 and 13 
are the pivotal points supporting the rulers. The displacement y.tga produced 
by a rotation a of ruler 8 for a picture ordinate y in the x direction is transformed 
by the inverter plane 14 into an identical displacement at y and then transmitted 
to pivotal point 13. If ruler 9 is displaced simultaneously by f from the zero or 
neutral position, sum y.tga + (x + Ps) tg B as the motion in the y direction will 
result at roller 15. This structural arrangement of the component parts has permitt- 
ed to eliminate any dead travel in the system by the provision of one single spring 
17 which is supported at the left. 
The Stereotop II can be easily enlarged into Stereotop III by the addition 
of the extra attachments. Thus, an equipment has been evolved which can suc- 
cessively handle any job ranging all the way from simple stereoscopic observation 
and examination of picture pairs to detailed stereoscopic plotting. 
IV.- Rectifier SEG V, 
Before designing a new rectifier it is advisable to make a careful examination 
of the requirements such equipment will have to meet to-day. Under the circumst- 
ances prevailing in Central Europe, the "intensive processes’ for the production of 
photographic plans or mosaics have always held first place. The areas with small 
variations of elevation, to which rectification is limited, are also the economically 
most profitable and most highly developed regions. Therefore, they require medium 
and large mapping scales and a high degree of accuracy for the plotting of points 
and planimetric features. A geometrically accurate rectification is based on 4 control 
points per aerial picture. On these, the projection of the picture is adjusted by 
systematic trials. That these high-precision processes (referred to as ‘point process- 
es”) are still holding their own and are even further developed is evidenced by the 
‘facet method’ which was recently evolved by the French cadastral service for the 
production of large-scale cadastral plans. 
For the problem to be solved mainly in overseas countries, that is: the pro- 
duction of photoplans of large areas on medium and small scales according to the 
extensive process, rectification has played only a minor role. The greater accuracy 
which could be obtained in comparison with the simpler mosaic processes, was 
frequently not required and did not warrant the time and labour involved. However, 
conditions seem to be changing. For a number of purposes, ‘uncontrolled mosaics’ 
do no longer meet rising requirements. On the other hand, it was found that 
fairly good photoplans can be obtained with ‘extensive rectification processes’. The 
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