180 
Ce procédé peut être appliqué aux levés cadastraux et aux sondages au 
voisinage des côtes. 
La restitution ponctuelle peut être effectuée avec un appareil simple du type 
von Orel. 
Aucune discussion n’a eu lieu à propos des publications ci-dessus. 
La séance du 23 juillet 1956, à 10.30 heures (d) 
H. Yzerman: The S.D.I. La séance est ouverte par le président. La parole est donnée à M. II. Yzerman, 
a new method for stereo- S u i sse> pour la communication »The S.D.I. a new method for stereoscopic 
scopie measurement . , 
and plotting, measurement and plotting». Kesume: 
(Publ. H Y l) Stereoscopic observation is an integral part in several photogrammetric proce 
dures. The accuracy of this delicate attribute of human effort is often overesti 
mated and consequently disappointing. Yet, if met with proper tools it can be 
still surprising. 
The Stereo-Direct-Invers is an attempt to realize these tools and to reduce 
the error of stereoscopic height to less than half its value. 
It is known, that interchange of ortho- and pseudo-stereoscopies improves the 
parallax measurements. The following errors can be eliminated: Firstly those of 
»Fixation disparity» (1), and secondly those caused by the »Fertsch-effect» (2). 
By means of a picture it is shown, that the interchange of stereoscopies results 
in inversion of the model in the direction of a point situated at the middle of the 
eye-base. Also, that for a small parallax the interchange of stereoscopies results 
in equal inversion of the model as to a reference plane, containing the floating 
mark. Thus, if caused by fixation disparity or Fertsch-effect, the parallax of the 
floating mark with a model point seems to be nil, then the interchange of stereo 
scopies would make the latent parallax visible in the order of twice its value. 
However it is indicated that the existing methods of interchanging stere 
oscopies are not fullproof in their results and uncomfortable in their practical 
application. 
The ideal way to interchange stereoscopies would be one that is: operated by 
merely pushing a button, timeless to the observer and without affecting in any 
way the position of the stereoscopic components. 
Stereo-photogrammetric instruments can be easily adapted to achieve before 
mentioned ideal way of interchanging stereoscopies. 
It is necessary to illuminate the photographs with alternate radiating light- 
sources and to separate the images that are received at both eyes by a shutter 
rotating synchronous with the alternate radiating lightsources. 
Interchange of stereoscopies is obtained by an electro-magnetic phase switch 
at the A. C. supply to the lamps circuit, while the circuit of the synchron shutter 
is left uneffected. 
Patents are applied for by the author. 
The tests were performed by five persons at Kern & Co., Aarau. The results 
that were derived from their measurements on an instrument of the mirror 
stereoscope type, show an average decrease of the mean square error in height 
of 50 % for Stereo-Direct-Invers as compared with single stereoscopic observa 
tion. The still more challenging result is the elimination of the subjective 
observation error. It opens the way to more reliable work, singularly as well 
as in shifts. 
Finally it is shown that the S.D.I.-method is superior to doubling the base/ 
height ratio.