(492) 
in most cases in order to determine the accuracy of instruments. There are several 
slightly different definitions for it. We use the one of them which is based on 
the theory of mean error: “The c-factor is defined as the ratio of flight height 
to contour interval” (Photogr. Eng. 1951 p. 371). The contour interval is in this 
case 3,33 times the mean error of height (Photogr. Eng. 1951 p. 359). When 
using the c-factor it is necessary to know previously the mean error of height in 
order to get the contour interval 3,33 times this error. But if we know this error 
we know also the accuracy of instrument, which should be afterwards derived 
also from the c-factor. The information given by the c-factor must be known 
before. That seems to be somewhat unsatisfactory. We would obtain a proof by 
a detour which could be obtained much better directly. The critics of the c- 
factor probably had this fact in mind. But in practice the c-factor seems to be 
used seldom in this direct manner for evaluating the accuracy of a plotting 
machine, it is used normally for relatively comparing their accuracy from ex- 
perience, it is a mean to which one is accustomed and in this respect it is without 
doubt useful at least for practice. 
But in other respect the c-factor is quite valuable as we have seen. It gives 
an essential and useful relation between the terrain and the photogrammetric 
contour lines. Especially we in Europe take into account in most cases the instru- 
ments and the special techniques of photogrammetry, but we tend to neglect a 
little the visible results of our photogrammetric work. In this respect the c- 
factor seems to be of positive value also for us. It is useful determining an ap- 
propriate contour interval for a given flight height and a known accuracy of 
instrument, the latter given by the c-factor of by the methods used in Europe. 
“THE ACCURACY OF PHOTOGRAMMETRIC CONTOUR-LINES 
AND THE AMERICAN C-FACTOR” 
par 
Prof. Dr. R. Finsterwalder. 
Un plan de courbes de niveau est examiné. Les résultats démontrent que 
l'équidistance doit étre 6 fois autant que l'erreur moyenne de hauteur pour les 
levers de carte à la manière intensive et 3,3 fois autant pour les levers de carte à 
la maniére extensive. Au dernier cas la hauteur de vol divisée par l'équidistance 
des courbes de niveau est équivalente au facteur C qui est employé en Amérique. 
Le profit du facteur C est discuté brévement. 
DISCUSSION ON PROF. FINSTERWALDER'S PAPER, 
*THE ACCURACY OF CONTOUR LINES AND 
THE AMERICAN C-FACTOR” 
by 
Robert E. Altenhofen. 
It is gratifying to note that Prof. Finsterwalder's paper draws the conclu- 
sion that the C-factor has the practical value of determining the contour inter- 
val for a given flight height and stereoplotting instrument accuracy. There has 
been much adverse comment on this oversimplified artifice for denoting the per- 
formance of plotters in the production of topographic maps. So, it is refreshing