(491) 
plex contour lines, a = 1,35 m) is almost the same contour interval on which the 
american c-factor is based. This standard interval is 3,33 times the mean height 
error (Photogr. Engineering 1951 p. 359), in our example it is 3,7 times this 
error. Thus we see that when using this standard interval differential uncer- 
tainty plays already a noticeable role, but it is still possible to take this influence 
into account. 
Fig. 5a shows the planigraph contour-lines, fig. 5b the multiplex contour- 
lines both constructed with an interval of 25 metres. Now also the planigraph 
contour lines show slight differential irregularities, one can easily make allow- 
ance for them. Ah is in this case 6a. The multiplex contour lines with the inter- 
Planigraph Multiplex 
Ifi A 
IN A 
Ah» 25m a=g4%om ah= 620 
  
a=135m ah=19a 
Fig. 5a ‘ Fig. 5b 
val Ah = 2,5 m = 1,9 a are affected to a high degree by the differential un- 
certainty, they begin to touch one another in several places and in one case the 
upper line crosses the lower. Measuring the 2,5 lines has brought no positive 
result but much trouble. It is absolutely uneconomic to measure contour lines 
with such a small interval. 
We see from that we have explained: The standard interval Ah = 3,33 a 
of the c-factor is quite appropriate, at a given accuracy of height measuring it 
is the smallest that may just be used. A considerable skill of the operator 1s 
necessary in order to get good results and contour lines which correspond to the 
reality. It is clear that one uses such a small contour interval only in extensive 
photogrammetric-work. — When working intensively we have to use a larger 
countour interval, we have seen at least Ah = 6 a in order to get the forms of 
contour lines with sufficient reliability and without noticeable differential 
uncertainty. 
We have touched upon the american c-factor. As it is not familiar to most 
european photogrammetrists some further words may be said about it. It is used