control upon which subsequent photogrammetric operations can 
be carried out with less precise equipment. 
If one accepts multiplex as a useful piece of photogram- 
metrical equipment, then an improvement in multiplex technique 
js worth recording. The combination drawing and tracing table 
described in this paper was designed to give multiplex better 
performance, improving and accelerating the plotting of small to 
intermediate scale maps where such might be economically plot- 
ted by multiplex. Members of this Society will appreciate the 
quantity of this type of mapping that still remains to be done in 
many parts of the world. 
In the conventional method of multiplex plotting, two com- 
plete photographs are projected. If the two images are resolved 
in correct orientation on a quinis white surface, one can view the 
whole stereo model. Immediately the tracing table is used the 
great advantage of being able to see this large area is lost, the 
view being restricted to the small area of the tracing table platen. 
Furthermore, the floating im of the tracing table is sufficiently 
remote from the plot so that the plot and image are difficult to 
co-relate. When detail becomes intricate the operator must 
refer back and forth between the stereo model and the drawing. 
If it were possible to make better use of the advantages of the 
projector-type plotter and combine these with the photo sketching 
method where spot heights are established and sketching of con- 
tours carried out under a stereoscope, such a technique should 
prove rapid and interesting and go a long way towards re-intro- 
ducing the art of the topographer into photogrammetry. In an 
attempt to achieve this, experiments were conducted at the Topo- 
graphical Survey of Canada in the fall of 1951. 
  
    
The first step was to devise a means of replacing the single 
floating mark of the conventional tracing table by a number of 
targets of equal elevation. Such an arrangement would accurately 
define a horizontal plane with respect to the terrain of the pro- 
jected model. Various shapes and colours of targets were 
attempted but none were satisfactory. The result was most en- 
couraging, however, when several tracing tables were set to the 
same height. It seemed possible to visualize the plane of the 
targets and predict the position of intersection of such a plane 
with the ground. Since this was very cumbersome, a small light 
box about eight inches square was mounted on sketchmaster legs