ed- 
grid card to cover a single stereo-model. This card had to be 
sufficiently opaque to hold back unwanted light and it also had to 
have a suitable white surface in order to properly resolve the 
projected images that formed the stereo-model. Certain practi- 
cal difficulties were encountered with this type of target illumi- 
nation, since each drawing had to be either on a specially pre- 
pared grid, which was very expensive, or on transparent 
materials. In the latter case, even the most transparent plastics 
affected the quality of both the projected images and the light tar- 
gets. 
The engineering model Type 301 subsequently produced by 
Photographic Survey Corporation Applied Research differs in two 
important respects from the original equipment. First, instead 
of illuminating a light dot grid from beneath the plotting surface 
and thus confining the plotting materials to transparencies, it 
projects the grid on to the resolving surface, which is also the 
horizontal reference plane surface. Spacing of the dots used to 
form the grid pattern can be varied to suit the nature of the ter- 
rain being mapped. The table surface can be opaque instead of 
plate glass but the material being used for plotting must have good 
light resolving characteristics and should lay flat to the surface 
of the table. The second modification incorporated in this 
engineering model provides for the vertical movement of the pro- 
jectors so that the plotting surface remains constant in position, 
an obvious advantage to the operator. 
The single point target of the conventional multiplex tracing 
table is primarily a measuring device and the operator is entirely 
dependent upon his visual acuity when employing it. When drawing 
topographical detail with the projector-type stereo-plotters, the 
operator should be able to make full use of the Panum area of his 
eyes and thus take advantage of his visual as well as his stereo- 
acuity. With this new approach to projection type mapping, the 
confining combination of a single target and small viewing area 
are eliminated and the operator has a better opportunity to size 
up the ground he is attempting to portray; consequently, the lines 
he draws will collectively be more representative of the terrain. 
Any. single contour line may not be more precise than a normal 
multiplex contour line but it should be in better sympathy with 
adjacent contours. An automatic check on lines is provided to the 
Operator since the line he has drawn appears in direct contact with 
the projected image of the topography and, consequently, plani-