Method 3: 
Filter spectacles and the plotting table are omitted. Coincidence of the red and green partial 
images of the object to be plotted is obtained directly by adjusting the pedal disk (clearing x-parallax). 
Then the planimetry is transferred to the manuscript with the aid of the pencil. 
Each of these three methods has its own advantages and disadvantages. Method No. 3 may at first 
glance seem unusual, but is quite useful for the compilation of planimetry, provided that there is 
sufficient contrast. 
The aforementioned three methods may above all be used for the following purposes: 
a) Compilation of new maps 
After the usual rigorous interior, relative and absolute orientation preference will probably be 
given to the plotting table — which is not adjustable vertically — and thus to the use of a luminous 
floating mark in the center of this table (Method 1). Under certain conditions, the work can be facili- 
tated, particularly in the rear half of the model, by inclining the screen of the table. Typical examples 
or useful photo and projection scales are: 
from 1 : 12,500 to 1 : 5,000 (directly) 
from 1 : 20,000 to 1 : 10,000 (directly) 
from 1 : 35,000 to 1 : 25,000 (via intermediate scale of 1 : 15,000), 
for example for 
topographic mapping 
route location and 
forestry mapping. 
b) Map revision 
Here, one possibility consists in using the rigorous method described under a). In this case, 
however, the plotting table will generally not be employed (Method 2 or Method 3), since every 
detail to be corrected will then directly become recognizable in the map to be revised by the 
discrepancy between the map and the projected image. 
In addition, the magnification range of approx. 1.3 to 3.0x given by the minimum base setting can 
be further extended towards 1.0x by omitting the true recovery of interior orientation: for this pur- 
pose, the photos are alternately shifted outwards by half the amount of the base, first for the left-hand 
half, then for the right- hand half of the model, and a suitable 1 : 1 lens is used. This approximate solu- 
tion should be entirely satisfactory for certain types of map revision where the planimetry is to be 
corrected in relatively limited areas only. 
As a general statement on the subject of photogrammetric map revision it may be said that the 
method of projecting the aerial photo directly onto the map gives better contrast for the eye (the map 
being illuminated by the “light from the air photo”) than the beam-splitting technique. In the latter 
the ‘“map light” is superimposed on the ‘‘air-photo light” so that the clarity of detail in the photo- 
graph is reduced, provided that “map positives” are employed. 
Extensive experiments have shown that the rigorous methods 2 and 3 should be given preference. 
Another important fact in this connection is that the operator need not glance back and forth between 
instrument and plotting table as in other methods, nor need photo and map be shifted in any way. 
c) Instruction 
The design principie of the DP-1 Double Projector is uncomplicated. It can be operated with 
relative ease and no calculations are necessary. Moreover, it should require practically no maintenance. 
Simultaneous observation of the scanning procedure in the model by instructor and students should 
be an advantage for teaching. It may therefore be expected to prove just as useful for training purposes 
as the former Multiplex. For the preparation of examples suitable for teaching, reference is made 
to [6]. 
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