208
l
Fig. 2. Diagram of the electronic photo transformer
The copies are mounted by superimposing the lines drawn through the check points
upon the corresponding lines on the cartographic base of the original. The displacement
should not be above 0.2 mm; gaps or overlappings due to deformation should not ex-
ceed 0.2 mm. If it is, the copies are extended or cut up into parts and mounted separa-
tely. This eliminates the danger of localising great distortions at the notches. It is also
to avoid too many notches, which cause similar distortions beyond the gaps. The possible
number of separate copies increases with the reduction of the scale of the maps being
compiled. In order to avoid too many small copies they are sometimes mounted in
groups before photographing.
Engraving
Engraving on transparent bases is widely used in the U.S.S.R. There are two types
of engraving: positive and negative. In the first case, a special thin soft semitransparent
or opaque layer of, say, asphalt, is put on viniprose, hostophan, astrafoil or other di-
mensional-stable transparent base. Depending on the technique, special instruments are
used for engraving on this base. If the engraving is done on a base covered with a soft
semi-transparent layer, the cartographic material reduced to the scale of the compilation
is placed under it; the image can be projected onto the base with the aid of an optical
System. If the engraving is done on an opaque layer, the layer is sensitized beforehand
and the cartographic material photographed upon it. After the engraving the engraved
picture is treated with an etching dye, while the soft layer is washed off. The original
thus obtained is used as a positive. In the second case, the engraved drawing remains
transparent, while the soft layer serves as a negative background, and so must have the
required photographic opacity. The original thus obtained is used as a negative.
The technique of compiling maps on a transparent plastic base with simultaneous en
graving for printing has been worked out recently. The plastic is covered with a white
layer on which a reduced p'hotograph of the cartographic material is copied, say, in a
