FOTOGRAMMETRIE
i83
H. G. JERIE,
A simple Contour Plotter (or Topographic Mapping
1. Introduction
A number of mapping agencies are using a simple procedure for
the production of small scale topographic maps, especially at scale
i200 ooo, whereby a considerable number of height control points
are determined for each stereomodel, either by ground survey
methods or during aerotriangulation and subsequently the contours
or form lines are interpolated by estimation under a mirror stereo
scope and plotted directly on the paper prints.
In a second working phase, these contours and other topographical
details are transferred from the paper prints to the map sheet
making use of a sketchmaster or similar instrument.
It is not intended to discuss here the merits of such a procedure.
However, if it is accepted as such, it seems worthwhile to design
a simple device by which the task of interpolation can be facilitated
and the number of necessary height control points can be reduced.
Based on a proposal by the author, a prototype of such a simple
contour plotter has been designed and constructed at the ITC and
has now been put at the disposal of the Topographic Survey of
Canada for testing and evaluation. A report about the practical
experience gained will be presented at some future stage.
2. Design principles
The principle of the construction is shown schematically in fig. i.
Essentially the plotter consists of a normal mirror stereoscope.
However, the mirrors i' and i" are half translucent (Deville
principle) allowing thereby to observe the string-surface (4) si
multaneously and stereoscopically over the mirror (3). In this way
the string surface (4) is virtually projected into the stereoscopic
model, formed by both aerial photographs, and seemingly inter
sects this stereoscopic model. This will enable the operator to
compile the contours directly on the left-hand paper print.
The surface (4) is materialized by a frame, filled with parallel
bright strings in front of a black background (fig. 2). It is of course
essential that vertical strings are used to form the surface, as any
other pattern would cause y-parallaxes. Using vertical lines, pro
jected virtually against the generally slightly disorientated photo
graphs, the observers eyes will be able to correlate identical lines
(however not along identical points of the lines) to give the im-
I.T.C.—Delft:
