General view of the automatic stereoplotting device installed on a Nistri Photomapper.
Fig. 3.
7.
and by R. W. Hutchins*. It is now installed on a Nistri Photomapper (see Fig. 3),
an anaglyphic type of stereoplotter that uses originalsize 230mm x 230 mm photo-
graphs. Aerial photographs are projected onto the scanning head of the tracing
table, which can be moved either manually (in an arbitrary direction in the xy
plane and in the z direction by rotating a screw) or automatically, by actuating
its three coordinates x, y, and z.
Images that are projected onto the scanning head (see Fig. 4) that is
equipped with a cathode ray tube are correlated by detecting points that have
similar density characteristics. The signals generated by the photocells in the
scanning process and processed by electronic circuitry may remove the vertical
parallax in the stereomodel by changing the elements of relative orientation inthe
projectors; move the tracing table in height until there is no horizontal parallax
at a given point; or move the tracing table by following points of equal horizontal
parallax or along a contour line. In other words, the device is able to measure
x- and y-parallaxes and to produce electronic signals that are proportional to
the parallaxes. These signals are received by servomotors that translate them
into a precise and rapid mechanical motion of the tracing table along the three co-
ordinates in the model space. Other servomotors placed on the projectors actuate
the elements of relative orientation x, d, and w. As a result, the present version
of the automatic stereoplotting device can be used to perform the relative orien-
tation, to measure spot elevations, to draw or record profiles along any arbitrary
line, and to draw contour lines.
Several buttons on the control panel operate the automatic plotting de-
vice. The equipment can be switched onto the evaluation of vertical or horizontal
parallaxes as required in the process of relative orientation and in the plotting
operations. Other controls are for automatic contouring, for automatic profiling
along parallellines, and for regulating the speed. Conveniently located on the
tracing table are additional buttons: to change the scanning pattern to a plotting
dot similar to that used on an anaglyphic type of plotter, and to switch on the
servocontrol of the orientation elements during the relative orientation.
The tracing table is equipped with a drawing pen that, if desired, can
draw contour lines or planimetric detail.
Relative Orientation
As previously mentioned the automatic stereoplotting device is able to
perform a semiautomatic relative orientation. For this purpose the orientation
components are equipped with servomotors that are actuated by the y-parallax
signals of the device. The selection of the appropriate orientation element is
made automatically by a bank of mercury switches attached to the space rod that
guides the lamps and photomultipliers. The inclination of this rod causes certain
switches to close and an accompanying wiring takes care of the selection.
Figure 5 shows schematically the areas within a model where the different orien-
tation elements are activated. It can be seen that close to position 6 both w, and
X| are active. This arrangement is made to provide over-correction. Different
ratios between these two motions will provide different degrees of over-correction.
*Hutchins, R.W. Electronic correlation of image detail in the stereo model.
Canadian Surveyor, XIV, No 5, 1958, 200-203.
