The alidade (fig. 11-31) is a straightedge with a
sighting device parallel to the edge. The more precise
types have telescopes for sighting, special retitles for
measuring distance, and graduated arcs for
measuring vertical angles. A new version also
includes a self-leveling, optical-reading system with
enclosed graduated arcs.
1. The open-sight alidade (fig. 11-31, view A),
which is very useful in sketching small areas, has a
collapsible open sight attached to a straightedge. A
level bubble is mounted on the straightedge for
keeping the alidade level. A trough compass is also
furnished for attaching to the sketch board. By
sighting through the peep sight, the operator can
determine a level line and the slope from the sighting
point. No magnification is provided, so the sight lines
are kept comparatively short. The distances can be
estimated by pacing or can be measured with a tape if
more accuracy is required. A 10-mil graduation that
is numbered every fifth tick mark from 0 to 40 runs
up on the right edge and down on the left edge of the
front sight for determining slopes.
2. The telescopic alidades (fig. 11-31, views B and
C) consist of straightedges with rigidly mounted
telescopes that can be rotated through a vertical
angle of ±30 0 . One type has a telescope set on a high
standard or post to raise it above the table. This
permits direct viewing through the telescope, which
is at a comfortable height. The other type has the
telescope mounted close to the straightedge. A right-
angle prism is attached to the eyepiece and permits
viewing through the telescope by looking down into
the eyepiece prism.
3. The telescope for the high standard is 16
power; for the low standard, 12 power. Both are the
inverting type with internal focusing. The prismatic
eyepiece inverts the image top to bottom, so that it
appears erect but reversed side to side. The line of
sight through the telescopes in a level position is
parallel to the straightedge on the base. The telescope
reticle has horizontal and vertical cross hairs and a
set of stadia hairs. As you already knew, the stadia
hairs are used to measure distances. The vertical
distance between the upper and lower stadia hairs is
carefully read and multiplied by the stadia interval
factor. This value is the straight-line distance
between the instrument and the rod.
4. A circular bubble and a magnetic compass
needle are attached to the base. These are used to
level the plane table and orient it to its proper
position. Since the ball-and-socket head does not
permit as fine a movement as the leveling screw, the
bubble is centered as accurately as possible. Then,
the wingnut (fig. 11-30, view B) is set firmly but not
tightly. When the plane table is tapped lightly on the
proper corner, the operator can refine the leveling
and then properly tighten the wingnut. To orient the
plane table, loosen wingnut F and rotate the table. It
is a good practice to draw a magnetic north line on
the cover sheet or on two pieces of tape attached near
the edges of the board. The straightedge is set on this
line during orientation. When the plane table is
rotated to face north, the magnetic needle is released
and will have room to swing in its case without
hitting the sides.
5. The telescopic alidades have two other
important features used for plane table surveying.
These are the detachable striding level and the
45.39
Figure 11-31.-Types of alidades.
11-34
