Surveying Instruments - Practical Boundary Surveying - page 90

Fig. 5.17 Wiggling In
Fig. 5.18 Wiggling In—pick a point
on a line between the two points. As there is no frame of reference, it is very diffi-
cult for most people standing between two points to determine whether or not they
are close to being on line.
In spite of these limits, we will do our best to guess at the location of the inter-
mediate point. An example of how this might look is shown in the overhead view
in Fig. 5.18 . The line between point 1 and point 2 is the line that we need to estab-
lish. It is a straight line between the two monuments.
The point Guess 1 has been selected as a trial location for the instrument. The
instrument is set up on point Guess 1 and point 1 is sighted. We always want to
sight the long line rather than the short line because projecting a line from a long
line to a short line will not magnify an error as much. The instrument is then
turned to an angle of 180°. A point is then set near point 2. In our example we will
call this point “A”.
Next, we measure the distance between A and point 2. We can see from
Fig. 5.18 that our point is 0.84 feet away from point 2. If our instrument were
halfway between points 1 and 2 it would be a simple matter to move the instru-
ment 0.42 feet—half the error. The next time we turned 180° we should be right
on point 2. In our example, we are about 2/3 (0.67) of the way between 1 and 2 so
we would want to move the instrument about 0.56 feet over (0.67 * 0.84 = 0.56).
After we moved the instrument over, we would sight point 1 again and turn 180°.
If we were still a little bit off point 2, we would measure the difference again and
readjust the instrument location. We would keep “wiggling in” as many times as
necessary until the instrument was exactly on line. In this example, we turned
180°, however we could have also double centered point A and used the averaged
result of the erect and inverted shots to establish point A.