Surveying Instruments - Practical Boundary Surveying - page 91

Fig. 5.19 Wiggling In—offset line
A more complicated example of wiggling in occurs when we need to estab-
lish an offset line between two monuments that are not visible from each other.
Knowing how to do this is very useful when working in urban or suburban loca-
tions where most streets have monuments at the intersections. One might think it
would be easier just to set up on one of the monuments and sight the other one,
but there are usually fences or hedges in the way which make sighting impossible.
An example of a 7 foot offset line is shown in Fig. 5.19 . It is the dashed line run-
ning between points 1 and 2. Notice that there is a monument at the intersection
of Harmony St. and Major Sevin St., shown as a small black square M1. Another
monument exists at the intersection of Harmony St. and Minor St. labeled M2.
These are the two monuments that we wish to offset.
If a surveyor was required to establish the lot corners of the parcel labeled
Locus , the 7′ offset would allow working on the sidewalk, outside of obstacles
which might exist on the street sideline. Once the offset line was established, the
surveyor could measure along the offset line to points opposite each of the lot cor-
ner and set points in the sidewalk—for example from point 1 to point 1A and 1B.
Points 1A and 1B are called Points on Line because they both lie on a straight
line—the line from 1 to 2. Both points also lie on projections of the property lines,
so once the instrument was set up on 1A or 1B it would be possible to sight down
the property line and set both the front and rear lot corners (providing there were
no obstructions on the property line). Short dashed lines are shown in Fig. 5.19
running from 1A and 1B to the property corners of the Locus.
The offset line from 1 to 2 is established by wiggling in. Wiggling in to estab-
lish an offset line is a bit more complicated than wiggling into a straight line
between two points, as in our previous example, but it is easily accomplished
nonetheless. A surveyor would proceed as follows. A tape would be used to meas-
ure out 7 feet from each of the monuments. Points would be set in the sidewalk
at the 7 foot offsets. In our example, we need to be exactly 7 feet off of the street
line, so the 7 feet must be measured perpendicular to the street line. The difficulty
lies in establishing points 1 and 2 at 90° to the street line. This is usually done by
eye or by using a small handheld right angle prism.