convenient
shore base
to triangulate offshore distances from a
line. No matter how you get offshore
distances, however, offshore points cannot be marked
like ground points with hubs or stakes. Therefore, in
the location of offshore points, there must usually be
coordination between a survey party on the beach and
a party afloat.
OFFSHORE LOCATION BY CHAINING
Figure 10-28 shows a situation in which offshore
locations of piles for a wharf were determined by
chaining. We will call each series of consecutive piles
running offshore a line and each series running
parallel to the shore a row. Alignment for each line
was obtained by transit—set up on a shore base line
offset from the inboard row of piles. In each line the
distance from one pile to the next was chained, as
shown.
In figure 10-28 the lines are perpendicular to the
base line, which means that the angle turned from the
base line was 90° and the distance from one transit
setup to the next was the same as the prescribed
distance between lines. If the lines were not
perpendicular to the base line, both the angle turned
from the base line, the distance from one transit setup
to the next, and the distance from the base line to the
first offshore pile in each line would have to be
determined.
Consider figure 10-29, for example. Here the
angle between each line and the base line (either as
prescribed or as measured by protractor on a plan) is
60°40´. You can determine the distance between
transit setups by solving the triangle JAB for AB, JA
being drawn from transit setup B perpendicular to the
Figure 10-29.—Offshore location in line oblique to the base
line.
Figure 10-28.—Offshore location by chaining
10-27
