Geology Reference
In-Depth Information
Fig. 6.11.
Direct projection of data from
structure contour map to cross
section.
Dotted lines
are right-
angle projection lines.
Circles
show projected points:
filled
circles
are projected from
known elevations; the
open
circle
is an interpolated eleva-
tion. The drafting tools are
shaded
interest. For many purposes, it is most convenient to construct a vertical profile. For
a cylindrical structure, a vertical profile is easily transformed into a normal section
(Sect. 6.5). The techniques of section construction will begin with vertical profiles.
Construction of an initially tilted profile is considered in Sect. 6.6.1.
The next step in constructing the cross section is to transfer the data from the map
to the profile. One convenient projection method is to align the cross-section graph
parallel to the line of section, tape the map and section together so that they cannot
slip, and project data points at right angles onto the cross section with a straight edge
and a right triangle (Fig. 6.11). Any type of map information can be transferred to the
cross section by this method. In computer drafting it is usually more convenient to
draw straight lines vertically or horizontally; therefore the map should be rotated so
that the projection direction is either horizontal or vertical. Data points are located at
their correct distances from the ends of the section and at their proper elevations. The
probable locations of turning points of the structure contour map are also marked as
points (Fig. 6.11), for example, between the two adjacent 600 contours. The location of
a turning point is constrained to be between the next higher and lower elevations.
An alternative method is to mark the locations of the data points on a strip of paper
for working by hand (Fig. 6.12a) or on a line drawn on top of the section line in a
drafting program. After marking, the line of data locations is rotated to be parallel to
the cross-section horizontal (Fig. 6.12b). In a drafting program, group the points be-
fore rotating. Then project the data points from the line onto the cross section
(Fig. 6.12c). The points can be projected with a right triangle as in the previous method,
or the overlay can be moved to the correct elevation on the section and each point
marked at the appropriate distance from the end of the section. After compiling the
