7. • All lines that were parallel to the original
horizontal and vertical axes remain parallel to
the corresponding new axes, and the scale
along these lines also remains unchanged.
However, lines which were not parallel to the
original axes will be distorted. Points along
such lines must be transferred onto the
projection by coordinates relative to the new
axes. A grid system is helpful in this process.
10. BBlloocckk DDiiaaggrraammss
• Along with the fence diagram, another type of
diagram illustrating three- dimensional geological
relationships is a block diagram (Figure 5).
• This type of figure is a two-dimensional
representation of a rectangular block. Two
intersecting cross sections often form the sides of the
block; the top of the block shows either a mapped
surface or relief on the uppermost geological
boundary shown in the cross sections. The block may
be rotated to achieve the most effective viewing
angle, and it may be drawn with or without the use of
perspective.
12. BBlloocckk DDiiaaggrraammss
• Isometric block diagrams do not employ perspective.
opposite sides of the block remain parallel, making the
distant end of the diagram appear to be larger (Figure
6 , Figure 7, and Figure 8).
The top of this type of block diagram may be given
any degree of tilt toward the viewer.
16. • A large amount of tilt emphasizes features on
the top surface of the block; a smaller degree of
tilt brings out details on the sides of the block.
Scales along the x, y, and z axes need not be
equal.
• The scale along the sides of the figure (y) is
sometimes chosen to be smaller than the scale
across the front and back edges (x), thus
creating an illusion of perspective.
• Perspective can be used in constructing block
diagrams (Figure 9).
18. In perspective blocks, all parallel lines with a
component in the y direction converge to a vanishing
point on the horizon.
Sets of parallel lines with different orientations
converge to different vanishing points.
The cross section on the front face of the block is
usually left undistorted by perspective. It is possible
both to rotate the block about the z axis or to display
it at different elevations relative to the horizon. Again,
the choice of block orientation is made to emphasize
the important features of the diagram.