Geology Reference
In-Depth Information
indicate an anticline with two local closures and a pronounced saddle between them,
similar to the inverse-distance result (Fig. 3.10b). Decreasing the grid spacing from
10
20 (Fig. 3.11c) increases the complexity of the surface, but
only slightly. Increasing the drift from linear (Figs. 3.11b,c) to quadratic (Fig. 3.11d)
greatly increases the complexity of the surface, resulting in numerous small closures
on the bigger structure, analogous to those produced by the equal-spaced contouring
style. An oblique view (Fig. 3.12) shows that the control points may lie at significant
distances from the interpolated surface. For further discussion of working with grid-
based computer contouring, see Walters (1969), Jones et al. (1986), and Hamilton and
Jones (1992).
×
10 (Fig. 3.11b) to 20
×
3.4.4
Adjusting the Surface Shape
In order to achieve the desired result (interpretive contouring) with computer con-
touring, it may be necessary to introduce a bias in the choice of nearest neighbors or
to introduce pseudopoints. A biased choice of neighbors is used in forming a TIN to
control the grain of the final contours or to overcome a poor choice of neighbors that
results from inadequate sampling of the surface. Pseudopoints can be used to insure
Fig. 3.12.
3-D oblique view to the NE of the kriged surface in Fig. 3.11d showing that some control
points (
squares
) lie above or below surface
Fig. 3.13.
Reinterpretation of the of the triangulation network in Fig. 3.8b.
a
Revised TIN network, nodes
are numbered.
b
Linear interpolation contouring of network in
a
