Geoscience Reference
In-Depth Information
Appendix A10: Mapping
Determination of true and apparent dip
When projecting structural data onto a cross-section, the true
dip of strata can be drawn on the section only for data with
strikes at 90° to the line of section. If the strikes of data are at a
lower angle, an apparent dip less than the true dip must be
determined. The trigonometric solution to this problem is often
very convenient (for values of
β
< 60°):
tan
α
=
tan
δ
sin
β
where
α
is the apparent dip (in the line of section),
δ
is the
true dip and
is the angle between the line of strike and the
line of interest (section).
β
The formula to calculate
α
in Microsoft Excel has the form:
=DEGREES(ATAN(TAN(RADIANS(B2))*SIN(RADIANS(B3))))
where B2 =
δ
and B3 =
β
.
Alternatively, you can use a nomogram (Figure A10.1), a
graphical approach to determining one of the three angles
above (
α
,
δ
,
β
) given the other two.
