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Fig. 2.16.
Equal-area, lower-hemisphere
stereogram. Measurement
error of 4° (radius of circles)
around true bedding poles
(points a and b )
the maximum expected for routine field measurements on a normally smooth bed
surface. Measurements on a rough surface may show an even greater variability. A
good average attitude from a rough surface can be obtained by making several mea-
surements and then separately averaging the strikes and dips or the trends and plunges
of the dips. A good field measurement procedure is to lie a flat field notebook or square
of rigid plastic on a rough bed surface to average out the irregularities.
The effect of the error is related to the attitude of the plane. If the bed is horizontal,
the pole is vertical (Fig. 2.16, point a) and the error means that the azimuth of the dip
could be in any direction, even though the true three-dimensional orientation of the plane
is rather well constrained. Small irregularities on a bed surface have the same effect
(Woodcock 1976; Ragan 1985). On a steeply dipping plane (Fig. 2.16, point b), the same
amount of error causes little variation in either the azimuth or the dip. Conversely, the
measurement of the trend of a line on a gently dipping surface is accurate to within a
few degrees, but the direction measured on a steeply dipping plane may show signifi-
cantly greater error (Woodcock 1976). A precision of about 2° is about normal for
calculations done using a stereogram. For greater precision, the calculations should be
done analytically by methods that will be presented later in the chapter.
2.3.3
Tangent Diagram
The other useful diagram for representing the attitudes of planes is the tangent dia-
gram (Fig. 2.17; an enlarged copy is given at the end of the chapter as Fig. 2.29). De-
veloped by Hubbert (1931), it has been popularized by Bengtson (1980, 1981a,b) in
the context of dipmeter interpretation. It is particularly valuable in the determina-
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