Strike and dip

Strike and dip are measured using a compass and a clinometer. A compass is used to measure the feature's strike by holding the compass horizontally against the feature. A clinometer measures the feature's dip by recording the inclination perpendicular to the strike. These can be done separately, or together using a tool such as a Brunton transit or a Silva compass.

Any planar feature can be described by strike and dip, including sedimentary bedding, fractures, faults, joints, cuestas, igneous dikes and sills, metamorphic foliation and fabric, etc. Observations about a structure's orientation can lead to inferences about certain parts of an area's history, such as movement, deformation, or tectonic activity.

Elements

When measuring or describing the attitude of an inclined feature, two quantities are needed. The angle the slope descends, or dip, and the direction of descent, which can be represented by strike or dip direction.

Dip

Dip is the inclination of a given feature, and is measured from the steepest angle of descent of a tilted bed or feature relative to a horizontal plane. True dip is always perpendicular to the strike. It is written as a number (between 0° and 90°) indicating the angle in degrees below horizontal. It can be accompanied with the rough direction of dip (N, SE, etc) to avoid ambiguity. The direction can sometimes be omitted, as long as the convention used (such as right-hand rule) is known.

A feature that is completely flat will have the same dip value over the entire surface. The dip of a curved feature, such as an anticline or syncline, will change at different points along the feature and be flat on any fold axis.

Strike

Strike is a representation of the orientation of a tilted feature. The strike line of a bed, fault, or other planar feature, is a line representing the intersection of that feature with a horizontal plane. The strike of the feature is the azimuth (compass direction) of the strike line. This can be represented by either a quadrant compass bearing (such as N25°E), or as a single three-digit number in terms of the angle from true north (for example, N25°E would simply become 025 or 025°).

A feature's orientation can also be represented by its dip direction. Rather than the azimuth of a horizontal line on the plane, the azimuth of the steepest line on the plane is used. The direction of dip can be visualized as the direction water would flow if poured onto a plane.

Apparent dip

While true dip is measured perpendicular to the strike, apparent dip refers to an observed dip which is not perpendicular to the strike line. This can be seen in outcroppings or cross-sections which do not run parallel to the dip direction. Apparent dip is always shallower than the true dip. If the strike is known, the apparent dip or true dip can be calculated using trigonometry:

${\displaystyle \alpha =\arctan (\sin \beta \times \tan \delta )}$
${\displaystyle \delta =\arctan (\tan \alpha ÷\sin \beta )}$

where δ is the true dip, α is the apparent dip, and β is the angle between the strike direction and the apparent dip direction, all in degrees.

Trend and plunge

The measurement of a linear feature's orientation is similar to strike and dip, though the terminology differs because "strike" and "dip" are reserved for planes. Linear features use trend and plunge instead. Plunge, or angle of plunge, is the inclination of the feature measured downward relative to horizontal. Trend is the feature's azimuth, measured in the direction of plunge. A horizontal line would have a plunge of 0°, and a vertical line would have a plunge of 90°. A linear feature which lies within a plane can also be measured by its rake (or pitch). Unlike plunge, which is the feature's azimuth, the rake is the angle measured within the plane from the strike line.