Glossary

The terms used in Cloud Structural Analysis and this manual, as the app uses them. “Reading a stereonet” explains how they fit together.

Alpha (α): For oriented core: the angle between the core axis and the structure’s plane, 0 to 90 degrees. How it is turned into a plane depends on the convention set in the app.
Aki-Richards: The standard way of finding a line from a plane and a rake. The app uses it for lines lying on a plane, such as slickensides (strike, dip and rake).
Azimuth: A compass direction, 0 to 360 degrees clockwise from north. Dip direction, trend and a hole’s azimuth are all azimuths.
Beta (β): For oriented core: the angle round the core, measured from the reference line (the bottom or the top of the core, set in the app) to the apex of the structure’s ellipse (the end furthest from, or nearest, the collar: also set in the app). It is clockwise looking down-hole unless you change it.
Bingham ellipse: An optional confidence ellipse drawn round the mean pole (for a cluster) or the fold axis (for a girdle). Unlike the circular Fisher cone it can be longer one way than the other, so it follows how the readings actually spread. Tick it under Statistics on the net (Advanced); it takes about 40 seconds.
Cluster: Poles that bunch together: one consistent orientation. The mean and its cone describe it.
Collar: The point where a drillhole starts: its east, north and elevation, and optionally its total depth. A collar table lets the holes be drawn in 3D.
Cone (95 % cone): The circle on the stereonet inside which the true mean direction lies with 95 % confidence, from Fisher statistics. A small cone means a well-determined mean.
Contours: Shading that shows where poles are densest. Kamb contours are in sigma above a uniform scatter; Schmidt and Fisher-kernel contours are in multiples of a uniform density.
Convention (alpha / beta): The rules that turn alpha and beta into a plane: where beta is measured from, which way it increases, whether alpha is measured to the plane or its pole, and which end of the ellipse is used. They are not standard, so the app checks them against the site’s own results.
Declination (magnetic): The angle between magnetic and true north, east positive. If you enter one, it is added to every azimuth after the quality checks.
Dip: How steeply a plane slopes, 0 degrees (flat) to 90 degrees (vertical), measured down from horizontal.
Dip direction: The compass direction the plane slopes down towards (an azimuth). With the right-hand rule it is the strike plus 90 degrees.
Domain: A structural domain, block or area, from the domain column of the file. A filter lets you analyse one domain at a time, because domains often have different patterns.
Equal angle (Wulff): A stereonet projection that keeps angles true, so it suits measuring and constructions.
Equal area (Schmidt): A stereonet projection that keeps the density of points true, so the app uses it for contours and statistics.
Fisher kappa (κ): How tightly poles cluster round their mean: the larger, the tighter. Above 10 the mean is reliable; for a girdle, kappa is not meaningful.
Fold axis: The line a fold is hinged on. For a girdle, the app takes it as the pole to the best-fit great circle. For unfolding it can be fitted to the readings or typed in.
Fracture frequency: The number of fractures per metre of hole, counted in windows along the hole from the depths of the readings.
Girdle: Poles spread along a great circle: folded layers. A single mean would be meaningless, so the app gives the best-fit circle and fold axis instead.
Great circle: The trace of a plane on the sphere, drawn on the stereonet as a curve. Easier to picture than a pole, but poles are used for counting and statistics.
Hemisphere (lower / upper): Which half of the sphere is projected. Geologists use the lower hemisphere; the upper is offered for other fields.
Hole orientation: The azimuth (trend) and dip (plunge) of the drillhole at the depth of a reading. It comes from a survey, from columns in the file, or from one value typed in for the whole file.
K and C (Woodcock): Woodcock’s shape and strength, from the eigenvalues of the orientation matrix. K above 1 is more cluster-like, below 1 more girdle-like; C is the strength of the fabric.
Kamb contours: Density contours in standard deviations (sigma) above what a uniform scatter would give. 2 sigma and above is conventionally significant. The app’s version is checked against Vollmer (1995).
Line: A linear feature measured by plunge and trend, such as a lineation, fold axis or slickenline.
Minimum curvature: The standard way of calculating a drillhole’s path between survey stations. The app uses it to place the hole and its structures in 3D.
Orientation set: A group of structures with similar orientation, such as a joint set. The finder fits a mixture model and chooses the number of sets with the BIC, allowing a background of scatter.
Pattern: What the readings form on the net: a cluster, a girdle, or no clear pattern. The app tells which from the eigenvalues of the orientation matrix.
Plane: A planar feature measured by dip and dip direction (or strike and dip), such as bedding, a joint or a fault.
Plunge: How steeply a line slopes down, 0 to 90 degrees below horizontal.
Pole: The line at right angles to a plane, plotted as a point. A pole to a plane dips 90 degrees less than the plane and points the opposite way.
Quality checks: The checks run when a file is read: values out of range, missing values, text that cannot be read and repeated readings. A row is excluded or only flagged, and the Data tab says which and why.
Rake (pitch): The angle of a line within a plane, measured in the plane from the strike direction (90 is straight down the dip).
RQD (rock quality designation): The percentage of each hole made up of core pieces 10 cm or longer between breaks, worked out here from the depths of the fractures.
Rose diagram: A circular histogram of directions. A plane has two ends, so strikes are drawn both ways. Planes can show strike or dip direction; lines show trend.
Sampling bias: A drillhole meets planes across it far more often than planes lying along it, so steep structures are under-counted in a vertical hole. See Terzaghi weighting.
Strike: The compass direction of the horizontal line in a plane. With the right-hand rule (the default) the plane dips to the right of the strike, so the dip direction is strike plus 90 degrees. The left-hand rule can be chosen.
Survey: A table of hole, depth, azimuth and dip along a drillhole. Between stations the app interpolates the hole’s orientation as the acQuire procedure does.
Terzaghi weighting: An optional correction for sampling bias. Each reading is weighted by 1 / |cos| of the angle between its pole and the hole, capped at a limit you can set, so the mean, kappa, cone and contours are less biased. It does not apply to lines or to the orientation-set finder.
Trend: The compass direction (an azimuth) a line plunges towards.
Unfolding: Turning every plane about the fold axis, each by its own angle, until the limbs lie flat. A cylindrical fold collapses to one point; what stays scattered is the departure from a cylinder.