geology · structural
Stereonet (Equal-Angle / Equal-Area Projection)
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Inputs
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Paste one row per line — separate columns with a comma or tab: Strike (°), Dip (°), Group (Set 1/Set 2/Set 3/Set 4/Set 5)
⚠ Needs at least 1 valid row to calculate — 0 so far.
Paste one row per line — separate columns with a comma or tab: Trend (°), Plunge (°), Group (Set 1/Set 2/Set 3/Set 4/Set 5)
⚠ Needs at least 1 valid row to calculate — 0 so far.
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Description
Plots planar (strike/dip) and linear (trend/plunge) structural measurements on a lower-hemisphere stereographic projection — the standard graphical tool for analyzing the 3D orientation of bedding, faults, joints, foliations, fold axes, and lineations. Both the Equal-Angle (Wulff) and Equal-Area (Schmidt) projections are supported, planes can be shown as poles (points), full great-circle (cyclographic) traces, or both at once, and Kamb-method density contours can be overlaid on either population.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Poles | — | Plane Poles & Great Circles |
| Lines | — | Line Points |
| Ngrp | — | Distinct Groups Pasted |
| Nshown | — | Groups Plotted (capped at 4) |
| Fisher | — | Fisher Mean Direction & Confidence Cone |
| Contours | — | Kamb Density Contours |
| Proj | — | 0 = Equal-Angle (Wulff/stereographic) projection — preserves angular relationships, the traditional choice for structural-geology stereonets. 1 = Equal-Area (Schmidt/Lambert azimuthal) projection — preserves areas, the standard choice when plotting many measurements to assess clustering/density without a systematic areal bias toward the net's center. This affects the actual projected coordinates, not just how they're displayed. |
| Poles | — | 1 = plot each plane's pole (a single point). Independent of Show Great Circles — both, either, or neither may be shown for the same set of planes. |
| GC | — | 1 = plot each plane's full great-circle (cyclographic) trace. Independent of Show Plane Poles — both representations are always computed regardless of which are currently shown, so toggling either is instant. |
| θ/δ | — | One row per planar measurement (bedding, fault, joint, foliation): strike and dip in degrees, right-hand-rule convention (dip is 90° clockwise of strike, looking in the dip direction), and an optional group label (e.g. a station or structural-domain name) for overlaying several datasets on one net with separate, color-coded pole/great-circle populations. Leave the group cell blank, or leave off the whole third column, for a group of "Set 1" (today's default single-population behavior). This platform's array-input schema has no optional-input mechanism for a whole INPUT (only for a trailing column within one), so at least one row is still required here even for a dataset with only line measurements — see this calc's own limitations. The Planes and Lines inputs share ONE combined pool of at most 4 simultaneously overlaid groups (this net's legible non-red, non-cone color limit — see this calc's own limitations) — a plane group and a line group with the same name are the SAME overlay group and share one color. |
| T/p | — | One row per linear measurement (fold axis, lineation, slickenline, borehole axis): trend and plunge in degrees, lower hemisphere (plunge 0-90, always downward), and an optional group label — see the Planes input's own description for the shared group-pool/color convention. At least one row is required even for a dataset with only planar measurements — see this calc's own limitations. |
| Poles Fisher | — | 1 = compute and plot a Fisher mean direction, concentration (kappa), and confidence cone for the Planes input's poles, treated as a single homogeneous population. Independent of Show Lines Fisher Mean/Cone — planes and lines are always computed as two separate Fisher populations, never mixed together into one statistic (tier2-session3-confidence-cones scoping). |
| Lines Fisher | — | 1 = compute and plot a Fisher mean direction, concentration (kappa), and confidence cone for the Lines input, treated as a single homogeneous population, independent of the Planes population's own Fisher statistic. |
| Conf | % | Confidence level for the Fisher confidence cone (e.g. 95% is the conventional default). Applies to both the Poles and Lines Fisher populations. The cone is only meaningful/computable for a population whose R-bar (mean resultant length) is >=0.65 and whose sample size supports this confidence level's cone formula — see this calc's own limitations for both guards. |
| Poles Kamb | — | 1 = plot Kamb density contours for the Planes' poles (same population scoping as the Fisher mean/cone). Disabled while more than one group is overlaid. |
| Lines Kamb | — | 1 = plot Kamb density contours for the Lines, independent of the Planes population's own contours. |
| σ | — | Kamb counting-circle size in standard deviations (1, 2, or 3; rounded to the nearest). Higher = more smoothing. The only contouring parameter exposed. |
Assumptions
- Each pasted strike/dip measurement uses the right-hand-rule convention (dip is 90° clockwise of strike, looking in the dip direction) — a quadrant-notation (e.g. N45E) or dip-direction/dip convention measurement must be converted to right-hand-rule strike before entry.
- All measurements are lower-hemisphere by construction (dip and plunge are entered 0-90°, always downward) — there is no upper-hemisphere mode, matching universal structural-geology convention.
- The Equal-Area (Schmidt) projection is appropriate when comparing the relative density/clustering of many measurements; the Equal-Angle (Wulff) projection is appropriate when angular relationships between individual planes/lines (e.g. measuring the angle between two lineations) must be read directly off the net.
Limitations
- This platform's array-input schema has no optional-input mechanism (the same gap rose_diagram's own limitations document), so both the Planes and Lines array inputs require at least one row — a dataset with only planes (or only lines) still needs a placeholder row in the other input; deferred cleanly rather than built as a half-finished optional-field hack.
- A Fisher mean direction, concentration (kappa), and confidence cone are computed per population (Planes' poles, Lines) but require at least 3 measurements in that population, a well-defined mean direction (R̄>=0.10), and — for the cone specifically — R̄>=0.65 plus a sample size that keeps the cone formula's own domain valid at the chosen confidence level; a population failing any of these guards shows a clear note instead of a silently wrong or missing result (tier2-session3b-confidence-cones-build). This is Fisher (point/pole clustering) statistics only — Bingham/girdle-distribution statistics (fitting a best-fit plane or cylindrical axis to a scattered set of poles/lines, e.g. finding a fold axis from folded-bedding poles) is a materially different statistical method and remains a genuinely separate, not-yet-built future extension, not silently folded into this feature.
- The background graticule (10° meridian/parallel spacing, degree tick marks every 10°/30° on the primitive circle) is a fixed-spacing reference grid, not a user-adjustable one — 10° was chosen as the legible spacing at this panel's ~260px net radius (2°, the traditional paper-net spacing, was tested and found illegibly dense at this size; 20° was tested and found comfortably legible but sparser). The grid is drawn at half opacity behind the crosshair and the user's own plotted planes/lines so it never competes with the actual data.
- Multi-group overlay (tier2-session4-multidataset-overlay) is capped at 4 simultaneously plotted groups (Planes and Lines share one combined pool) — a dataset naming more groups than that still computes group_count correctly, but only the first 4 (in order of first appearance) are plotted, with a flag identifying which groups were dropped. Confidence cones, density contours, and multi-group overlay are MUTUALLY EXCLUSIVE by design, not merely a display choice: whenever more than one group is present, fisher_stats and contour_stats both come back empty regardless of their toggles, since both describe only a single combined population — showing one net-wide cone or contour field next to group-colored data would misrepresent something never computed per group. Per-group Fisher/contour statistics remain a separate, not-yet-built future extension (same boundary as the Bingham/girdle-distribution gap above).
- Kamb density contours (one population at a time, 3+ measurements needed) use only exponential-smoothing weighting (Vollmer's own default) with σ (1/2/3, default 3) as the sole exposed parameter — grid resolution and the fixed [1σ,2σ,3σ] levels aren't adjustable, and every measurement is an unsigned axis, never a directed vector. A contour level is standard deviations by which local density departs from uniform, NOT a confidence percentage ('3σ' ≠ '99.7% confidence') — per mplstereonet's density_grid() docs, an open reimplementation of Vollmer 1995. Higher σ smooths more (same source), which can blur distinct tight clusters together.
Use Cases
- → Fault/fracture orientation and kinematic analysis: Plot fault-plane strike/dip measurements as great circles alongside slickenline trend/plunge measurements as lines to visualize the geometric relationship between fault surfaces and their associated slip lineations — a standard structural-geology field and lab technique.
- → Bedding/foliation orientation mapping: Plot bedding or metamorphic foliation strike/dip poles from multiple outcrops as points to visually assess fold geometry, structural domains, or a regional dip pattern — clustering of poles reveals the fold axis or dominant structural trend.
Related Calculations
Region Notes
Global
Always state which projection (Equal-Angle vs. Equal-Area) and hemisphere convention (lower, used exclusively here) were used alongside a stereonet plot — the two projections place the same measurement at visibly different radii for any plunge strictly between 0° and 90°, which can matter when comparing a plot against a hand-plotted net from older field notes or a different software package.
References
Primary source
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