Strike and dip are the two angular measurements that fully describe the orientation of any planar geological surface — bedding, foliation, joints, faults, or veins — in three-dimensional space. Strike is the compass bearing of a horizontal line on the plane (the intersection of the plane with a horizontal surface). Dip is the angle that the plane descends from horizontal, measured perpendicular to strike, together with the general direction it dips toward.
Why it matters
These measurements are the language of structural geology. They let a geologist project surface observations into the subsurface, build cross-sections, interpret folds and faults, and assess slope stability where bedding or jointing controls failure. A bedding plane recorded as strike 045°, dip 30° SE trends northeast–southwest and tilts 30° down toward the southeast.
Representations and conventions
Two notations are common. Quadrant notation gives strike relative to north/south (e.g. N45°E) with dip amount and quadrant (30°SE). Azimuth notation uses a 0–360° bearing. The widely used right-hand rule removes ambiguity: when you walk along the strike azimuth, the plane dips to your right, so dip direction is always 90° clockwise from the recorded strike. Map symbols show a strike line with a short tick mark on the dip side and the dip angle in degrees.
In GIS
Strike and dip are stored as point features with numeric attributes (strike azimuth, dip angle, optional dip direction). They are usually symbolized with a rotated strike-dip marker whose rotation is driven by the strike field, and labeled with the dip value. Storing dip direction explicitly avoids right-hand-rule confusion when data come from mixed sources.
Common pitfall
Mixing right-hand-rule strike with quadrant data, or rotating symbols by dip direction instead of strike, produces map symbols pointing the wrong way. Always confirm whether a dataset's strike follows the right-hand rule and whether dip direction is stored separately.