A borehole is a narrow, usually vertical or inclined shaft drilled into the ground to investigate the subsurface. It is used to recover core or cuttings, run downhole geophysical logs, measure groundwater, or install monitoring and extraction equipment.
Why it matters
Boreholes are the primary source of direct, depth-controlled subsurface data. Where surface maps and remote sensing only see the top, a borehole records the actual sequence of lithologies, contacts, fault zones, water levels, and sample assays at known depths. In GIS, boreholes anchor 3D geological models, cross-sections, and resource estimates to real ground truth, so the quality of their location and depth data drives the reliability of everything built on top.
A concrete example
A borehole is typically stored as a point with a collar (the surface location and elevation, e.g. X/Y in EPSG:32631 and Z in meters above a vertical datum) plus related tables: a survey table (downhole depth, azimuth, dip for deviated holes), a lithology/interval table (from-depth, to-depth, unit), and assay or geotechnical tables. Software such as QGIS plugins, Leapfrog, or a PostGIS schema joins these on a borehole ID to reconstruct the hole in 3D.
Common pitfall
Treating a borehole as a single 2D point loses its depth dimension; intervals must keep from/to depths and, for deviated holes, a survey to compute true 3D positions. Another frequent error is mixing vertical datums or units between collar elevations and the surrounding DEM, which makes boreholes float above or sink below the modeled surface.