OR/13/001 Glossary

DTM	Digital Terrain Model — Model of surface of the solid Earth (generally the boundary between geosphere and atmosphere or hydrosphere). This is traditionally derived from OS contours and spot heights and should therefore exclude all buildings, trees, hedges, crops, animals etc. Sometimes also referred to as ‘bald earth’ models.
Envelope	Defined here as the extent of a geological deposit in plan view (2D): forming a distribution map of the particular unit, a presence — absence map.
GSI3D	Geological Surveying and Investigation in 3D.