We present software tools and methods applicable to the geological modelling of sparse spatial and structural data within a 3-D digital environment. Free-form surfaces derived from section-style control frames and constrained by field-based structural measurements are employed as partially automated design aids intended to speed up and streamline the 3-D geological model building process. Some design degrees of freedom such as NURBS tension (or weights), knot sequencing and tying surface features are also discussed with examples drawn from spatial and structural data collected in Baffin Island by the Geological Survey of Canada and near-mine exploration data from Canadian mines. Interpolation of field-based structural measurements along the boundary of an unknown surface is also demonstrated. This work is potentially relevant to regional mappers and others dealing with sparse spatial and structural data, and/or conceptual surface modelling.