Understanding Students' Spatial Thinking in Nias Culture-Based Geometry Learning: A Qualitative Study Integrating Van Hiele Theory and Problem-Based Learning

Background: Spatial thinking is essential for geometry learning because it enables students to interpret, represent, and reason about three-dimensional objects. However, geometry instruction often remains procedural and offers limited opportunities for students to develop meaningful spatial understanding. Although ethnomathematics, Van Hiele's theory, and Problem-Based Learning (PBL) have each been shown to improve geometry learning, little is known about how their integration supports the development of students' spatial thinking. This study explored how geometry learning integrating Nias cultural contexts, Van Hiele's theory, and PBL facilitated students' spatial thinking. Methods: This qualitative descriptive study involved 24 eighth-grade students from a junior secondary school in South Nias, Indonesia. Learning was conducted across six instructional meetings. Data were collected through classroom observations and students' responses to six spatial ability tasks. Three representative tasks and the written responses of six representative students were analysed to examine five dimensions of spatial ability. Data were analysed through data reduction, data display, and conclusion drawing. Results: Students' spatial thinking developed progressively throughout the learning process. They progressed from recognising geometric forms in Nias cultural artefacts to analysing geometric properties, constructing mathematical relationships, and communicating geometric reasoning. This progression was reflected in the spatial tasks, demonstrating the development of mental rotation, spatial visualization, spatial relations, spatial perception, and spatial orientation. Although individual differences remained, most students successfully coordinated multiple spatial representations when solving contextual geometry problems. Conclusions: Integrating Nias cultural contexts, Van Hiele's theory, and PBL established a coherent learning trajectory that supported the progressive development of students' spatial thinking. This study provides qualitative evidence of how culturally responsive geometry learning fosters multiple dimensions of spatial thinking and offers an alternative framework for designing meaningful geometry instruction.