Minimum spanning trees for valley and ridge characterization in digital elevation maps

Texture synthesis employs neighbourhood matching to generate appropriate new content. Terrain synthesis has the added constraint that new content must be geographically plausible. The profile recognition and polygon breaking algorithm (PPA) [Chang et al. 1998] provides a robust mechanism for characterizing terrain as systems of valley and ridge lines in digital elevation maps. We exploit this to create a terrain characterization metric that is robust, efficient to compute and is sensitive to terrain properties.

Terrain regions are characterized as a minimum spanning tree derived from a graph created from the sample points of the elevation map which are encoded as weights in the edges of the graph. This formulation allows us to provide a single consistent feature definition that is sensitive to the pattern of ridges and valleys in the terrain Alternative formulations of these weights provide richer characteristicmeasures and we provide examples of alternate definitions based on curvature and contour measures.

We show that the measure is robust, with a significant portion derived directly from information local to the terrain sample. Global terrain characteristics introduce the issue of over- and underconnected valley/ridge lines when working with sub-regions. This is addressed by providing two graph construction strategies, which respectively provide an upper bound on connectivity as a single spanning tree, and a lower bound as a forest of trees.

Efficient minimum spanning tree algorithms are adapted to the context of terrain data and are shown to provide substantially better performance than previous PPA implementations. In particular, these are able to characterize valley and ridge behaviour at every point even in large elevation maps, providing a measure sensitive to terrain features at all scales.

The resulting graph based formulation provides an efficient and elegant algorithm for characterizing terrain features. The measure can be calculated efficiently, is robust under changes of neighbourhood position, size and resolution and the hybrid measure is sensitive to terrain features both locally and globally.

Renewing assessment practices: literacy teaching and learning in digital environments

Interpretations of “literacy” and approaches to literacy pedagogy and assessment are under renewal as meaning-making and learning are increasingly situated in digitized environments. While the implications of these shifts are in part technological, they are also relational, as students are increasingly positioned as interactive with participatory roles in self-knowledge and increased responsibility for their learning. However, while shifts are occurring in understandings of literacy and approaches to literacy pedagogy, the same cannot be said for the way in which assessments of digital literacies are undertaken. There is a lack of valid, reliable, and practical assessments of new literacies to inform and help students to become better prepared for study, work, and citizenship in digital environments. This article maps five characteristics of effective formative assessment in print-based classrooms with seven affordancesin digital learning and assessment to suggest an analytical framework for examining teacher and student assessment in digital environments. Drawing on data from a research project in which a team of teachers introduced a one-to-one computing program and worked to renew their literacy assessment practices, this article discusses how each of the seven affordances are enacted in the assessment practices in a years five and six primary school classroom. The findings from this research project show that educational technologies have the potential to enable new approaches to teaching, learning, and assessment that better align with the needs of twenty-first century literacy learners. The findings alsosupport approaches to formative assessment that value print and multimodality and engage students in more flexible and differentiated ways. They can enable teachers and students to be re-positioned as designers, knowledge producers, and collaborative learners. The seven affordances provide a framework that holds rich possibilities for teacher learning and planning as prompts to support reflection on formative assessment practices, critique habitual practices, and considernew opportunities.