A multi-scale DAG for cartographic objects
Sabine Timpf, Andrew U. Frank · Repository for Publications and Research Data (ETH Zurich) · 1995
Geographic Information Systems manage data with respect to spatial location and that data is presented graphically as a map or sketch.A database of objects with some geometric properties is used to render these objects graphically for different tasks.Typically these tasks require graphical presentations at different levels of detail, ranging from overview screens to detailed views [Herot et al., 1980].Practically a base map is stored and its scale changed graphically.Without major distortions, only changes to twice or half the original scale are feasible by simple numeric scale change.A function to draw cartographic sketches quickly and in arbitrary scales is needed.We propose a multi-scale hierarchical structure where renderings of spatial objects with increasing detail are stored: a directed acyclic graph (DAG).These are used to compose a topographic map at a particular scale.We assume that the object renderings for the DAG already exist.Methods to select objects for rendering are based on the importance of the object for a particular taskand on the principle of equal information density.We propose a method for determining information density, namely measuring the ink content of the map.