Lecture 09: Data Representation (VII) Topics 3. Data Models 3.3 Spatial data models 3.3.3 Triangular Irregular Network (TIN) 3.3.4 Summary of Spatial Data Models Readings on the topics Chapter 8 in the text, pp. 189-190 Chapter 3 in Bonham-Carter’s (1994), pp. 78-81 Other readings Chapter 6 in Aronoff’s (1993), pp. 177-180

Outlines 3. 3 Spatial Data Models: 3.3.3 Triangular Irregular Network (TIN): 3.3.3.1 What is a TIN Triangle is the basic unit of representing the world, each triangle is defined by three points and its surface. (The TIN Figure) The area covered by a triangle is said to be in uniformity in its changes. Each of the three edges represents the change of this uniformity. Mostly used for terrain representation

3.3.3.2 Representation of TIN Consists of two tables: Triangle Node Table The Node Attribute Table (The TIN representation Figure) 3.3.3.3 Creation of TIN 3.3.3.3.1 How to pick points break points and break lines (The Hard Line and Soft Line Figure) 3.3.3.3.2 How to connect points into triangles Equal lateral triangles are preferred One approach is the Delaunay Triangulation (Also called as the Thiessen Polygon approach) (The Delaunay Triangulation Figure)

3.3.3.3 Creation of TIN (Continued…) 3.3.3.3.4 How to model the surface a) using a plane surface (linear) (The Dane Elevation Model with TIN Imprints Figure) b) using curved surface 3.3.3.4 Evaluation of TIN Representation 3.3.3.4.1 Pros: a) compact, no redundancy b) facilitate the computation of surface attributes 3.3.3.4.2 Cons: a) many possible triangulations from a same set of points b) slow to create one c) difficult for spatial overlay operation (the compact nature and difficulty for spatial overlay make it a data model for terrain data storage)

3.3.4 Summary of Spatial Data Model: 3.3.4.1 Comparison (The comparison table) 3.3.4.2 Converting between data models (1) Data quality – always lower precision spatial variability (2) Topology 3.3.4.3 Choosing a spatial data model (1) spatial variability (2) topological requirements (3) the nature of the project (budget, accuracy)

Questions: 1. Given a TIN diagram, please construct the Triangle-Node table. Where would one store the elevation data and the x-y coordinates for each node? 2. Compare and contrast the representation of digital elevation data using a raster data model and that using a TIN model 3. Compare and contrast the three basic spatial data models (raster, vector and TIN).