Vector Based Data

1. Vector Based Data CS 128/ES 228 - Lecture 5b

2. Great Rivalries in History • Lincoln vs. Douglas • “The first great Presidential Debates” • Trekkies vs. Jedis • Red Sox vs. Yankees • Vector vs. Raster CS 128/ES 228 - Lecture 5b

3. Spatial data models • Raster • Vector • Object-oriented Spatial data formats: CS 128/ES 228 - Lecture 5b

4. Vector format Figure 3.9, p. 78 • Spatial precision limited by number format • Discrete features explicitly represented • Surfaces shown by contours rather than continuous values CS 128/ES 228 - Lecture 5b

5. Layers Figure from previous edition, not found (by me) in our text • Vector data is generally stored in layers • Layers contain ONE type of entity • Some layers may be raster-based CS 128/ES 228 - Lecture 5b

6. Sources of Vector Data • Digitization of raster data • Computer analysis of raster data • Direct measurement (by GPS, formal surveying, “field work”, etc.) CS 128/ES 228 - Lecture 5b

7. Advantages of Vector Data “A place for everything, and everything is in its place” CS 128/ES 228 - Lecture 5b

8. More Specific Advantages of Vector Data • Each “item” corresponds to a real-world feature • Items can be “annotated” with other (non-spatial) data • Items can be selected (or hidden) CS 128/ES 228 - Lecture 5b

9. An Example of Annotation CS 128/ES 228 - Lecture 5b

10. Storage – Rasters are (inherently) inefficient • Every pixel must be described • A 300x300 image (using 24-bit color) takes up 270,000 bytes CS 128/ES 228 - Lecture 5b

11. Storage – Vectors are more “storage appropriate” • Only “items” are described, e.g. “filled yellow circle, (100,100,40)” • This image would require less than 50 bytes! CS 128/ES 228 - Lecture 5b

12. Resolution Figure 3.10, p. 79 • Rasters are limited by the size of the raster (the pixel) • Vectors are limited by the number of points (along a line or polygon body) CS 128/ES 228 - Lecture 5b

13. Do these line segments intersect? Topology • Topology is the study of shapes • In GIS, it is taken to mean the information about intersections and adjacencies. CS 128/ES 228 - Lecture 5b

14. Maintaining Topology • …is a difficult problem from a “technical” point of view • Topology must be established at the time of input and maintained as the data is edited • Shapefiles contain NO topological information CS 128/ES 228 - Lecture 5b

15. Topological Problems • Vertices don’t match • Lines (do or) don’t intersect • Polygons don’t close CS 128/ES 228 - Lecture 5b

16. Fixing Topology is a “snap” • When two entities (point or line) are within a specified tolerance, we can “snap” them to the same point. • Tolerance is determined on the screen, not directly by real-world distance “Snap!” “you drive a Chevy” CS 128/ES 228 - Lecture 5b

17. Applications of Topology • Voronoi Diagrams (also called Thiessen polygons) • Can be used to • Interpolate • Solve nearest-neighbor problems • Find “empty” regions CS 128/ES 228 - Lecture 5b

18. Summary • Vector format allows one-to-one matching between real-world objects and data items. • Vector format allows maintenance of topological information CS 128/ES 228 - Lecture 5b

19. Summary, continued • Vector format supports inclusion of attribute data • Vector format tends to require less storage • Vector format makes certain forms of queries MUCH easier CS 128/ES 228 - Lecture 5b

20. Raster vs. Vector • Art vs. Math? • Distant vs. Up close and personal? • Gluttonous vs. Efficient? • Available vs. Desirable? It depends on the problem! CS 128/ES 228 - Lecture 5b

21. Historical Footnote • Lincoln/Douglas was NOT the first great Presidential debate • Lincoln and Douglas did partake in epic debates, but only while running for a Senate seat from Illinois • (That said, many folks make the mistake on slide 2.) CS 128/ES 228 - Lecture 5b