CS 128/ES Lecture 5b1 Vector Based Data

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 Lecture 5b2

3 Spatial data models 1.Raster 2.Vector 3.Object-oriented Spatial data formats:

CS 128/ES Lecture 5b4 Vector format  Spatial precision limited by number format  Discrete features explicitly represented  Surfaces shown by contours rather than continuous values Figure 3.9, p. 78

CS 128/ES Lecture 5b5 Layers Vector data is generally stored in layers Layers contain ONE type of entity Some layers may be raster-based Figure from previous edition, not found (by me) in our text

CS 128/ES Lecture 5b6 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 Lecture 5b7 Advantages of Vector Data “A place for everything, and everything is in its place”

CS 128/ES Lecture 5b8 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 Lecture 5b9 An Example of Annotation

CS 128/ES Lecture 5b10 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 Lecture 5b11 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 Lecture 5b12 Resolution 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) Figure 3.10, p. 79

CS 128/ES Lecture 5b13 Topology Topology is the study of shapes In GIS, it is taken to mean the information about intersections and adjacencies. Do these line segments intersect?

CS 128/ES Lecture 5b14 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 Lecture 5b15 Topological Problems Vertices don’t match Lines (do or) don’t intersect Polygons don’t close

CS 128/ES Lecture 5b16 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 Lecture 5b17 Applications of Topology  Voronoi Diagrams (also called Thiessen polygons)  Can be used to  Interpolate  Solve nearest- neighbor problems  Find “empty” regions

CS 128/ES Lecture 5b18 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 Lecture 5b19 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

Raster vs. Vector CS 128/ES Lecture 5b20 Art vs. Math? Distant vs. Up close and personal? Gluttonous vs. Efficient? Available vs. Desirable? It depends on the problem!

Historical Footnote CS 128/ES Lecture 5b21 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.)