CS 128/ES Lecture 5b1 Vector Based Data

CS 128/ES Lecture 5b2 Spatial data models 1.Raster 2.Vector 3.Object-oriented Spatial data formats:

CS 128/ES Lecture 5b3 Vector format  Spatial precision limited by number format  Discrete features explicitly represented  Surfaces shown by contours rather than continuous values

CS 128/ES Lecture 5b4 Layers Vector data is generally stored in layers Layers contain ONE type of entity Some layers may be raster-based

CS 128/ES Lecture 5b5 Sources of Vector Data  Digitization of raster data  Computer analysis of raster data  Direct measurement (by GPS?)

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

CS 128/ES Lecture 5b7 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 5b8 An Example of Annotation

CS 128/ES Lecture 5b9 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 5b10 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 5b11 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)

CS 128/ES Lecture 5b12 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 5b13 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 5b14 Topological Problems Vertices don’t match Lines do (or don’t) intersect Polygons don’t close

CS 128/ES Lecture 5b15 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

CS 128/ES Lecture 5b16 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 5b17 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 5b18 Summary, continued Vector format supports inclusion of attribute data Vector format tends to require less storage space Vector format makes certain forms of queries MUCH easier