Vector Based Data CS 128ES 228 Lecture 5

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

Spatial data models Spatial data formats: 1. Raster 2. Vector 3. Objectoriented CS 128/ES 228 - Lecture 5 b 2

Vector format § Spatial precision limited by number format § Discrete features explicitly represented § Surfaces shown by contours rather than continuous values CS 128/ES 228 - Lecture 5 b 3

Layers n Vector data is generally stored in layers n Layers contain ONE type of entity n Some layers may be raster-based CS 128/ES 228 - Lecture 5 b 4

Sources of Vector Data § Digitization of raster data § Computer analysis of raster data § Direct measurement (by GPS? ) CS 128/ES 228 - Lecture 5 b 5

Advantages of Raster Data “A place for everything, and everything is in its place” CS 128/ES 228 - Lecture 5 b 6

More Specific Advantages of Vector Data n Each “item” corresponds to a realworld feature n Items can be “annotated” with other (non-spatial) data n Items can be selected (or hidden) CS 128/ES 228 - Lecture 5 b 7

An Example of Annotation CS 128/ES 228 - Lecture 5 b 8

Storage – Rasters are (inherently) inefficient n Every pixel must be described n A 300 x 300 image (using 24 -bit color) takes up 270, 000 bytes CS 128/ES 228 - Lecture 5 b 9

Storage – Vectors are more “storage appropriate” n Only “items” are described, e. g. “filled yellow circle, (100, 40)” n This image would require less than 50 bytes! CS 128/ES 228 - Lecture 5 b 10

Resolution n Rasters are limited by the size of the raster (the pixel) n Vectors are limited by the number of points (along a line or polygon body) CS 128/ES 228 - Lecture 5 b 11

Topology n Topology is the study of shapes n In GIS, it is taken to mean the information about intersections and adjacencies. Do these line segments intersect? CS 128/ES 228 - Lecture 5 b 12

Maintaining Topology n …is a difficult problem from a “technical” point of view n Topology must be established at the time of input and maintained as the data is edited n Shapefiles contain NO topological information CS 128/ES 228 - Lecture 5 b 13

Topological Problems n Vertices don’t match n Lines do (or don’t) intersect n Polygons don’t close CS 128/ES 228 - Lecture 5 b 14

Fixing Topology is a “snap” n When two entities (point or line) are within a specified tolerance, we can “snap” them to the same point. n Tolerance is determined on the screen, not directly by real-world distance CS 128/ES 228 - Lecture 5 b 15

Geocoding n Using a combination of geographic and attribute data, we can “look up” locations based upon external (to the map) information such as street addresses. CS 128/ES 228 - Lecture 5 b 16

The Ultimate Geocoder? Camp Winadu CS 128/ES 228 - Lecture 5 b 17

Applications of Topology § Voronoi Diagrams (also called Thiessen polygons) § Can be used to § Interpolate § Solve nearestneighbor problems § Find “empty” regions CS 128/ES 228 - Lecture 5 b 18

Summary Vector format allows one-to-one matching between real-world objects and data items. n Vector format allows maintenance of topological information n CS 128/ES 228 - Lecture 5 b 19

Summary, continued Vector format supports inclusion of attribute data n Vector format tends to require less storage space n Vector format makes certain forms of queries MUCH easier n CS 128/ES 228 - Lecture 5 b 20