CS 128/ES 228 - Lecture 5b 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. JedisRed Sox vs. Yankees

Vector vs. Raster

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Spatial data models

1. Raster

2. Vector

3. Object-oriented

Spatial data formats:

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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 228 - Lecture 5b 5

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

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Sources of Vector Data Digitization of raster

data

Computer analysis of raster data

Direct measurement (by GPS, formal surveying, “field work”, etc.)

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Advantages of Vector Data

“A place for everything, and everything is in its place”

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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

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Storage – Rasters are (inherently) inefficient Every pixel must

be described

A 300x300 image (using 24-bit color) takes up 270,000 bytes

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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!

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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

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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?

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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

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Topological Problems Vertices don’t

match

Lines (do or) don’t intersect

Polygons don’t close

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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

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Summary Vector format allows

one-to-one matching between real-world objects and data items.

Vector format allows maintenance of topological information

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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

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Raster vs. Vector

•Art vs. Math?

•Distant vs. Up close and personal?

•Gluttonous vs. Efficient?

•Available vs. Desirable?

It depends o

n 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.)