cs 128/es 228 - lecture 5b1 vector based data. great rivalries in history lincoln vs. douglas “the...

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

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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. Jedis

Red Sox vs. Yankees

Vector vs. Raster

CS 128/ES 228 - Lecture 5b 3

Spatial data models

1. Raster

2. Vector

3. Object-oriented

Spatial data formats:

CS 128/ES 228 - Lecture 5b 4

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

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 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 228 - Lecture 5b 13

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 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 o

n the p

roble

m!

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.)