CS 128/ES 228 - Lecture 5b1 Vector Based Data. Great Rivalries in History Lincoln vs. Douglas “The...
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Transcript of CS 128/ES 228 - Lecture 5b1 Vector Based Data. Great Rivalries in History Lincoln vs. Douglas “The...
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 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!