CS 128/ES 228 - Lecture 6a 1

Attribute Data

CampusID Name Type Floors Footprint

6 Murphy Academic 2 2001

9 Hopkins Support 2 946

12 Maintenance Support 1 1848

15 Hickey Support 2 2367

17 Shay-Loughlen Dorm 3 1298

CS 128/ES 228 - Lecture 6a 2

Why Databases?

One of the advantages of vector-based data is that each datum corresponds to a (portion) of a “real” object.

BUT… Objects are more than geographic locations

CS 128/ES 228 - Lecture 6a 3

Database “Review”

Databases consist of tables Each table holds records

Records are in rows Each record consists of fields, i.e.

individual data items Fields are in columns

Fields that have unique values within a table are called “keys”

CS 128/ES 228 - Lecture 6a 4

Sequential Databases

Once upon a time, computer scientists tried to put all the data for an application in one table

It didn’t work

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Flaws of Sequential Databases

Not all data is homogeneous

Monolithic structure leads to contention problems

Separation of data into “sub” databases leads to duplication (and inconsistencies)

CS 128/ES 228 - Lecture 6a 6

Relational databases

Use many (related) tables of data, with minimal duplication

Tables are “linked” through common values in particular fields

“Queries” permit rich “data mining”

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Relational Databases in GIS One table traditionally holds

geographic information Other tables hold data about other

attributes Tables are linked through “object

Ids” (Object Ids should be independent of

the software)

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Relational Databases in GIS Consider a “Buildings” layer

“GIS” data includes location (including coordinate system), symbology, internal bookkeeping

“External” data includes name of building, date built, purpose, etc.

CS 128/ES 228 - Lecture 6a 9

Sometimes they are mixed

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Queries

Information is gained by linking tables through “joins”

Queries can involve computed quantities, etc.

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Storing Topology Shapefiles contain no topological

information

But topology is important

SOLUTION

Store the topology elsewhere

CS 128/ES 228 - Lecture 6a 12

Data Structure for Topology

1

9

8

7

6

4

5 3

2

10A

B

C

D

Zone 1

Zone 2

Bound.

A B 4

A C 5

B C 10

C D 8

CS 128/ES 228 - Lecture 6a 13

Practicality

GIS must maintain topology (to be useful)

GIS can’t store topology (in shapefiles)

SO, GIS must compute topology

CS 128/ES 228 - Lecture 6a 14

Topological Operations Merge two regions into one

A

B

C

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Topological Operations Split one region into two

A B

C

CS 128/ES 228 - Lecture 6a 16

Topological Operations Clip an image

CS 128/ES 228 - Lecture 6a 17

Topological Operations Erase an image

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Other operations

Union

Intersect

“Snaps”

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How this is used

Find nearest neighbor

Find largest “empty” region

CS 128/ES 228 - Lecture 6a 20

What distinguishes (some) GIS’s

The types of these operations that they can perform.

Both Database and Topological