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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. Attribute Data. Why Databases?. - PowerPoint PPT Presentation
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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”
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 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
CS 128/ES 228 - Lecture 6a 5
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”
CS 128/ES 228 - Lecture 6a 7
What Color Hair Does Student 234 Have?
Mary
Dawn
Pete
Pat
117 Mary 3.8
201 Dave 2.5
234 Dawn 3.1
235 Pete 3.2
382 Ted 2.6
378 Jane 3.0
444 Ann 4.0
This is an example of a JOIN operation
CS 128/ES 228 - Lecture 6a 8
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)
CS 128/ES 228 - Lecture 6a 9
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 10
Sometimes they are mixed
CS 128/ES 228 - Lecture 6a 11
Queries
Information is gained by linking tables through “joins”
Queries can involve computed quantities, etc.
CS 128/ES 228 - Lecture 6a 12
Storing Topology Shapefiles contain no topological
information
But topology is important
SOLUTION
Store the topology elsewhere
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
Data Structure for Topology
1
98
76
4
5 3
2
10 AB
C
D
Zone 1
Zone 2
Bound.
A B 4
A C 5
B C 10
C D 8
Select Boundary Where Zone1=A
Ans: 4,5
CS 128/ES 228 - Lecture 6a 15
Topological Operations Merge two regions into one
A
BC
…and change the appropriate table
CS 128/ES 228 - Lecture 6a 16
Topological Operations Split one region into two
A BC
…and change the appropriate table
CS 128/ES 228 - Lecture 6a 17
Other Topological Operations Clip an image
CS 128/ES 228 - Lecture 6a 18
Topological Operations Erase an image
CS 128/ES 228 - Lecture 6a 19
Other operations
Union
Intersect
“Snaps”
CS 128/ES 228 - Lecture 6a 20
How this is used
Find nearest neighbor
Find largest “empty” region
CS 128/ES 228 - Lecture 6a 21
What distinguishes (some) GIS’s
The types of these operations that they can perform.
Both Database and Topological