GIS Data Models

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GIS Data Models

• GEOG 370
• Christine Erlien, Instructor

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GIS Data Models Why?

• Knowing how GIS data are structured helps us to
  use GIS programs more effectively
• Basic computer file structures
• Database structures

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Basic computer file structures

• What is where?
• Computer file structures allow the computer to
  store, order, search data
• Types
• Simple list
• Ordered sequential
• Indexed file

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Basic computer file structures Simple list

• Simple List
• Most basic
• No order, no organization
• Input is simple ? just add on
• Searching difficult inefficient
• Example If my class roster were ordered based
  on when you added this class

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Basic computer file structures Ordered
sequential files

• Ordered sequential files
• Records ordered by alphabetic or numerical
  character sequence
• How? Algorithm divide and conquer
• Record compared to records preceding following
  to determine which 1/2 to search
• Repeat until done
• Inserting a record is slow
• Searching more efficient than simple list

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Basic computer file structuresOrdered
sequential files

• To add
• Maggie Valley
• Whats the process?

• Example file
• Chapel Hill
• Cary
• Durham
• Graham
• Greensboro
• Raleigh

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Basic computer file structures

• Indexed files
• Database index
• Can be built for field that uniquely identifies a
  record (primary key) or other fields
• Used to determine the location of rows in a file
  that satisfy some condition
• Keys indexes can be extracted sorted and
  original file accessed faster than the original
  file could be sorted
• Types
• Direct Each record searched for particular
  properties
• Inverted Index based on anticipated search
  criteria

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Indexed files
Direct index
Inverted index
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Basic computer file structuresIndexed files

• Advantages
• Quicker (i.e., reduces computational time)
• Disadvantages
• Inverted
• Requires knowledge of likely search criteria
• Data additions require recalculation of index

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Databases Database Structures

• What is where?
• Geographic searches ? data retrieval
• Data retrieval requires data organization

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Databases Database Structures

• Database Collection of multiple files
• Requires more elaborate structure for management
• DBMS Database Management System
• Database structure types
• Hierarchical data structures
• Network systems
• Relational database systems

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Database Structures Hierarchical

• Hierarchical data structures
• One-to-many (parent-child) relationship
• Requires relationship be defined before structure
  decision rules developed
• Advantage
• Easy to search
• Disadvantage
• Knowledge of all questions that might be asked
  necessary
• Unanticipated criteria make search impossible
• Large index files ? memory intensive, slow access

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Hierarchical Database Structures
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Database Structures Network Systems

• Network Systems
• Allow users to move from data item to data item
  through a series of pointers
• Pointers Computer structures that direct a piece
  of data to all others to which it relates
  (connect one file location to another)
• Pointers indicate relationships among data items

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Database Structures Network Systems
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Database Structures Network Systems

• Advantages
• Less rigid than hierarchical structure
• Can handle many-to-many relationships
• Reduce data redundancy
• Greater search flexibility
• Disadvantages
• In very complex GIS databases, the number of
  pointers can get quite large ? storage space

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Database Structures Relational Databases

• Predominant in GIS
• Tuples Ordered records/rows of attribute values
• Primary Key Unique identifier for each record in
  a relational table

Lu_code Crop type Status Cost
010001 Row crops Active 1000/ha
020001 Orchards Dormant 1500/ha
021001 Rangeland Active 900/ha
010001 Row crops Active 1100/ha
010404 Garden farms Active 1250/ha
010001 Row crops Dormant 1050/ha
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Database Structures Relational Databases

• Joining tables ? Relational join
• Matching data from one table to corresponding
  data in another table
• How? Link the primary key to the foreign key
• Primary Key Unique identifier in 1st table
• Foreign key Column in 2nd table to which primary
  key is linked

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Database Structures Relational Databases
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Relational DB Normal Forms

• Normal forms A set of rules established to
  indicate the form tables should take
• Goal Reduce database redundancy ? database
  performance is better
• First normal form
• Table must contain columns rows
• Columns will be used for searches, so only one
  value per cell

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Relational DB Normal Forms

• Second normal form
• Every column that is not the primary key should
  be dependent on the primary key
• On the entire primary key if primary key is
  comprised of more than one column

PART WAREHOUSE QUANTITY
WAREHOUSE-ADDRESS Key Part Warehouse
together Address ? only dependent on warehouse
portion of key
PART WAREHOUSE QUANTITY
WAREHOUSE WAREHOUSE-ADDRESS
Example from William Kent, "A Simple Guide to
Five Normal Forms in Relational Database Theory",
Communications of the ACM 26(2), Feb. 1983,
120-125.
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Relational DB Normal Forms

• Third Normal Form
• Nonprimary keys must depend on primary key
• Primary key does not depend on any nonprimary key
• EMPLOYEE DEPARTMENT LOCATION
  
• Key field Employee
• Location is redundant not dependent on key
  field
• EMPLOYEE DEPARTMENT DEPARTMENT
  LOCATION

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Normalization of Database Tables

• Normalization Process of organizing data in a
  database
• Creating tables establishing relationships
  between them according to rules of normal form
• Goal Make the database more flexible by
  eliminating redundancy and inconsistent dependency

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Normalization of Database Tables

• Problem with data redundancy
• Wastes disk space
• Creates maintenance problems
• If data existing in more than one place must be
  changed ? must be changed the same way in each
  case

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Normalization Normal Forms

• Describing databases
• If the 1st rule is observed, the database is said
  to be in "first normal form."
• If the first 3 rules are observed, the database
  is considered to be in "third normal form."
• Additional levels of normalization are possible,
  but 3rd normal form is considered the highest
  level necessary for most applications

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Recap

• File types
• Simple list
• Ordered Sequential
• Indexed
• Databases Many files
• Structure necessary ? access to data in 1 or more
  files easier
• Database types
• Hierarchical
• Network
• Relational