GEOG70

1
Spatial Data Models and Structure
2
Part 1 Basic Geographic Concepts

• Real world -gt Digital Environment
• GIS data represent a simplified view of physical
  phenomena
• These data contain
• Locational Information
• Non-spatial attributes

3
Symbolization

• In a GIS, we represent real world phenomena in a
  digital format

1 0 0 1 0 0 1 1 1 1 1 0
4
Vocabulary

• Real-World Entities or Phenomena
• Data Objects
• Cartographic Objects

5
Terminology

• Entities or Phenomenon -- real world features to
  be represented in a database

6
Terminology

• Data Objects -- digital representations of
  entities or phenomena

Pasture
House
Road
7
Terminology

• Cartographic Objects -- real-world entities as
  depicted on maps

8
Real World ? Data Objects

• Attributes
• Information about object (e.g., characteristics)
• Location/Spatial information
• Coordinates
• May contain elevation information
• Time
• When collected/created
• Why? Objects may have different attributes over
  time

9
Real World ? Cartographic Objects

• Real world objects differ in
• Size
• Shape
• Color
• Pattern
• These differences affect how these objects are
  represented on maps
• Where possible the cartographic objects (i.e.,
  map symbols) can relate to the entities they are
  representing (e.g., water blue)

10
Topology

• The spatial relationships between data objects

11
Conceptualizing Topology

• Primary
• Adjacency
• Connectivity
• Containment
• Secondary
• Direction
• Proximity (distance)

12
Adjacency
Springfield
Shelbyville
13
Connectivity
These roads are connected at the black points.
14
Containment
Springfield
Moes
Kwik-E-Mart
Nuclear Plant
15
Direction
Moes is Northeast of the Kwik-E-Mart The
nuclear plant is Southeast of the Kwik-E-Mart
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Proximity
Homer lives near Ned Homer lives far from Grampa
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Complex Case Overlap
Blue Lake
Springfield
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Part 2 GIS Data Models

• Entities in the real world are represented as one
  of the following in a GIS
• Raster data
• Pixels in an array
• Vector data
• Points
• Lines
• Areas (or polygons)

Key concept!
19
Continuous Discrete

• The continuous field view represents the real
  world as a finite number of variables, each one
  defined at every possible position.
• The discrete object view represents the
  geographic world as objects with well-defined
  boundaries in otherwise empty space.

20
Continuous Discrete

• Some data types may be presented as either
  discrete or continuous
• Example
• Population at a point (discrete)
• Population density surface for an area
  (continuous)

21
Continuous Discrete

• Continuous
• Data values distributed across a surface w/out
  interruption
• Key words What varies and how smooth?
• Examples elevation, temperature
• Discrete
• with well-defined boundaries in otherwise empty
  space
• Examples
• Points Town, power pole
• Lines Highway, stream
• Areas U.S. Counties, national parks

22
Continuous or Discrete?
www.regional.org.au/au/asa/2003/i/6/walcott.htm
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Continuous Discrete

• In computer databases
• Raster data models represent continuous data
• Vector data model are used for discrete objects

24
Raster Data Model
The raster data model represents the Earths
surface as a two-dimensional array of grid cells,
with each cell having an associated value
1 2 3 5 8
4 6 8 3 9
3 5 3 3 1
7 5 4 3 9
2 2 4 5 2
rows
25
Raster data example
Elevation data each cell contains a number
representing the elevation of that cell.
26
Part 3 The vector data model
27
Vector Data Objects

• Geographic building blocks
• Points
• 0 dimensional
• Lines
• 1 dimensional
• Polygons
• 2 dimensional

28
Spatial Objects

• Data objects in the vector data model can be
• A point can represent
• Tree, airport, a city, street intersection, a
  movie theater, a benchmark
• A line is a data object, made up of a connected
  sequence of points. It can represent
• Roads, rivers, regional boundaries, fences,
  hedgerows, power lines
• A polygon is an enclosed area. Examples
• A census tract, Saunders building, boundary of
  Chapel Hill, a lake, a watershed, a city

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Object example oak tree
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Thought question

• How are you going to represent the California
  OAK tree in digital format?
• A point? A polygon? Or a pixel?
• It will depend on
• Scale of observation
• Purpose of your research
• The type of data you have access to in the GIS

31
Thought questions

• When do you want to represent Chapel Hill as a
  polygon object instead of a point object?
• When do you want to represent a river as a
  polygon instead of a line?

32
The vector data objects
(x,y)
(x,y)
(x,y)
(x,y)
(x,y)
(x,y)
(x,y)
(x,y)
(x,y)
(x,y)
(x,y)
point
line
polygon(area)

• The vector data model represents geographic
  features similar to the way maps do
• A point recorded by a pair of (x,y)
  coordinates.
• A line recorded by joining more than one point,
• A polygon recorded by a joining multiple points
  that enclose an area

33
Vector Data Storage in Computers Points
Data Storage
Points
Point ID Coordinates
4
1 1, 1 2
4, 2 3 6, 2
4
2, 4
2
3
1
0
34
Vector Data Storage in Computers
Lines (Sometimes called arcs)
Note In GIS, this is considered a line (a
connected set of individual lines).
35
Vector Data Storage in Computers Polygons
36
The Arc-Node Data Structure
Benefit
The arc-node structure allows efficient data
storage for vector data
How does it work?
It stores data so that nodes construct arcs, and
arcs construct polygons
Nodes define the two endpoints of an arc. They
may or may not connect two or more arcs. An arc
is the line segment between two nodes. The points
between two nodes defining the shape of an arc
are called vertices. Nodes and vertices are
represented as x, y coordinates.
37
The Arc-Node Data Structure
2
Arc ?, ?, ? Nodes 2, 5 Vertices 1, 6 for arc
? 3, 4 for arc ?

3
1
?
?
B
A
?
4
5
6

• Points
• 1 x1,y1
• 2 x2,y2
• 3 x3,y3
• x4,y4
• 5 x5,y5
• 6 x6,y6

38
Arc-Node Data Structure enables topology
definition
Topology defines spatial relationships. The
arc-node data structure supports three major
topological concepts Connectivity
Arcs connect to each other at
nodes Area definition
Arcs that connect to surround an area define a
polygon Contiguity Arcs
have direction and left and right sides
39
Topology Connectivity
Connected arcs are determined by searching
through the list for common node numbers.
Arc-node list
10
11
12
?
?

• Arc From-Node To-Node
• 10 11
• 11 12
• 11 13
• 13 15
• 13 14

?
?
13
14
?
15
Because of the common node 11, arcs 1, 2, and 3
all intersect. The computer can determine that
it is possible to travel along arc 1 and turn
onto arc 3. But it is not possible to turn
directly from arc 1 to arc 5.
40
Topology Area Definition
Polygon-Arc Topology
1
8
Polygon Arc List B
1,5,8,4 C 2,6,9,5 D
6,3,4,7 E
9,7,8
B
5
C
2
4
E
9
D
6
7
3
Polygons are simply the list of arcs defining its
boundary, arc coordinates are stored only once,
therefore, reducing the amount of data and
ensuring that the boundaries of adjacent
polygons dont overlap
41
Topology Contiguity
Two geographic features which share a boundary
are called adjacent. Contiguity is the
topological concept which allows the vector data
model to determine adjacency.
An Arc
left
From-Node
To-Node
right
Direction
1
8

• Arc Left Right
• Polygon Polygon
• C B
• E C
• ? ?
• 1 ? ?

B
5
C
2
4
E
9
D
6
7
3