GEOG 60 – Introduction to Geographic Information Systems
Professor: Dr. Jean-Paul Rodrigue
Topic 3 – Geographical Data Structures
A – Geographic Data Models
B – Inputting Spatial Information
A
Geographic Data Models
■ 1. Raster Representation
■ 2. Vector Representation
■ 3. The ArcView Shapefile Model
1
Spatial Objects
■ Definition
• Delimited geographic areas.
• Ex. park, golf course, wildlife
reserve.
• Associated attributes.
• Attributes are qualities or
characteristics of spatial
objects.
• Ex. census data, types of
vegetation, heights or depths.
Grass, 1 feet
Shrubs, 6 feet
Forest, 25 feet
■ Types of geographical
delimitation
• Three conventional delimitations.
• Points, lines and polygons.
1
Raster Representation
■ Issue
• A model for a computer to represent real geographical elements
as graphical elements (on screen or on paper).
■ Two models of representation possible:
• Raster (grid-based)
• Vector (line-based)
Representation
1
Raster Representation
■ Cellular organization
•
•
•
•
•
Divides space in a series of units.
Each unit is generally similar in size to another.
Grid cells is the most common raster representation.
Features are divided into cellular arrays.
Coordinate (X,Y) is assigned to each cell, as well as a value.
• Allows for registration with geographic reference system.
• JPEG, GIF, BMP and TIF are raster formats.
■ Tessellation
• Geometric shapes that can completely cover an area.
• Squares / Rectangles.
• Triangles.
• Hexagons.
1
Raster Representation
Point
Column
Real world
Line
Row
Value
=0
=1
=2
=3
Raster
Grid
Area
Triangles
Hexagons
1
Raster Representation
700 meters
■ Advantages
• Easy to conceptualize.
• Overlay operations are easy.
• A two-dimensional array forms a
coverage.
■ The problem of resolution
Size = 7x7x4 = 196
Cell = 10 m x 10 m = 100 m2
• For a small grid:
• Coarse resolution but limited
storage space.
• For a large grid:
• Fine resolution but large
storage space.
Size = 10x10x4 = 400
Cell = 7 m x 7 m = 49 m2
1
The Mixed Pixel Problem
2
Vector Representation
■ Concept
Point
• Assumes that space is continuous,
rather than discrete.
• Infinite (in theory) set of coordinates.
(X,Y)
(X2,Y2)
(X4,Y4)
(X3,Y3)
(X,Y)
Line
(X5,Y5)
■ Points
• Spatial objects with no area but can
have attached attributes.
• A single set of coordinates (X and Y)
in a coordinate space.
■ Lines
(X,Y)
(X2,Y2)
Polygon
• Spatial object made up of connected
points (nodes).
• Have no width.
■ Polygon
(X5,Y5)
(X4,Y4)
(X3,Y3)
• Closed areas that can be made up of
a circuit of line segments.
• Line segments that make up a portion
of a polygon.
2
Vector Representation
Node 1 Node 2
Node 4
Node 3
Arc B
Node 5
4
Node 8
Node 6
1 Arc C 2 Arc D
Arc E
5
Arc H
Arc F
Arc G
Arc A
8
6
Node 7
3
Arc J 7 Arc I
Node
X
Y
Arc
From
To
1
2
3
4
5
6
7
8
12
22
53
24
5
36
17
38
4
16
42
17
9
43
21
44
A
B
C
D
E
F
G
H
I
J
6
4
1
2
5
4
7
3
8
7
4
1
2
3
2
5
5
8
7
6
2
Vector Representation
1
B
4
2
C
A
5
B
6
J
3
C
H
E
F
A
D
G
8
7
Poly # of arcs
A
B
C
4
4
5
I
Arc list
B,C,E,F
A,F,G,J
E,D,H,I,G
2
Vector Representation
Point Table
Line Table
Pt. ID
X
Y
Ln. ID
Pt. 1
Pt. 2
1
2
3
4
5
...
24.5
24.8
27.8
30.1
14.2
...
27.4
24.1
22.5
29.9
30.1
...
35
36
37
38
39
...
1
4
6
2
8
...
3
2
8
10
11
...
Relational Links
Poly. Table
Attrib. Table
Pol. ID Ln. ID
Pol. ID Attrib.
74
74
74
75
75
...
38
35
29
28
42
...
74
75
76
77
78
...
104.2
100.1
105.7
102.7
106.1
...
2
GIS Vector Models – 3 Major Models
Unique identifier
Hybrid
Coordinate and
Topological files
Attribute tables
Relational database
(Features) - Relational join – (Attributes)
Integrated
Relational database
Element – Class – Attribute
Object-Oriented
Object store
3
The ArcView Shapefile Model
■ Format
Hybrid data model.
Store spatial information in a vector format.
Sequential list of features.
Less processing intensive (faster drawing time).
Two-dimensional (x,y) and three-dimensional (x,y,z) features
supported.
• Each shapefile represent one shape type.
•
•
•
•
•
• Point, Polyline, and Polygon are the most common.
• Three major components:
• Main file (*.shp): Store the list of geographical features.
• Database file (*.dbf): Store the attributes in a table.
• Index file (*.shx): Links the database and geographical features.
3
The ArcView Shapefile Model
Main file
(*.shp)
Index file
(*.shx)
dBase
table
(*.dbf)
a
a
b
b
c
c
d
d
3
The ArcView Shapefile Model
geometry
object identifier (optional)
geometry tracking field (optional)
geom
id
shp_len
type
surface
width
lanes
name
101
102
103
104
105
...
4507.4
3491.1
2321.8
682.9
1279.1
...
2
1
3
5
4
...
asphalt
concrete
asphalt
gravel
asphalt
...
85.3
45.1
75.9
35.2
60.3
...
4
2
4
2
4
...
abc
def
ghi
jkl
mno
...
Predefined fields
custom fields
C
Inputting Spatial Information
■ 1. The Input Subsystem
■ 2. Choosing What to Input
■ 3. Editing Vector Objects
1
Input Subsystem
■ Digitizing
• Most difficult and time consuming task in mapping and GIS.
• Takes about 75% of the time in a mapping project.
• About 75% of the costs of operating a GIS system.
■ Digitizing and errors
• Since digitizing is very time consuming, you must get it right on
the first time.
• Error correction is excessively long and costly.
• The larger the file, the bigger the number of potential errors.
• Each model, raster or vector, requires special digitizing
equipment.
1
Input Subsystem
■ Mouse
• The mouse is the most basic input system.
• A rolling ball with two sensors, one of X, one for Y.
• It continuously sends a set of X,Y coordinates to the CPU.
Buttons are sending interrupts to the CPU.
• By itself, it cannot be used to encode mapping information, but it
is suitable for tracing.
X,Y
Pointer
1
Input Subsystem
■ Tracing and digitizing
•
•
•
•
Tracing is a form of digitizing.
Mainly imply using a scanned image.
Easier for less experienced users.
Precision is limited to the resolution of the scanned image.
1
Input Subsystem
■ Digitizer
• Table containing a matrix of very small cables.
• A mouse-like device, often called a puck (cursor), moves over the table.
• Creates an electromagnetic field disruption on the grid, the center of
which is the X,Y coordinate.
• One of the most precise digitizing technique.
• Varies according to:
•
•
•
•
•
Stability: Tendency of coordinates to change with temperature.
Repeatability: With the same location, are the X,Ys exact?
Linearity: Keeping up with movements of the cursor.
Resolution: The smallest unit or measure it can handle.
Skew: Differences between variation of Xs and Ys.
1
Input Subsystem
■ Scanner
• An horizontal light makes a pass and each line is read by a
photoelectric cell (like a photocopy machine).
• Resolution:
• Number of pixels per units of surface the cell can read.
• 1,200 DPI (Dots per inch) is common.
• Color depth:
• Number of different colors the cell can read.
• 1-bit only supports B&W, 4-bit 16 colors (or shades of gray), 8-bit 256
colors and 12-bit 16.7 million colors.
• Two types:
• Drum scanners (rotating drum),
• Flat-bed scanners.
2
Choosing What to Input
■ Some Rules
• Find what are your goals.
• Digitize the information you really need:
• Most base maps contain a lot of information.
• Try to choose a conventional source of spatial data.
• Use the level of accuracy corresponding to your task:
• High levels of accuracy equal high levels of diminishing returns.
• Input data as separate themes:
• Each theme should be specific.
• Each theme has a specific geographic feature (point, line or polygon).
2
Choosing What to Input
Type of Feature Information
Points
Lines
Polygons
Streets
Store Locations
Parks
Highways
Themes
2
Choosing What to Input
■ How Much to Input
• Choosing the right amount of information to encode is difficult.
• Depends on the level of accuracy.
Not enough
Too Many
Good Solution
2
Base Map (Air Photograph)
2
Digitizing Major Roads and the Hydrography
2
Digitizing Lots
2
Final Product
3
Editing Vector Objects
■ Points
•
•
•
•
•
•
Simply changing the coordinate.
Dragging and dropping the most common.
Lines
Changing the coordinate of one or more points.
Splitting a line in two.
Merging lines.
■ Polygons
• Changing the coordinate of one or more points (the last point is
also the first point).
• Splitting a polygon in two.
• Using a boundary to draw another polygon.
• Merging polygons.
• Creating an island in a polygon.
• Creating an intersection.
3
Editing Vector Objects
Intersection
Moving a point (vertex)
Merging lines
Line B
Line A
Splitting a line
3
Editing Vector Objects
Polygon A
Polygon B
Moving a point (vertex)
Splitting a polygon
3
Editing Vector Objects
Using a boundary
Merging polygons
3
Editing Vector Objects
Creating an island
Creating an intersection
3
Editing Vector Objects
■ Snapping
•
•
•
•
Make a vertex take the coordinates of a reference.
Spanning tolerance defines the “search space”.
Avoid overshoots and undershoots for lines.
Avoid gaps and overlaps for polygons.
■ Snap to Vertex
• Snaps the next vertex to the nearest vertex in an existing line or
polygon.
■ Snap to Boundary
• Snaps the next vertex to the nearest line segment in an existing
line or polygon boundary.
3
Editing Vector Objects
■ Snap to Intersection
• Snaps the next vertex to the nearest node common to two or
more lines or polygons.
■ Snap to Endpoint
• Snaps the next vertex to the nearest endpoint of an existing line.
• For lines only.