GUS: 0265Applications in GIS
Lecture Presentation 1:
Vector Data Model and Operations
Jeremy Mennis
Department of Geography and Urban Studies
Temple University
Vector Data Models
Geometric Primitives:
Points, lines, polygons
Two Primary Types:
Spaghetti
Topologic
(and TIN)
Spaghetti Vector Data Model
Each point, line, or polygon is stored as a record in a file that consists of that entity’s ID and a list of coordinates that define geometry.
ID Coordinates
1 3,4
2 5,5
1
2
For Points:
Spaghetti Vector Data Model
Each point, line, or polygon is stored as a record in a file that consists of that entity’s ID and a list of coordinates that define geometry.
ID Coordinates
1 (0,1), (3,4), (5,6)
2 (3,1), (5,2), (4,3)
1
2
For Lines:
Spaghetti Vector Data Model
Each point, line, or polygon is stored as a record in a file that consists of that entity’s ID and a list of coordinates that define geometry.
ID Coordinates
1 (2,4), (4,3), (3,6) , (2,4),
2 (3,1), (5,2), (4,3), (3,2), (3,1)
1
2
For Polygons:
ESRI Shapefile
Designed by ESRI for ArcView
Implementation of the spaghetti vector model
An individual layer stores a single type of geometry (i.e. point, line, polygon)
No topology (but it can be calculated on the fly...)
Draws relatively fast
‘Open’ file format
ESRI Shapefile
Three primary files in a shapefile: .shp, .shx, and .dbf
All files must share the same prefix for one shapefile,
e.g. road.shp, road.shx, and road.dbf
.shp : stores the feature geometry (binary)
.shx : index for .shp file
.dbf : attribute data stored in dBASE format
Spaghetti Vector Data Model
Advantages
simple
efficient for display and plotting
Disadvantages
inefficient for most types of spatial analysis
Vector Topologic Data Model
Composed of points, lines, and polygons
Node: a point at the intersection of three or more lines
In addition to coordinate locations, the topologic relationships among geometric features are explicitly recorded
A
B
Ca1a2
a3 a4
n1
n2
ID Arcs
A a1, a2
B a2, a4
C a3, a4
Polygon Topology
Node Arcs
n1 a4, a2, a1, a3
n2 a2, a4, a3, a1
Node Topology
Arc Start End Left Right
a1 n1 n2 A
a2 n1 n2 A B
a3 n1 n2 C
a4 n2 n1 C B
Arc Topology
Arc StartXY IntermediateXY EndXY
a1 4,5 (4,8), (8,8), (8,1), (4,1) 4,3
a2 4,5 (6,7), (6,3) 4,3
a3 4,5 (1,3) 4,3
a4 4,3 4,5
Arc Coordinate Data
Vector Topologic Data Model
Vector Topologic Data Model
Planar Enforcement:
No two individual features can overlap.
There are no ‘holes’ or ‘íslands’ that are not themselves features.
Every feature is represented as a record in the attribute table.
Vector Topologic vs. Spaghetti
Spaghetti: can encode as 2 or 3 polygons (and have 2 or 3 records in the attribute table)
Topologic: must be encoded as 3 polygons (and have 3 records in the attribute table)
ESRI Coverage
Designed by ESRI for ArcInfo
Implementation of the vector topologic data model
‘Closed’ file format
Each coverage is a directory, with numerous files that store feature geometry, projection, registration, etc.
Attribute data is stored in a separate INFO directory, which stores all attribute data for all coverages in its parent directory.
Triangulated Irregular Network (TIN)
Triangulated Irregular Network
(TIN)
Vector Operations
Classification
Buffer
Overlay
Classification
Line Dissolve (Map Dissolve)
1 grain crops 2 orchards 3 residential 4 commercial
1 agricultural 2 non-agricultural
2
1
3
4
1 2
Buffer
Definition of what is within/without a given proximity
Point buffer
Line buffer
Polygon buffer
Overlay
An overlay operation takes two or more data layers as input and results in an output data layer
Three types of overlay:
Point in polygon
Line in polygon
Polygon (polygon on polygon)
Point in Polygon Overlay
A
BC
ID Tree
A Elm B Maple C Elm
Point Table
ID Tree Cover
A Elm Rural B Maple Rural C Elm Urban
Point Table
ID Cover
1 Rural 2 Urban
Poly Table
1 2
+ A
BC=
Land CoverTrees NewTrees
Line in Polygon Overlay
A
B
C
ID Street
A Race B Race C Arch
Line Table
ID Street Cover
A Race Rural B Race Urban C Arch Urban D Race Urban
Line Table
ID Cover
1 Rural 2 Urban
Poly Table
1 2
+ =
Land CoverStreets NewStreets
A
B
C
D
Polygon Overlay
Intersection (and) Union (or) Identity
Polygon Overlay: Intersection
Agriculture
A
B
A
Land Cover
ID Owner
A Brown B Smith
ID Cover
A commercial B industrial
B
Area of intersection
New node
<Intermediate>
Polygon Overlay: Union
Agriculture
A
B
A
ID Owner
A Brown B Smith
ID Cover
A commercial B industrial
B
Area of union
New node
<Intermediate>Land Cover
Polygon Overlay: Identity
Agriculture (input layer)
A
B
A
Land Cover (identity layer)
ID Owner
A Brown B Smith
ID Cover
A commercial B industrial
B
Area of identity
New node
<Intermediate>