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>