Presented by

Rehana Jamal

(GIS Expert & Geographer)

Dated: 02.12.14

Advance Applications of RS/GIS in Geo-Environmental Conservation

Subject

Lecture# 9&10

M.Phil & P.hd

Topic

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Spatial Data Models and Structure

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Spatial database is a spatial Model of Reality

Introduction

The real world is too complex for our direct understanding so we create “Models” of reality having similarity with selected aspects of the real world.

Models are in form of layersLayer is a heterogeneous collection of geometries having the same attribute set.

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Spatial Data Models facilitate

Early analysis of properties, e.g. storage cost, querying ability

Reuse of shared data among multiple applications

Exchange of data across organization

Conversion of data to new software / environment

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Why Spatial Data Model?

Data structures are complex for GIS because they must include information pertaining to entities with respect to:

positiontopological relationships attribute information.

It is the topologic and spatial aspects of GIS that distinguish it from other types of databases.

First

Second

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Raster data-- represent the landscape as a rectangular matrix of square cells. Or Set of cells on a grid that represents an entity (entity -- symbol/color -- cells).

Vector data --represent features as discrete points, lines, and polygons. Or an entity is represented by nodes and their connecting arc or line segment (entity -- points, lines or areas -- connectivity)

Object data– an entity is represented by an object which has as one of its attributes spatial information.

Types of Spatial Data Structure

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Three types of representations for geographic data

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Raster data models incorporate the use of a grid-cell data structure where the geographic area is divided into cells identified by row and column. This data structure is commonly

called raster.

Raster data Model The term raster implies a regularly spaced grid other tessellated data structures do exist in grid based GIS systems.

Raster data structure represents geographical space by dividing the space into a series of units known as pixels(picture element).

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VECTOR VECTOR CHARACTERISTICSCHARACTERISTICS

POINT ×

LINE

POLYGON

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× × × ×

× ×

×

×

×

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Point

Line

Polygon

Vector dataRaster data

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DATA MODELS OF RASTER AND VECTORDATA MODELS OF RASTER AND VECTOR

REAL WORLD 1 2 3 4 5 6 7 8 9 10

1

2

3

4

5

6

7

8

9

10

GRID RASTER VECTOR 802 Dec-14

Pond

Lake

River

Reality overlaid with a grid

1 1 0

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1 1 1

11 1

2

2

22

2

2

11

0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0

0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0

Resulting raster

Creating a RasterCreating a Raster

0 = No Water Feature1 = Water Body2 = River

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Creating a VectorCreating a Vector

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A point is represented by an explicit x,y coordinate in vector format, but as a raster, it is represented as a single cell —the smallest unit of a raster. By definition, a point has no area but is converted to a cell representing area.

Point

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In vector format, a line is an ordered list of x,y coordinates, but in raster format it is represented as a chain of spatially connected cells with the same value. When there is a break between the chain of same-valued cells, it represents a break in the line feature, which could represent different features such as two roads or two rivers that do not intersect.

Lines

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A vector polygon is an enclosed area defined by an ordered list of x,y coordinates in which the first and last coordinates are the same, thereby representing area. By contrast, a raster polygon is a group of contiguous cells with the same value that most accurately portray the shape of the area.

Polygonal, or area, data is best represented by a series of connected cells. Examples of polygonal features include buildings, ponds, soils, forests, swamps, and fields.

Polygons

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Raster data model

Divide the world into square cells

Register the corners to the earth

Represent discrete objects as collections of one or more cells

Represent fields by assigning attribute values to cells

Oak savannah

Grassland

Mixed conifer

Douglas fir

Legend

Fig. Raster representation, each colour represents a different value of a nominal-scale field denoting land cover class02 Dec-14

grids (ArcGIS & ArcInfo specific)

graphical images (TIFF, JPEG, BMP, GIF, etc.)

USGS DEM (Digital Elevation Model)

remotely-sensed images (Landsat, SPOT, Quick bird, World View 1, AVHRR etc)

Types of Raster Data

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Spatial resolution refers to the dimension of the cell size representing the area covered on the ground OR spatial resolution refers to the cell size (the area covered on the ground and represented by a single cell). Therefore, if the area covered by a cell is 5 x 5 meters, the resolution is 5 meters. The higher the resolution of a raster, the smaller the cell size and, thus, the greater the detail.

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Coarser Spatial resolution

High Spatial resolution

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Discrete Continuous

Discrete objects/data and Continuous surfaces/objects/data

Data Structure for Continuous Surface Model

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Vector data model

x

y

Point Line/arc Polygon

Node

Vertex

×

×

×

×

×

×

Line segment Node

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Topological data Structure (model)

TopologyTopology is that branch of mathematics used to define spatial relationships between entities (ESRI 1992). For example, an area or polygon is defined by a set of lines which makes up its boundaries. In this case the line is the border between two polygons.

The connectivity or contiguity of these features is referred to as their topology structure (ESRI 1992). By sorting information about the location of a feature relative to other features, topology provides the basis for many kinds of geographic analysis without having access to the absolute locations held in the coordinate files (ESRI 1992).

Topological data Structure

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Topological data Structure (model)

The Topological data model is termed Arc-Node data model.

Arc the basic logical entity, a series of point that starts and end at a node.

Node is an intersection point where two or more arcs meet. A node can also occur at the end of a dangling arc i.e. and arc that is not connected to another arc such as the end of

a dead-end street.

Polygon is comprised of a closed chain of arcs that represents the boundary of the area.

Point is encoded as a single XY co-ordinate pair. Point is considered as the polygon with no area.

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Vector GIS data models

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Spaghetti model

Topological vector model

TIN (Triangular Irregular Network)

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Spaghetti model

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0 10 20 30 40 50 60 70

70

60 50

40

30

20

10

0

a6.D. N5

a7

a7

a7

a7

N6

N1

N2

N3

N4

a1

a2

a2

a2

a3

a4

a4

a5

A

B

C

E

E

X-axis

Y-a

xis

Spatial data encoding

oNode Topology

oArc Topology

oPolygon Topology

oArc coordinate data

Topological Data Model or Structure/Arc-Node Data Model

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A 1,4,5

B 2,4,6

C 3,5,6

A

B C

i. 1 4

ii. 4 4

iii. 6 4

iv. 4 1

Node X Y

1 2 3 4 5 6

2

3

4

5

6

1

iii

iii

iv

1 i iii O A

2 i iv B O

3 iii iv O C

4 i ii A B

5 ii iii A C

6 ii iv C B

Line From To Left Right

Poly Lines

O = “outside” polygon1

2 3

4 5

6

Topological Data Model or structure/Arc-Node Data Model

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The Triangulated Irregular Network (TIN) data model is an alternative to the raster and vector data models for representing continuous surfaces.

The Triangulated Irregular Network (TIN) data model

TIN Data Model

Four Tables for TIN Model

     Node Table it lists each triangle and the nodes which define it.

   Edge Table it lists three triangles adjacent to each facets. The triangles that border the boundary of the TIN show only two adjacent facets.

     XY Co-ordinate Table it lists the co-ordinate values of each node.

     Z Table it is the altitude value of each node. 02 Dec-14 25

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Topological Relationships between Spatial Objects orSpatial relationship in Topological data model

Containment

Contiguity/Adjacency

Connectivity

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Raster dataVector data Object data

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Three types of representations for geographic data

Summary

Spatial Data Models and Structure

Spaghetti model

Topological vector model

TIN (Triangular Irregular Network)

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Possible Exam Questions

Q1.Differentiate the following: 5 Marks each

i.Raster and Vector data modelii.Data model and Data structureiii.Spaghetti and Topological data modeliv.Raster and Vector data model

Q2.Write a note on the following: 5 Marks each

i.TIN data modelii.Spaghetti data modeliii.Raster and Vector data model

Q3.Describe in detail with diagrams the “Spatial data models”. 15 Marks

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References:

www.yahoo.com

www.esri.com

“Geographic Information System” by Stan Aronoff

(http://www.olemiss.edu/depts/geology/courses/ge470/RasterDataModel.htm#8b.1)

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