Data models in geographical information system(GIS)

Index
 Introduction
 Components of GIS
 Data types in GIS
 Data representation
 Raster data model
 Vector data model
 Differences between Raster and Vector
 Triangulated Irregular Network (TIN)
 Digital Elevation Model (DEM)
 Conclusion
 Reference

Introduction
 Geographic Information System is an
integrated system of computer
Hardware and Software, coupled
with procédures and human analyst
which together support the capture,
management, manipulation, analysis,
modelling and display of spatially referenced data.
 GIS can store many different types of data on one map.
 This enables people to more easily see, analyze and
understand patterns and relationships.

Components of GIS
GIS consists of 5 key components:

 Hardware: it consists of a computer system on which the
GIS software will run.
 Software: GIS software provides the functions and tools
needed to store, analyze, and display geographic
information.
 People: GIS users range from technical specialists who
design and maintain the system to those who use it to help
them perform their everyday work.
 Method: a successful GIS operates according to a well-
designed plan and business rules, which are the models
and operating practices unique to each organizations.
 Data: Geographic data and related tabular data that can be
collected in-house. Digital map forms the basic data input
for GIS.

Data Types in GIS
The data in a GIS can be classified into two main
categories:
1. Spatial data
Describes the absolute and relative location of
geographic features.
2. Attribute data or Non-spatial data
Describes characteristics of the spatial features.
These characteristics can be quantitative and/or
qualitative in nature.

The Data Model
 Data model is a conceptual description (mental
model) of how spatial data are organized for use by
the GIS.
 The data model represents a set of guidelines to
convert the real world (called entity) to the digitally
and logically represented spatial objects consisting
of the attributes and geometry.
 The attributes are managed by thematic or semantic
structure while the geometry is represented by
geometric-topological structure.

There are two major types of geometric data model ; vector
and raster model.
a. Vector Model
Vector model uses discrete points, lines and/or areas
corresponding to discrete objects with name or code
number of attributes.
b. Raster Model
Raster model uses regularly spaced grid cells in specfic
sequence. An element of the grid cell is called a pixel
(picture cell). The conventional sequence is row by row
from the left to the right and then line by line from the top
to bottom. Every location is given in two dimensional
image coordinates ; pixel number and line number, which
contains a single value of attributes.

0 1 2 3 4 5 6 7 8 9
0 R T
1 R T
2 H R
3 R
4 R R
5 R
6 R T T H
7 R T T
8 R
9 R
Real World
Vector Representation
Raster Representation
Concept of
Vector and Raster
line
polygon
point

Data models in GIS
 RASTER DATA MODEL
 VECTOR DATA MODEL
 TRIANGULATED IRREGULAR NETWORK
MODEL(TIN)
 DIGITAL ELEVATION MODEL (DEM)
 NETWORK MODELS

The term raster implies a regularly spaced grid . Raster data
consists of rows and columns of cells (or pixels). In this format a
single value is stored against each cell. Raster data can represent a
multiplicity of things including:
• Visual images (that is colour and/or hue)
• Discrete value, such as land use
• Continuous value, such as rainfall
• Null values if no data is available.

Cell Size of Raster Data
 The level of detail represented by a raster is often
dependent on the cell (pixel) size or spatial resolution
of the raster. The cell must be small enough to capture
the required detail but large enough so computer
storage and analysis can be performed efficiently.

Smaller cell size
 Higher resolution
 Higher feature spatial
accuracy.
 Slower display
 Slower processing
 Large file size
Larger cell size
 Lower resolution
 Lower feature spatial
accuracy
 Faster display
 Faster processing
 Smaller file size

Advantages of Raster
 It is a simple data structure.
 It has the ability to represent continuous surfaces and
perform surface analysis.
 The ability to uniformly store points, lines, polygons
and surfaces.
 The ability to perform fast overlays with complex
datasets.

Disadvantages of Raster
 There can be spatial inaccuracies due to limits
imposed by raster dataset cell dimension.
 Raster datasets are potentially very large. Resolution
increases as the size of cells decreases. Accordingly
cost and disk space used also increases.
 There is also a loss of precision that accompanies
restructuring data to a regularly spaced raster cell
boundary.

 Vectors are graphical objects
that have geometrical
primitives such as points,
lines and polygons to
represent geographical
entities in the computer
graphics.
 A vector refers to a
geometrical space which has
a precise direction, length
and shape
 Points, Lines and Polygons
can be defined by the
coordinate geometry.

 A vector spatial data model uses two-dimensional
Cartesian (x, y) coordinate system to store the shape of
a spatial entity.

 In vector world the point is the
basic building block from which
all spatial entities are
constructed.
 The simplest spatial entity, the
point, is represented by a single
(x, y) coordinate pair.
 Line and area entities are
constructed by connecting a
series of points into chains and
polygons.

Point
 A point is a 0 dimensional
object and has only the
property of location (x,y)
 Points can be used to
Model features such as a
well, building, power pole,
sample location etc.
 Other names for a point are
vertex, node, 0-cell.

Line
 A line is a one-dimensional
object that has the property of
length
 Lines can be used to represent
road, streams, faults, dikes,
marker beds, boundary, contacts
etc.
 Lines are also called an edge,
link, chain, arc, 1-cell
 Connected multiple lines are
called polylines.

Polygon
 Polygon features are made of
one or more lines that
encloses an area.
 A polygon is a two-
dimensional object with
properties of area and
perimeter represented by a
closed sequence of lines.
 A polygon can represent a
city, geologic formation,
dike, lake, river, etc.

Advantages of Vector
• Requires less disk storage space.
• Efficient for topological relationship
• Graphical output more closely resembles hand-drawn
maps.
• Easy to edit
• Accurate map output
• Efficient projection transformation

Disadvantages of Vector
• Complex data structure.
• Less compatibility with remotely sensed data.
• Expensive software and hardware.
• Not appropriate to represent continuous data
• Overlaying multiple vector are often time consuming.

Difference between Raster and Vector
Raster Vector
 It is a simple data structure.
 Overlay operations are easily
and efficiently implemented.
 High spatial variability is
efficiently represented in a
raster format.
 The raster format is more or
less required for efficient
manipulation and
enhancement of digital images.
 More complex data structure.
 Overlay operations are more
difficult to implement.
 The representation of high
spatial variability is inefficient.
 Manipulation and
enhancement of digital images
cannot be effectively done in
the vector domain.

 The raster data structure is
less compact.
 Topological relationships are
more difficult to represent.
 The output of graphics is less
aesthetically pleasing because
boundaries tend to have a
blocky appearance rather
than the smooth lines of
hand drawn maps. This can
be overcome by using very
large number of cells, but it
may result in unacceptably
large files.
 Vector provides a more
compact data structure.
 Provides efficient encoding of
topology.
 The vector data model is
better suited to supporting
graphics that closely
approximate hand-drawn
maps.

Triangulated Irregular Network

What is a TIN?
 TIN stands for Triangular Irregular Network,
which is a vector approach to handling a digital
elevation model. TIN’s are used to interpolate
surfaces using multiple triangles.
 TIN’s are able to interpolate surfaces by selecting
representative points that are usually data points.
TIN’s connect these points to form a set of
continuous and connected triangles. The data
points consist of X, Y and Z values. The final result
gives users a TIN surface.

Advantages of TIN
 TIN’s give researchers the ability to view 2.5D and 3D
at an area that was interpolated from minimal data
collection.
 Users can describe a surface at different levels of
resolution based on the points that were collected.
 TIN interpolation gives GIS users greater analytical
capabilities. TIN models are easy to create and use.
 They provide users a simplified model that represents
collected data points.
 Using a TIN surface in conjunction with ArcMap
extensions such as Spatial Analysis and 3D Analyst,
TIN users can also derive slope, aspect, elevation,
contour lines, hillshades, etc.

Different Types of TIN Methods
and Processes
There are many different types of TIN
interpolation methods.
Some of the most popular TIN methods include
 Natural Neighbour,
 Krigging,
 Spline,
 Nearest Neighbour and
 Inversed Distance Weighting.
 These TIN interpolation methods use
mathematical algorithms in order to generate
interpolated surfaces. Each of these methods will
produce different types of surfaces.

DEM
 Digital Elevation Model is a data model which
represents the surface of a terrain in 3 dimension.
 DEM can be represented as a raster or as TIN.
 The TIN DEM dataset is also referred to as a primary
DEM or measured DEM.
 Raster DEM is referred to as secondary DEM or
computed DEM.

3D rendering of DEM of Tithonium Chasma on Mars

Conclusion
 Geographic Information System is a computer system for
capturing, storing, checking, and displaying data related to
positions on Earth’s surface.
 A data model in GIS is a mathematical construct for
representing geographic objects or surfaces as data.
 The two basic data models of GIS are Raster and Vector. Other
important data models are TIN (Triangulated Irregular
Network) and DEM (Digital Elevation Model).
 Raster consists of matrix of cells organized into rows and
coloumns where as vector represents data using points, lines
and polygons. In TIN data is represented by irregular triangles.
DEM can be represented as a raster or as TIN.

Reference
 M. Anji Reddy, Textbook of Remote Sensing And
Geographical Information System, IV edition, BS
Publications, pp. 323-355.
 S. Kumar, Basics of Remote Sensing and GIS,
University Science Press, pp. 69-100.
 http://bgis.sanbi.org/GIS-primer
 www.education.nationalgeographic.org
 www.wikipedia.org > Geographical Information
System

Data models in geographical information system(GIS)

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