DATA in GIS and DATA Query

Data in GIS & Data Query
MASHHOOD ARIF

Contents
Types of Data
Layers in GIS
Querying GIS Data
Data Integration: Overlay

Types
Spatial & Attribute Data
Raster & Vector
Geo-referencing Data
Layers of Data

Attribute & Spatial Data
ATTRIBUTE
 Says what the feature is
 Eg: statistics, text, images,
sound etc.
SPATIAL
Says where the feature is
Coordinate based
Vector Data
Discrete Functions
Points
Lines
Polygons
Raster Data
Continuous Surface

Attribute & Spatial Data Criteria
Attribute
 Explains about spatial data
 Relevant non-spatial data
 Words or numbers
 Qualitative methods
 Quantitative methods
Spatial
 X-Y coordinates
 Shape
 Area / Shape
 Perimeter
 Distance
 Neighborhood

Attribute Data
 Attributes can be numeric or alpha-numeric
 Data that can be assigned to a point, line or area spatial features
 Example Attributes… Stand ID, Compartment No., Vegetation type,
Name of the Forest Block, Types of Road, VSS code etc.

Spatial Data
Nature of Spatial Data
 Spatial component
Relative position between objects
Coordinate system
 Attribute component
Explains spatial objects characteristics
 Spatial relationship
Relationship between objects
 Time component
Temporal element
Characteristics of Spatial Data
 Location
Description
Grid Reference
Latitude / Longitude
 Geometry
Shape
Route
Landscape
 Topology
Connected to
Within
Adjacent to
North to

GIS Data Formats
There are two formats used by GIS systems to store and retrieve geographical
data:
 Raster
 Vector

Raster Format
 A grid (or raster) system stores data as a string
of characters in which each character
represents a location
 Data are divided into cell, pixels, or elements &
each cell/Pixel is assigned only one value
 Cells are organized in arrays and array of Pixels
form the entity-Point, Line, Area and surface
 The shape and size of the array determines the
basic Resolution
 Polygons can be formed indicating areas of
homogeneous characteristics
 The most common example of raster data is a
digital image
Vector Format
 A vector system usually stores data as
coordinates
 Data are associated with points, lines, or
boundaries enclosing areas
 In a vector based system every point is recorded
by a pair of x and Y coordinates
 Straight line segments called vectors are
displayed to indicate line based data ( roads
rivers wells)
 Most spatial features can be displayed as:
Points-Line- Polygons

Comparison between Raster & Vector
RASTER
 Raster formats are efficient when comparing
information among arrays with the same cell
size
 Raster files are generally very large because
each cell occupies a separate line of data,
only one attribute can be assigned to each
cell, and cell sizes are relatively small
 Raster representations are relatively coarse
and imprecise
VECTOR
 Vector formats are efficient when comparing
information whose geographical shapes and
sizes are different
 Vector files are much smaller because a
relatively small number of vectors can
precisely describe large areas and many
attributes can be ascribed to these areas
 Vector representations of shapes can be very
precise

Geo-referenced Data
Capturing Data
Scanning: all of map converted into raster data
Digitising: individual features selected from map as points, lines or polygons
Geo-referencing
 Initial scanning digitising gives co-ordinates in inches from bottom left corner of
digitiser/scanner
 Real-world co-ordinates are found for four registration points on the captured
data
 These are used to convert the entire map onto a real-world co-ordinate system

Layers in GIS
 Data on different themes are stored in separate “layers”
 As each layer is geo-referenced layers from different sources can
easily be integrated using location
 This can be used to build up complex models of the real world from
widely disparate sources

Layers in GIS (Contd..)
Importance
 Geographic data = representation of reality
 Reality is complex
 GIS uses a layer approach
 Each layer includes information about one type
of phenomenon
 Data layers must be aligned with one another
 Proximity: Finding what is near or within a
distance from a certain location or feature
 Overlay: Combining two layers to create new
information (eg: habitat based on veg, elevation & temp)

Querying GIS Data
Attribute query
Select features using attribute data (e.g. using SQL)
Results can be mapped or presented in conventional database form
Can be used to produce maps of subsets of the data or choropleth maps
Spatial query
Clicking on features on the map to find out their attribute values
Used in combination these are a powerful way of exploring spatial
patterns in your data

Spatial data: Registration
Districts, 1/1/1870
Attribute data: Mortality rate
per 1,000 from lung disease
among men aged 45-64
Source: Registrar General’s
Decennial Supplement, 1871
Query: Select areas where
mortality rate > 58.0
Attribute query: Lung disease in the 1860s

District: Alston with Garrigill
County: Cumberland
M_rate: 68.4
Spatial query: Lung disease in the 1860s

Mapping through attribute query

Data Integration: Overlay
Joins two layers to create
a new layer
The output layer will
contain both the spatial
AND attribute data from
both of the input layers

Conclusion
 Data are the “heart” of any geographic information system
 Data becomes valuable when it is analyzed and acted upon
 Positioned by its known spatial coordinates
 Input and organized (generally in layers)
 Stored and retrieved
 Analyzed (usually via a Relational DBMS)
 Modified and displayed

DATA in GIS and DATA Query

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