This document discusses key concepts related to data in GIS systems. It describes the different types of spatial and attribute data as well as vector and raster data formats. It explains how data is organized into layers and how those layers can be queried and overlaid to integrate information from different sources and analyze spatial patterns in the data.

1. Data in GIS & Data Query
MASHHOOD ARIF

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

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

4. 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

5. 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

6. 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.

7. 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

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

9. 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

12. 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

13. 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

14. 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

15. 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)

16. 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

17. 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

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

19. Mapping through attribute query

21. 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

22. 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