Geographical Information System (GIS)

INTRODUCTION TO
GEOGRAPHICAL INFORMATION SYSTEM
UNIT – II
Dr. Rambabu Palaka, Professor, BVRITOct. 2020

Topics
1. Introduction to GIS
2. Components of GIS
3. Types of Data
o Spatial Data
o Non-Spatial Data
4. GIS Operations
5. Coordinate Systems
6. Datum
7. Map Projections
8. Raster Data Compression Techniques
9. GIS Software
10. Free GIS Data Resources

Data vs Information
 Data, by itself, generally differs from information.
 Data is of little use unless it is transformed into information.
 Information is an answer to a question based on raw data.
 We transform data into information through the use of an Information System.

What is GIS?
A Geographic Information System (GIS) is a
computer system for capturing, storing,
checking, and displaying data related to
positions on Earth's surface.

Geographic Information Systems (GIS)
Geography – The study of where features are located on the Earth’s surface.
Geographic coordinate systems
latitude
longitude
- Represent exact positions on the Earth
- 42.3216, -71.089118 (lat. long. for Boston)
Georeferencing / Geocoding – The process of assigning geographic
coordinates to features to represent their location.

GIS Functions
• Capture: Paper maps, digital data, coordinates, GPS, all together = gis data
• Store: Vector (database consisting of points, lines and polygons) or raster
formats (grid cells or pixels)
• Query: Identifying specific features, identify features based on conditions
(counties with population greater than 500,000)
• Analyze: Proximity (parcels 100 feet from the road), overlay, network analysis
(how linear features are connected
• Display: Maps, graphs, Reports
• Output: Paper maps, image

• Common Manipulation
• Query
• Reclassification
• Map Projection changes
• Spatial Analysis
• Buffering
• Overlay
• Network
Spatial Data Manipulation and Analysis

Types of Data
1. Attribute data:
Says what a feature is
Ex.: statistics, text, images, sound, etc.
2. Spatial data:
Says where the feature is
 Vector data: Discrete data
o Points
o Lines
o Polygons (zones or areas)
 Raster data: Continuous data

Representation of Spatial Elements

Data Model
A geographic data
model is a structure for
organizing geospatial
data so that it can be
easily stored and
retrieved.
Geographic coordinates
Tabular attributes

File-based Data Models
 Coverages
 Developed for
workstation Arc/Info
~ 1980
 Complex structure,
proprietary format
 Attributes in Info
 Shapefiles
 Developed for ArcView
~ 1993
 Simpler structure in
public domain
 Attributes in dBase
(.dbf) tables
Geographic coordinates and attributes
are stored in separate but linked files
Arc
Info

Geodatabase model
 Stores geographic coordinates as one
attribute (shape) in a relational database
table
 Uses MS Access for “Personal
Geodatabase” (single user)
 Uses Oracle, SQL Server, MySQL or other
commercial relational databases for
“Enterprise Geodatabases” (many
simultaneous users)
Shape

Raster Data - Compression Techniques
Raster data is comprised of grid cells. Vector data is comprised of vertices and paths.
These are the two primary spatial data types in GIS.
Raster Data:
Accuracy depends on cell size. Cell size must be small enough to capture the required detail. And as
resolution increases, the size of the cell decreases. But this comes at a cost for speed of processing and data
storage.
Raster storage in its raw state is inefficient because it is normally stores values row-by-row from the top left
corner.
The way to improve raster size efficiency is through image compression. And there are several ways to use
GIS compression to reduce file size and still maintain quality of data.

Full raster coding (no-compression)
By convention, raster data is normally stored row by row from the top left
corner.

Types of Compression Techniques
Types of Compression Techniques:
1. Run Length Encoding – Grouping Rows of Data
2. Block Coding – Grouping Blocks of Data
3. Chain Coding – Defining the Exterior Boundary
4. Quadtree Encoding – Subdividing Data Into Quarters

1. Run Length Encoding – Grouping Rows of Data
Run length encoding stores cells on a row-by-row basis. Instead of recording each
individual cell’s values, run length encoding groups cell values by row.
Take this line of data:
AAAAAABBBBCCCCCCCCC
It can be rendered as:
6A4B9C
This image encoding method reduces data volumes because each line is recorded
more efficiently. Even though the same information is being held, values that are the
same are stored as a string.

In this example, the first row is blank and is stored as (0,8). This means there are
8 cells and they are all zeros. In the second row, there are 4 consecutive zeros so
it gets a value of (0,4). After this, we have three consecutive cells with the value 1
so it gets a value of (1,3). This continues until it reaches the bottom-right cell.

2. Block Coding – Grouping Blocks of Data
The block coding raster storage technique assigns areas that are blocks to reduce
redundancy.
The block coding raster image compression method subdivides an entire raster
image into hierarchical blocks. It’s an extension of the run length encoding
technique, but extends it to two dimensions.

In the example above:
Instead of storing 64 grid cells, all it takes is just 7 blocks. Using block coding, it
requires one 3×3 block, two 2×2 blocks and four 1×1 cell blocks to encode this raster
image.
In this block coding example, the top-left corner is used as a reference for each block.

3. Chain Coding – Defining the Exterior Boundary
Chain coding defines the outer boundary using relative positions from a start point.
The sequence of the exterior is stored where the endpoint finishes at the start point.
During the encoding, the direction is stored as an integer. However, in this example
we use cardinal directions for simplicity. For example, the value 0 is north and 1 is
east.

During the encoding, the direction is stored as an integer. However, in this example
we use cardinal directions for simplicity. For example, the value 0 is north and 1 is
east.
In the example, we start at position (5,2). From here we define the border using
cardinal directions and number of movements. We move east 3 positions until we hit
the edge. At this location, we move south 4 positions. This process continues until the
end point hits the start point.
Note: Only for the purpose of this exercise, we used north, east, south and west as

4. Quadtree Encoding – Subdividing Data Into Quarters
Quadtrees are raster data structures based on the successive reduction of
homogeneous cells. It recursively subdivides a raster image into quarters. The
subdivision process continues until each cell is classed.
It reduces raster storage requirements. It also is
dependent on the complexity of the feature and
the resolution of the smallest grid cell.
In the example, the top-left and bottom-right 8×8
grids do not need to be subdivided further
because they are homogeneous. The top-right
8×8 grid is subdivided into three 4×4 grid. The
remaining 4×4 grid is separated into 4 individual
classes.

GIS Software
GIS Software:
Google Earth Pro – To View and Download Historical Satellite Image Data
Two Versions:
Web & Desktop
1. QGIS – Open Source
2. ArcGIS – ESRI’s Product - Commercial Software
3. AutoCAD Map – Autodesk’s Product (No Support for Analysis, But useful for Data Creation)
- Commercial
4. Global Mapper – Free and Commercial
5. MapInfo - Commercial
6. Small World - Commercial
7. Bentley Map - Commercial

Free GIS Data Resources
Free GIS Data Sources:
ISRO-NRSC Bhuvan Website - https://bhuvan.nrsc.gov.in (for Indian Satellite Data)
USGS - https://www.usgs.gov (for World Satellite Data)
Google Earth Engine - https://earthengine.google.com
(For World Satellite Data Processing & Analysis using JavaScript / Python script without Downloading Data
and without GIS Software)
Free GIS Data Viewers:
Google Earth (Web Version) - https://earth.google.com/web
Google Earth Pro (Desktop Version) - https://www.google.com/earth/versions/#download-pro
(To view and download Historical Image Data)

Reference
Textbook of Remote Sensing and
Geographical Information Systems
M. Anji Reddy
BS Publications

Geographical Information System (GIS)

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Geographical Information System (GIS)