GPS & GIS – An

Introduction

Where Will This Take Us?• What is GPS? What is GIS?• How do GPS and GIS work?• How will they help us?

? Find

This!

What is GPS?

• GPS stands for Global Positioning System

• GPS is a satellite navigation system designed to provide accurate position, velocity, and time information almost anywhere in the world

• the present system is known as NAVSTAR - NAVigation Satellite Timing And Ranging

• the GPS program operated and controlled by the US Department of Defense

How does GPS work?

• Each satellite broadcasts its orbital position as a modulated signal

• Receiver determines the difference between the time signal from the satellite and its own internal clock

• The distance from the satellite can be calculated using the speed of light (300,000 km/second)

• Position and distance from 3 or 4 satellites allows triangulation of receiver’s position

Major GPS Components

Space Segment

• A constellation of 24 orbiting satellites with 3-4 spares

• Orbit at a distance of 11,000 miles

• Each satellite contains several very precise atomic clocks

• orbit roughly every 12 hours, but they are not geosynchronous

Control Segment

• There are five Control Stations around the world• Stations monitor satellites as they pass

overhead by measuring the distance from the Control Station to the satellites

• Master Control Station determines satellite conditions (orbit, clock, health status) and calibrates the satellite’s clock

User Segment• Accuracy is dependent on the kind of receiver • Navigation (Recreation) grade – least

accurate• Mapping grade – accurate• Survey grade – most accurate• Costs range from $100 to $30,000

Error in Satellite Signals

DOP = Dilution of Precision (This means that the signal degrades)

Error in Satellite Signals

DOP projection for 23 January 2009 (Our data collection day)

Geographic Information Systems - GIS

• GIS is a spatial database with a strong visual display component

• GIS is not primarily a mapping program

• GIS was developed in the early 1980s, but its acceptance and development outside of the environmental sciences has been slow

GIS Data Models

• Raster Model– The first GIS model developed– Based on grids of cells that are assigned

values and grouped into layers

• Vector Model– Uses points, lines, and polygons define

data classes– Grouped into themes or layers

GIS – Raster Data Model

COLUMNS

GridCell

ROWS

GIS – Raster Data Model (cont)

House

Archaeological Site

Road

GIS – Raster Data Model (cont)

GIS – Raster Data Model (cont)

S S

S

S

S

S

S

S

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R

R

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R

R

R

H

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GIS – Vector Data Model

House

Road

Archaeological Site

GIS – Vector Data Model (cont)

House

Road

Archaeological Site

GIS Models - Combined

• We’ll use a combination of raster imagery and vector layer files to build our geo-database

• Let’s see a real-world application