Lecture 4 - Geovisualization

Module 2

A. Data types

B. Data collection

C. Effective communication

D. Data visualization

A. B. C. D.

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A. Refers to techniques to

encode data as visuals

B. Helps choosing the best

design

C. A way to present data

A. B. C.

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A. Make it colourful

B. Make the message clear

and effective using

graphics

C. Hide the facts from the

audience

A. B. C.

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A. True

B. False

A. B.

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A. True

B. False

A. B.

50%50%

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A. Number and title

B. Number, title and column

headings

C. Title and column

headings

D. Source, column headings,

title and number

A. B. C. D.

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15

A. True

B. False

A. B.

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A. Chart type design, axis scale and labels, data labels, formatting captions

B. Chart type recommended by Excel

C. Chart axis, data labels and right colour

A. B. C.

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“Geo”: from Greek “gē” - earth

… or Geographic visualization:

maps and graphics are used to gain insight

from geographic information (supported by

spatial data)

"Hekateaus7m" by Dariusz Ciach, 2003. Licensed under CC BY-SA 2.5 via Wikipedia - http://en.wikipedia.org/wiki/File:Hekateaus7m.jpg#/media/File:Hekateaus7m.jpg

Hecatæus's map (500 BC)

Describes the earth as a circular plate with an encircling Ocean and

Greece in the centre of the world.

As many other early maps in antiquity his map has no scale.

Units of measurements: "days of sailing" on the sea and "days of

marching" on dry land.

The purpose of this map was to accompany Hecatæus's geographical

work that was called Periodos Ges, or Journey Round the World.

John Snow’s map showing the clusters of

cholera cases in the London epidemic of 1854

Dr. Snow was able to establish a geographic

association between cholera cases and a water

pump by visual analysis of its location.

This pattern was then used to infer the relationship

between the disease and drinking water.

https://www.youtube.com/watch?v=Nun95_cQzBg

Maps provide a spatial context that aids

information-processing and visualization.

This has a huge impact on the world today

where most decisions are data-driven and rely

on visuals to facilitate that process.

Step 1 – Consider what the real world distribution of the phenomenon might look like

Step 2 – Determine the purpose of the map and its intended audience

Step 3 – Collect data appropriate for the map’s purpose

Step 4 – Design and construct the map

Step 5 – Determine whether users find the map useful and informative.

(Slocum, McMaster, Fritz & Howard, 2009)

Spatial is often used interchangeably with geographic:

geographic refers to the Earth’s surface and near-surface whereas spatial refers to any

space, not only the space of the Earth’s surface.

Geospatial data is a compromise between these two concepts.

Geospatial data must contain a location and that could be an address or coordinates.

Geospatial data can be geographically referenced or georeferenced.

Georeferencing: act of assigning locations to data

Examples:

• Giving names to places is the simplest form of georeferencing (e.g. street

address)

• Metric georeference (geographic coordinate systems)

• Geographic information (values, character strings or symbols that convey to the

user information about the location of the feature being observed)

• Temporal information (a record of when the data was collected)

• Thematic information (description of the real-world feature to which the data refers

- attributes)

The ingredients we need for a Geospatial Revolution!

• Environmental resources

• Utilities (gas, water, electricity

lines, cables namely the location

of these networks)

• Administrative, survey and census

data

• Topography

• Remotely sensed data (data

captured by satellites)

Objects (vector data in GIS – points, lines, polygons)

Or

Layers (raster data in GIS - cells/pixels with a specific width and height, displayed

in a grid)

Adequate representation depending on the application:

urban planning, civil engineering, public services, medical and humanitarian

organizations, business

• Geospatial data can be generated from already available sources such as

satellite images and maps or it has to be collected in the field.

Remotely sensed data (satellite images, LIDAR, aerial photographs) and digitized

and/or scanned maps provide geospatial data for larger areas and/or when specific

thematic data is required.

Field data needs to be collected when data does not exist in any other format or it is

not provided with the required spatial and temporal resolution.

Two important types of field data are survey data and Global Positioning Systems

(GPS) data.

Using remote sensing (satellite

images) for land use mapping –

thematic geospatial data (raster)

http://www.earthzine.org/wp-content/uploads/2014/07/photo-333.png

Surveying equipment (theodolite) Field survey collecting data to define a parcel boundary

http://cdn.screenrant.com/wp-content/uploads/James-Bond-Gadget-GPS.jpg

http://jamesbond.wikia.com/wiki/Goldfinger?file=Bon

d_18_Now_Pay_Attention_%28Goldfinger%29

https://www.youtube.com/watch?v=Kns0-V_sXLQ

http://www.pushing-pixels.org/wp-content/uploads/2013/04/skyfall-maps.jpg

https://storyfountain.files.wordpress.com/2012/11/darker-2.jpg

GPS data is collected using a portable device that

can be easily carried around

GPS devices are used for many

purposes and not only to collect field

data

GPS applications can be grouped into

five broad categories:

location, navigation, tracking, timing

and mapping

Location (“Where am I?”)

Knowing the precise location of

something or someone

GPS are integrated within modern

smartphones to support location-based

services and apps

(Zipf & Jost, 2012)

Navigation (“Where am I going”?)

GPS technology is broadly used for navigation on water, in the air and on the land.

It provides navigation information to establish routes to reach specific locations.

Tracking (“Where is it going?”)

GPS tracking is the process of monitoring it as it moves along.

GPS used in conjunction with other software can provide the backbone for systems

tailored to applications in agriculture, mass transit, urban delivery, public safety,

and vessel and vehicle tracking

http://www.bbc.co.uk/britainfromabove/stories/visualisations/taxis.shtml

Timing ("When will it all happen?")

GPS is used to disseminate precise time, time

intervals, and frequency. GPS makes the job

of "synchronizing our watches" easy and

reliable.

Mapping (“Where is everything else?”)

Mapping uses geospatial data collected by

GPS to create maps and models of the real

world from “mountains, rivers and forests”, to

“roads, routes and city streets” and even to

“endangered animals, precious minerals and

natural disasters”. http://www.avenza.com/pdf-maps/features

Global Positioning Systems (GPS) use

signals from satellites to uniquely identify

a location anywhere on Earth, 24 hours a

day, in any weather condition

https://doms.csu.edu.au/csu/thumbs/

ae5d79f0-a5e0-4768-8883-

3ba17231e855/1/3cda58d8-da76-437f-

a279-e77274ce7bde

SATELLITES

• Satellite is an artificial body placed in orbit round the Earth to collect location data

• Launched as rockets to a very precise orbit

• Positions are exact and are constantly monitored (by ground stations)

• Powered by solar energy

• Weights approximately 900 kg and is about 5 m across with the solar panels

extended

• Transmit radio signals to communicate with receivers

About 40 years ago, GPS was created as the first

satellite navigation system (NAVSTAR).

It was developed by the US Department of

Defence (it was originally intended for military

applications but, in the 1980s, the government

made the system available for civilian use).

https://doms.csu.edu.au/csu/thumbs/5ea5fd2c-be78-47fd-a24e-b7f655341a1f/1/a5c5d1cb-41c2-4459-8ff2-83a65fa17047

Other additional systems have been developed by

other countries so, today, many other satellites

populate the Earth’s orbits forming a satellite

constellation known as the Global Navigation

Satellite Systems (GNSS).

Multi-constellation GNSS first became widely available in 2010/2011, but only as two

constellations: GPS and GLONASS (the Russian system).

In 2012, the European system Galileo and the Chinese system BeiDou started

operating too.

• More satellites are available to send signals to the receivers therefore increasing

the accuracy of positioning.

• On average there are between 5 and 8 satellites visible in the sky at any time from

any point on Earth.

• With the development of the multi-constellation GNSS we can expect this number

increase to up to 20 satellites by 2020.

Monitor GPS satellites, check their

vitality and exact position in space,

calibrate their signals for time and

place, manage accuracy available

to users

https://doms.csu.edu.au/csu/thumbs/c1d705b7-f478-4b1a-840f-d3e39637acaa/1/7ebc04dd-299c-4bbf-bf31-568ba7052820

https://doms.csu.edu.au/csu/thumbs/cd63f304-04ca-4189-9adf-df92ca692caa/1/20782c83-2def-4096-8e00-5792a29ab66b

• A handheld, or a device mounted on a vehicle, plane,

boat; or a unit displaying position, speed and

navigation.

• Using GPS satellites in space as reference points, a

GPS receiver can determine its precise position on

the Earth’s surface.

• GPS data includes horizontal location based on a

geographic (latitude, longitude) or projected

(eastings, northings) coordinate system and, if

available, the height (z) of the point location.

Satellites and receivers transmit similarly coded radio signals.

The time delay between transmission and receipt of the signals gives the distance between the

satellite and the receiver.

• The time difference tells the GPS receiver how far away the satellite is.

• With distance measurements from a few more satellites, the receiver can determine the

user's position and display it on the unit's electronic map.

• A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D

position (latitude and longitude).

• With four or more satellites in view, the receiver can determine the user's 3D position

(latitude, longitude and altitude).

• A GPS user will see a position fix displayed on their receiver. The accuracy obtainable from

GPS receivers ranges from 100 m to as little as a few millimetres depending on the type of

technology installed.

https://doms.csu.edu.au/csu/thumbs/ed6ece58-c9a8-47b7-b2ce-8c632303cca6/1/b3f4a506-7b12-43f9-a50a-ecf10820095c

The basis of GPS is triangulation from satellites

(or trilateration - method of determining the

relative positions of objects using the geometry of

triangles)

To "triangulate," a GPS receiver measures distance

using the velocity and travel time of radio signals.

Distance = Velocity * Time

Velocity of light Time it takes for the

signal to arrive at

the receiver

DISTANCE

• If we know the distance from one satellite to the

receiver, then that puts us somewhere on a sphere

that is centred on the satellite.

• Knowing the distance from two satellites to the

receiver places you on a circle that’s between the

two satellites – the intersection of the two spheres.

• Distances from three satellites will intersect at two

points on that circle.

• Three satellites are enough to pinpoint a location

on earth

• Both the satellite and the receiver are synchronised – the difference (in sync) of

the receiver time and the satellite time tells us the travel time.

• Each satellite has a unique code (a Pseudo Random Code) – this code is used to

figure out the difference in synchronisation.

• Timing on the satellite side is almost perfect because they have incredibly

precise atomic clocks on-board. Clocks in receivers are less accurate.

• An error of a thousandth of a second translates to 320km.

• Three satellites can locate a unique point on earth. Adding a fourth satellite can

correct for timing errors.

Despite 24-hour global coverage, GPS use can be hampered by certain factors

that can degrade the GPS signal and thus affect accuracy.

• Atmospheric errors

• Signal multipath

• Receiver clock errors

• Orbital errors

• Number of satellites visible

• Satellite geometry/shading

Accuracy errors around 10 m

• Atmospheric errors

The speed of light (velocity) is used to calculate the distance between satellites and

receivers. A GPS signal has to pass through the ionosphere and the troposphere,

which introduce errors in the calculation.

• Signal multipath

The GPS signal is reflected off objects such as tall buildings, trees or large rock

surfaces before it reaches the receiver. This increases the travel time of the signal

thereby causing errors.

• Receiver clock errors

A receiver's built-in clock is not as accurate as the atomic clocks onboard the GPS

satellites which introduces very slight timing errors.

• Orbital errors

These include inaccuracies of the satellite's reported location (to be corrected by

ground stations).

• Number of satellites visible

The more satellites a GPS receiver can "see," the better the accuracy. Buildings,

terrain, electronic interference, or sometimes even dense foliage can block signal

reception, causing position errors or possibly no position reading at all.

• Satellite geometry/shading

This refers to the relative position of the satellites at any given time. Ideal satellite

geometry exists when the satellites are located at wide angles relative to each other.

Poor geometry results when the satellites are located in a line or in a tight grouping

• Many of the errors can be removed or

reduced using Differential GPS (DGPS).

• Differential GPS techniques require two

receivers, one fixed at a known location (the

base station) and the other at an unknown

location (the roving receiver):

one receiver measures the timing errors and

provides correction information to other

receivers that are roving around.

https://doms.csu.edu.au/csu/thumbs/1c21d3a8-0cfe-42a7-b65b-58ccd4ccdede/1/f4457424-7c30-4c01-b518-8d59b37205c8

• Using its known position the reference

receiver can calculate what the travel time of

the GPS signals should be.

• The reference receiver computes error

correction factors for all visible satellites.

These correction factors are then available to

GPS receivers covered by the reference

station

Next lecture:

Geographic coordinate system

Projected coordinate system

Effective maps

- This lecture is included in

Module 2.

- GPS apps for outdoor

activity:

Polaris Navigation GPS

(Android)

GPS Receiver HD (iphone)