L6 GIS Data Sources – Part 1

Highlights of Chapters 5 & 6

Introduction

• The most expensive part of a GIS project is gathering the data and the construction of the database.

• In the past, people frequently began by digitizing paper maps.

• Today we begin by looking online for data.• However data collection is still important

as public data may not meet your particular needs.

Primary vs. Secondary Data

• Primary data is data that you collect yourself.– A questionaire keyed to an address– GPS coordinates– Survey data– Photographs

• Secondary data is data obtained from another source.

GIS Applications

GIS APPLICATION

DigitizingFrom Maps or

PhotosGPS &

Surveying Data

CAD Input/ Conversion

Images

DownloadedGIS Data

AddressesTabular

Data

MAPS Tabular Information

Digital Information

Coincidental Data

Manual Digitization – Map Digitization

Digitizing TabletOn-screen Digitizing/

Heads-up Digitizing

Field MeasurementCoordinate Surveying

GPS

(courtesy NGS)

Global Positioning System

• Initially developed by the U.S. Department of Defense for military use.

• Still maintained by DOD today, but can also be for civilian use.– Navigation in your car.– Finding your favorite fishing hole.– Mapping trails or roads.– Collecting data.

Global Positioning System

• Location, Speed, and Timing...The 3 keys to GPS.• The GPS receiver must have 3 things to calculate

distance:– 1.) Exact location of the satellites– 2.) The Speed at which the radio signal from the satellites is

traveling– 3.) Very accurate timing to track the time it takes for the signal

to go from satellite to receiver

How does GPS work?

• Just as we depend on satellites for cellular phones and TV broadcasts, we also rely on satellites for GPS.

• In fact, GPS relies on 24 satellites that orbit the earth in very precise paths.

www.stoller-eser.com

How does GPS work?

• Imagine that a GPS unit communicates with only one of these 24 satellites.

• Then, the GPS can only make a large and general “You Are Here”.

www.stoller-eser.com

How does GPS work?

• With two, and even three and four satellites tocommunicate with, theGPS unit can make the“You Are Here” smallerand more precise.

How does GPS work?

• Communication with four satellites is usually enough to improve location accuracy to within about 10 meters.

• Some GPS units can provide location within 1 meter!

• Postprocessing can provide centimeter accuracy

Components of a GPS

• GPS uses three parts:– Space Segment

• satellites

– Control Segment• base stations

– User Segment• fighter jet• surveyor• hikerwww.aero.org/publications/GPSPRIMER/GPSElements.html

Space Segment

• 24 satellites in 6 orbital planes

• Satellites orbit at 11,000 nautical miles

(12,659 statute miles)• Each satellite orbits the

earth in 12 hours.• Each satellite broadcasts

a signal.www.stoller-eser.com

EXACT LOCATION OF SATELLITES

• The U.S. military creates a "master plan" for each of the satellites to set their orbits.

• This plan is constantly monitored for ephemeris (orbital) errors.

• Receivers download almanacs that tell them the exact location of the satellites.

SPEED OF THE RADIO SIGNAL

• Satellites actively send out a radio signal, which is assumed to be the speed of light.

• The speed of light is assumed to be constant, but in reality, it is not due to atmospheric interference.

SPEED OF THE RADIO SIGNAL

• When the radio signal reaches Earth, it can get reflected off objects (buildings, trees) before hitting the receiver. 

• This bouncing off objects other than the receiver is called Multi-pathing, and increases the travel time of the signal (thus creating error in the distance measurement).

Very Accurate Timing

• Receiver has a precise internal clock so it knows when it generated a signal, and when the satellite signal was received. Subtraction yields the time, which can be converted to distance.

• Note we assume satellite receiver clocks are in synch. Must make this assumption to calculate travel time.

• Unfortunately, this is not the case. Basically, receiver clock may be biased, so we need extra measurements.

• Satellites have extremely accurate atomic clocks, (cost $100k each). These are monitored and synchronized among satellites.

GPS

Position is estimated based on range measurements

Range = speed of light x travel timeRange = c(t1 – t2)

(c =299,792,458 meters per second)

Control Segment

• Base station receives signal and monitors each satellites exact location in space.

• Base station also maintains an atomic clock for precise measurements of signal travel time.

www.stoller-eser.com

GPSUser Segment

Trimble

Users with a device that records data transmitted by each satellite and processes this data to obtain three dimensional coordinates

Garmin Etrex

The Handheld GPS Receiver

Garmin eTrex Legend

http://silentflix.com/gps/features.html

Setting Up the Receiver

• Time –12 hour, US-Eastern• Units

– Position Format – UTM UPS– Map Datum – NAD83– Distance/Speed – Metric– Elevation – Meters– Vertical Spd – m/sec– Depth - Metric

Setting Up the Receiver

• Heading– Display – Cardinal Letters– North Reference - True

• Interface – Garmin

• System – GPS –Off/On– WAAS - Enabled

WAAS Wide Area Augmentation System

• GPS alone does not meet the FAA's navigation requirements for accuracy, integrity, and availability.

• The Federal Aviation Administration (FAA) and the Department of Transportation (DOT) are developing the WAAS program for use in precision flight approaches.

WAAS

• WAAS corrects for GPS signal errors caused by ionospheric disturbances, timing, and satellite orbit errors, and it provides vital integrity information regarding the health of each GPS satellite.

• WAAS consists of approximately 25 ground reference stations positioned across the United States that monitor GPS satellite data.

WAAS

• Two master stations, located on either coast, collect data from the reference stations and create a GPS correction message.

• This correction accounts for GPS satellite orbit and clock drift plus signal delays caused by the atmosphere and ionosphere.

• The corrected differential message is then broadcast through one of two geostationary satellites, or satellites with a fixed position over the equator.

GPS Accuracy

• Accuracy depends on:– Quality of equipment– Time over which observations are made.

• Source Amount of Error– Satellite clocks: 1.5 to 3.6 meters– Orbital errors: < 1 meter– Ionosphere: 5.0 to 7.0 meters– Troposphere: 0.5 to 0.7 meters– Receiver noise: 0.3 to 1.5 meters– Multipath: 0.6 to 1.2 meters– Selective Availability – User error: Up to a kilometer or more

GPS Accuracy

• Accuracy is best when the satellites used are widely spaced

www.stoller-eser.com

Sources of Signal Interference

Earth’s Atmosphere

Solid StructuresMetal

Electro-magnetic Fields

GPS Accuracy

Differential GPS

CORS – Continuously Operating Reference Stations

Remote Sensing measures electromagnetic energy reflected or emitted from objects –

airborne or satellite-based instruments

Aerial Photographs common relatively inexpensive easy to interpret small area coverage can be geometrically

corrected

Satellite Imageslarge area coveragebroader spectral rangedigital formatsinexpensive for large

areasgeometrically accurate

Two Main Image Types

What Information Can Be Remotely Sensed ?

Fundamental Variables• Planimetric (x,y) location and dimensions• Topographic (z) location• Color (spectral reflectance)• Surface Temperature• Texture• Surface Roughness• Moisture Content• Vegetation Biomass

Spectral Reflectance Curves

Wavelength (m)

% of energy reflected

Simple example of spectral signature (visible light only)

9,0,0

0,0,9

0,9,0

Red,Green,Blue

5,3,0

9,9,0(real sunlight is 9,9,9)

Source: http://imagers.gsfc.nasa.gov/teachersite/UL2ans.htm

A = Absorbed     R = Reflected

Object description Color reflected

Red Green

Blue

Ex. Red apple red R A A

Yellow block yellow R R A

Green block green A R A

Pinkish colored block (magenta)

magenta R A R

Blue block blue A A R

Turquoise colored block (cyan)

cyan A R R

Source: http://imagers.gsfc.nasa.gov/teachersite/UL2ans.htm

Visible Light

Simple example of spectral signature (visible light only)

9,0,0

0,0,9

0,9,0

Red,Green,Blue

5,3,0

9,9,0(real sunlight is 9,9,9)

Source: http://imagers.gsfc.nasa.gov/teachersite/UL2ans.htm

Aerial Photographs

Photos are usually scanned and converted to digital images for on-screen display and measurements

Scale most commonly controlled by 1. flying height2. lens focal length

Scale is approximately equal to f / Hf = focal lengthH = flying height

Scale is NOT Constant

• Can be over flat terrain with perfectly vertical photographs - rarely occurs

• Terrain - some objects are closer to lens, hence larger scale

• Tilt - causes perspective distortion

Terrain Variation – Causes Relief Displacement

Features aredisplaced radiallyfrom their planimetricposition due to differences in relative elevation

Tilt measured as the angle between a line perpendicular to the film and a line perpendicular to the datum.

Typically specified to be less than 3o on vertical aerial photos.

Tilt Distortion

Parallax -Relativeshift in pos-ition with achange in viewing loc-ation. Closer (taller) objectsshiftmore.We measureparallax toestimate height.

Overlapping Stereophotographsto create parallax shifts

Source:http://www.2spi.com/catalog/stereo-3D/mstereo.html

Source:http://www.funsci.com/fun3_en/stscp/stscp.htm

Softcopy Photogrammetric Workstations

Orthophoto production

Perspective vs. Orthographic Views

An orthophotograph or orthoimage – tilt/terrain distortion removed

Normal angle lens (1500 m) True Orthophoto

http://www.sharpgis.net/page/true-orthophoto-generation.aspx57

LIDAR

• Lidar (also written LIDAR or LiDAR) is a remote sensing technology that measures distance by illuminating a target with a laser and analyzing the reflected light. The acronym LIDAR comes either from combining the words light and radar (Wikipedia)

Besides geometric fidelity, we are also interested in the photo information content How do we interpret the photographs? 

Select a photographic system appropriate to the task,

 i.e., scale, coverage, time of year, and film type which best renders the details of interests

Black and White Image

True Color Image

Color Infrared Image

Color Infrared

Image Classification

• Converts an image into a raster that can be analyzed within a GIS.

• Types of classification methods:– Supervised– Unsupervised

Satellite Image

Classified Satellite Image

Characteristics used include:

•Shape•Size•Color (or tone)•Texture•Shadows•and Context

Use characteristics of the objects observed, plus knowledge of acquisition (scale, time of

year, film type) to identify features

Advantages

- High view, little relief displacement- Ultra-stable satellites, little tilt distortion- Extended spectral range, from radar to

far infra-red- Low cost per unit area (for large study areas)- Digital images, which may be easily enhanced, integrated into a GIS

Satellite Images

Disadvantages

•Limited acquisition flexibility, fixed schedules•Expensive for small areas, due to fixed frame size•Limited scale/resolution•Requires sophisticated, moderately expensive systems to take advantage of digital image

Satellite Images

Most common useful applications•Landcover mapping, large arease.g., wetlands, urban, forest

•Disaster evaluation, management•Crop monitoring•Change detection (for example, deforestation)

•Snow monitoring, runoff estimation•Geologic prospecting•Vegetation health monitoring

Satellite vs. Photos – Which to Use?

Satellites• Lower detail (barely)• Expanded spectrum• Inherently digital• Stable platform• Higher flight path• Inexpensive for large

areas

Aerial Photos• Higher detail• Less expensive for

small areas• Flexible repeat time• Fly under clouds• Simple handling