Post on 13-Dec-2015
Global Positioning Systems
Jeff Blossom, Senior GIS SpecialistCenter for Geographic Analysis
Harvard University gis.harvard.edu
September 13, 2015
Guest presentation to GOV 1008 Intro to GIS
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What is GPS?
The Global Positioning System
Constellation
• GPS is a global navigation satellite system developed by the U.S. Department of Defense, managed by the U.S. Air Force. Free for anyone to use.
• Provides geographic locations at any time, anywhere.
Latitude
Longitude
Geographic coordinate system
WGS 1984 Geographic Coordinate System Notation
Degrees, Minutes, Seconds (DMS) and Decimal Degrees (DD)
0° 0’ 0”
N 45° 0’ 0”
0° 0
’ 0”
N 90° 0’ 0”
S 90° 0’ 0”
S 45° 0’ 0”
0.0
45.0
90.0
-90.0
-45.0
E 9
0° 0
’ 0”
E 1
80°
0’ 0
”
W 9
0° 0
’ 0”
W 1
80°
0’ 0
”
0.0
90.0
180.
0
-90.
0
-180
.0
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The Global Positioning System
Constellation
• GPS is a global navigation satellite system developed by the U.S. Department of Defense, managed by the U.S. Air Force. Free for anyone to use.
• Provides geographic locations at any time, anywhere.
• 24 - 32 earth orbiting satellites
A GPS satellite
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Solar panels – powerAtomic clock – precisely records time in UTCRadio transmitter – broadcasts a continuous signal containing the time, and ephemeris (identification and positional information)
San Diego Aerospace Museum
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GPS satellite on display
The Global Positioning System
Constellation
• GPS is a global navigation satellite system developed by the U.S. Department of Defense, managed by the U.S. Air Force. Free for anyone to use.
• Provides geographic locations at any time, anywhere.
• 24 - 32 earth orbiting satellites
• GPS receivers• Contain a clock, computer, radio
wave receiver.• Dime sized to dinner plate sized• Receive signals from GPS satellites• Computes distance to satellite by
comparing time: signal sent vs. received.
• Receiving four satellite signals allows a position calculation using trilateration
Trilateration – location determination through the measurement of distances.
If the distance to 3 satellites is known, where these 3 spheres intersect can be calculated.
Trilateration is the method used by a GPS receiver to determine one’s position on earth.
Watch the explanation by a NASA scientist:
• http://www.youtube.com/watch?v=3zRlbboMvb0
GPS uses – Navigation (land, air, water)
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GPS uses - Commercial
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Precision agriculture Fleet management
GPS uses: Military
Weapon targeting and guidance
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Troop deployment and navigation
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GPS use at Harvard – mapping rural households for a public health study in Indonesia
GPS use at Harvard – Mapping roads and air pollution
Accra, Ghana
Boston, MA
GPS / GIS integration
Geographic Information System (GIS) defined:
A collection of computer hardware and software designed for capturing, storing, updating,, manipulating, analyzing, displaying, and publishing all forms of geographically referenced information.
• People performing different roles are required.• Common methodologies are applied within a GIS.
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GPS / GIS integration
• Load Geographic Information System (GIS) maps and data onto a GPS.
• Display basemap• Collect feature attributes
ID Type Diameter Health
1 coniferous 14" poor
2 deciduous 8" good
3 coniferous 10" poor
4 deciduous 10" good
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GPS use at Harvard – Child physical activity study
GPS / GIS integration
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Recreational GPS useGeocaching Collaborative mapping
opencyclemap.org
Recreational GPS use – Mountain biking in Utah
Good examples / graphics of GPS uses
Penn State University Geospatial Revolution Videos, Episodes 1 and 2
http://geospatialrevolution.psu.edu/episode1(minutes 2:00 – 2:45)http://geospatialrevolution.psu.edu/episode2(whole episode)
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https://www.strava.com/activities/88401986
Map yourself with GPS
25http://gis.harvard.edu/services/blog/creating-interactive-story-map-contains-narrated-geotagged-photos-and-gps-track
Real time mapping with GPS
Boston Subway: http://sites.harvard.edu/~wuensch/T/subway-map.htmlBoston commuter rail: http://sites.harvard.edu/~wuensch/T/commuter-rail-map.html
GPS Sources for ErrorGPS radio wave distortion
Atmosphere – Charged ionic particles and water vapor slow the signal.
Ground – Deflects GPS signals.
Buildings, tree canopy – Deflect and obstruct GPS signals.
• The lower a satellite is on the horizon, the more atmosphere its satellite has to pass through.
• Overall error in precision is recorded and noted as Precision Displacement of Position (PDOP)
• Well distributed satellite geometry minimizes PDOP.
• GPS receivers vary in their ability to filter out GPS signal ‘noise’.
GPS AccuracyGPS accuracy depends on:1) The number of signals received. More signals = better accuracy2) The receiver’s ability to filter noise. Better filtering = better accuracy.More signals can be acquired by: Accessing the L1 and L2 radio bands from GPS satellites. Accessing other Global Navigation Satellite Systems:
GLOSNASS (Russia), Galileo (European Union China (Compass/Beidou) India (IRNSS – not yet operational) Japan (QZSS – proposed)
Base station access – public and private subscription based. Receivers exhibit a broad range of noise filtering sophistication.
Mission Planning to aid accuracy
GPS devices – Survey, mapping, recreational
Survey grade – Very sophisticated receivers, able to receive many signals. Centimeter accuracy possible. Cost: several thousand $$. Mapping grade – ~1 meter accuracy, possible.Cost: $700 and up.
Recreational – User friendly, 10 meter accuracy. Cost: $100 and up.
GPS Device Vendors
• Trimble: http://trimble.com/– High-end surveying and mapping receivers
• Leica: http://leica.com/– High-end surveying and mapping receivers
• Magellan: http://www.magellangps.com/– Consumer GPS: in-car navigation systems, low-end
handheld receivers• Garmin: http://www.garmin.com/
– Consumer GPS: in-car navigation systems, low-end handheld receivers
• TomTom: http://www.tomtom.com/– Consumer GPS: in-car navigation systems
GPS Summary• 24 - 32 satellites in orbit at 20,200 km• Continuous 24-hour, worldwide coverage• Position and time data transmitted from satellites
through coded radio waves.• Accuracy is depends on the sophistication of the
receiver, number of signals received, and ranges from 10 meter to centimeter accuracy.
• Satellites controlled by U.S. Dept. of Defense (clock adjustments, activation, signal scrambling)
• WGS 1984 coordinate system used• Wide range of applications
Some TerminologyWaypoint – GPS term for a single geographic location.
Track – GPS term for a linear geographic location.
WGS1984 – The geographic coordinate system GPS uses.
Mapping routes/lines: http://www.strava.com/Free, android and iOS.
Mapping point features, and survey collection:
Fulcrum app: http://fulcrumapp.com/ android and iOS, but only free for 30 days.
KoBo Toolbox – free, but android only. http://www.kobotoolbox.org/
Map with your smartphone as well?
Survey control point location from the field exercise:http://www.upo.harvard.edu/campusprojects/survey/nywj10.htm
Collect waypoint data and information with your iPhone / Android:http://web.fulcrumapp.com/users/sign_upCreate at least one “app” at the Fulcrum website
On your phone, search for “Fulcrum GPS” at your app store.
Download the free app onto your phone.
Differential Correction – an enhancement to GPS using fixed reference stations.
Reference Stations Continuously receives GPS signals through a high end receiver. An accurate survey determines a precise location of the receiver. Unlike a roving GPS receiver, stations know exactly where they
are, and can figure out what the travel time of each signal should be.
In the U.S. there is a network of reference stations known as CORS – Continuously Operating Reference System - and is managed by the U.S. Geodetic Survey.
Differential Correction – an enhancement to GPS using fixed reference stations.
Calculated times are compared with the actual times. The difference is an "error correction" factor.
Mapping and Survey grade GPS receivers can be combined with software that corrects mapped positions based on reference station information.
Differential correction is possible with Survey grade, and some mapping grade GPS receivers
Survey grade – Very sophisticated receivers, able to receive many signals. Centimeter accuracy possible. Cost: several thousand $$. Mapping grade – ~1 meter accuracy, possible.Cost: $700 and up.
Recreational – User friendly, 10 meter accuracy. Cost: $100 and up.
Differential correction possible
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Conclusion - Tips for the fledgling GPS Mapper
Do you need to use GPS? Check if the data you require is already mapped. Is what you want map visible on Google Earth or another imagery
dataset? If so, it’s much simpler and less costly to map it from Google Earth or the imagery source (make sure to verify the accuracy).
To determine what type of GPS may be necessary, ask yourself: What level of accuracy is required for your mapping purpose? What type of information will be collected?
When planning your field mapping, think about: What are the environmental conditions of the field site? Is there a time of day when satellite geometry will be the best for my
area?
Center for Geographic Analysis contact: http://gis.harvard.edu/contactus
Jeff Blossom: jblossom@cga.harvard.edu
This presentation and the GPS lab are available at:http://gis.harvard.edu/training/non-credit-training/past-workshops
The CGA has GPS devices you can borrow for free. See a list at:http://gis.harvard.edu/tools/hardware/global-positioning-systems