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Introduction to GPS and mapping. Global Positioning System. What is GPS ?. Department of Defense developed for navigation Standard positioning service (public uses) Precise positioning service Launches began in 1970s Full operational capability in mid 1990s

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Presentation Transcript
what is gps




What is GPS?
history of gps

Department of Defense developed for navigation

    • Standard positioning service (public uses)
    • Precise positioning service
  • Launches began in 1970s
  • Full operational capability in mid 1990s
  • ‘Selective Availability’ turned off 2000
    • Degraded accuracy to ~100m
History of GPS
gps system

Space segment – (satellite life = 10 years)

    • Several generations of satellites in use now
  • Control segment
  • User segment
  • US spends $400,000,000 yearly
    • Free to you – tax dollars at work again
  • Other countries operate their own systems too
GPS system
space segment
Space Segment
  • 24 satellites with spares in 6 orbital planes (4 in each)
  • ~12,500 miles elevation
  • 55 degree inclination
  • Each one circles Earth every 12 hours (7000 mph)
  • At least 4 visible at every point on the earth at all times
control segment

Master Control Station (MCS) in Colorado

  • 5 Monitoring stations
  • Ground control stations (Ground Antennas)
    • Unmanned
    • Enable MCS to control the satellites
Control Segment
user segment



Base map

Record tracks, waypoints, distance

User Segment
how does it work


Need D+1 satellites to determine position = 2D needs 3 satellites, etc

How does it work?
sources of error ionosphere
Sources of error ionosphere
  • Dual frequency mode of more advanced receivers corrects
sources of error satellite geometry
Sources of error - Satellite geometry

Geometric Dilution of Precision (GDOP) – the higher the value, the poorer the measurement (very good = <4, bad = >6)

how accurate

Recreation grade = ~8m

  • Mapping grade = <1m
  • Survey grade = several cm
  • Accuracy versus Precision
    • Accurate measurement versus true location
    • Precise measurements close to each other
How accurate?

Geodetic Datums: What are they?

  • Define the size and shape of the earth
  • Used as basis for coordinate systems
  • Variety of models:
    • Flat earth
    • Spherical
    • Ellipsoidal
  • WGS 84 defines geoid heights for the entire earth
Geographic Coordinate Systems (based on Datums)

Latitude, Longitude of curved surface

Projections make math easier

USGS Map Projections

Projected Coordinate Systems

Cartesian (X, Y) with designated origin and distance from origin

projection and registration
Projection and registration

Datum used effects positional errors

state plane coordinates

UTM is still not accurate enough for small area surveying.

During 1930s, each US state adopt its own projection and coordinate system, generally known as State Plane Coordinates (SPC).

Each state chose its own projection based on its shape to minimize distortion over the area of the state.

Some states have more than one internal zone.

The North American Datum 1983 (NAD83) is commonly used for SPC.

State Plane Coordinates