Data Sources and the Global Positioning System

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Data Sources and the Global Positioning System

• cs 189
• Sept 27, 2007

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Outline

• Finish up data sources
• Surveying
• Coordinate Geometry
• Transformations
• Global Positioning System
• Demo (if there's time)

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Surveying

• The basis for a lot of GIS data, particularly
  data requiring high accuracy such as property
  lines, governmental unit boundaries, utilities,
  etc.
• Uses highly accurate measurement devices to
  determine distances and angles from a starting
  point (station), usually a known control point
• A distance and angle make a traverse, which can
  be linked together

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Coordinate Geometry

• Coordinate geometry (COGO) is a set of connected
  traverses (distances and bearings)
• Using basic trigonometry, each station can be
  calculated
• Stations become the vertices that define lines or
  areas of interest
• These can be input into a GIS to create the lines
  or polygons of a GIS layer

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Coordinate Geometry
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Coordinate Transformation

• Brings spatial data into an Earth-based map
  coordinate system so each data layer aligns with
  every other data layer
• Uses known locations on the Earth for control
  points
• These can be survey or GPS locations
• Match these locations to features in a GIS layer
  (vector) or visible places on imagery (raster)

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Coordinate Transformation
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Coordinate Transformation

• Book discusses affine transformation and the
  formula not important for this class
• Just know that using two sets of coordinates a
  function is used to transform all locations
• Also discusses RMSE
• Raster resampling
• Metadata

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The Global Positioning System

• The Global Positioning System consists of 24
  satellites that broadcast high-frequency radio
  signals, called peusdo-random code, that contain
  the precise time to ground based stations and
  handheld GPS receivers.

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GPS Segments

• GPS is made up of three segments

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How GPS Works

• GPS determines positions on the Earth by using
  precise time and distance measurements and a
  method of triangulation
• Specifically, GPS receivers measure distance
  using the time travel of the satellite radio
  signals
• To do this, GPS relies on very accurate clocks
  and knowledge of where the satellites are at all
  times

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Triangulation

• Using a single distance measurement from a
  satellite, one's position relative to the
  satellite can be narrowed down to the surface of
  a sphere with the satellite at the center

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Triangulation

• Using two distance measurements from two
  satellites, one's position can be narrowed down
  to where two spheres intersect

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Triangulation

• Using three distance measurements from three
  satellites, one's position can be narrowed down
  to one of two points one of which usually makes
  no sense because its nowhere near the earths
  surface

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Measuring Distance

• Distance is determined by measuring how long the
  satellite signal takes to get to the receiver
• To make the measurement we assume that both the
  satellite and our receiver are generating the
  same pseudo-random codes at exactly the same time
• The satellite signal will not match the receiver
  signal, however, due to travel time through the
  atmosphere

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Measuring Distance

• By comparing how late the satellite's
  pseudo-random code appears compared to our
  receiver's code, we determine how long it took to
  reach us
• This time, multiplied by the speed of light (the
  velocity of the radio signal) gives us the
  distance to the satellite
• Distance speed of light travel time

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Measuring Time

• Since the time for a signal to travel from a
  satellite to the Earth's surface is roughly 0.06
  seconds, depending on where the satellite is,
  very accurate clocks are needed
• The satellites are equipped with atomic clocks,
  which are very expensive
• Receivers would be too expensive to equip with
  atomic clocks, so a fourth satellite is used

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Satellite Positioning

• Measuring the distance to a satellite requires
  knowing where the satellite is
• Receivers are loaded with almanacs which contain
  the positions of all the satellites at all times
  (since their orbits are known)
• Over time, satellites are pulled slightly out of
  orbit by gravitational forces
• These changes are called ephemeris errors and
  are monitored and corrected for

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Satellite Positioning

• The DoD monitors these errors and redetermines
  the location of the satellites
• New locations are then sent up to the satellites
• The peusdo-code also contains a navigation
  message that includes information about the
  ephemeris errors

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Types of Errors

• Ephemeris error uncertainty about exact
  location of satellite
• Atmospheric error interference with the radio
  signal as it travels through the atmosphere
• Multipath errors signal is reflecting off
  objects
• Minute discrepancies in the atomic clocks of the
  satellites
• Dilution of Precision position of available
  satellites are too close together

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Multipath Error

• You can recognize a mulitpath error when the
  position or speed jumps around or when you get no
  signal.
• Giving your antenna a clear view of the sky will
  help minimize the error
• Try stepping away from buildings or holding your
  antenna up higher
• The more satellites available the more likely the
  error will be minimized.

Any object that can reflect the radio signal can
cause an error. Buildings, mountains, trees,
houses, or the ground. All cause multipath errors.
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Dilution of Precision (DOP)

• DOP describes the geometric strength of the
  satellite configuration
• GPS receivers measure this as PDOP (Positional
  DOP) and display a skyplot of the satellites
  positions
• Many high-end GPS receivers have a setting that
  only allows them to take waypoints when the PDOP
  is within a certain range
• Most recreational receivers don't have this
  functionality instead they calculate an
  estimated error

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Postitional DOP
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Correcting Errors

• WAAS A system of satellites and ground stations
  that provide GPS signal corrections for better
  accuracy. Your GPS receiver must have WAAS
  capability to us this system. Newer recreational
  grade receiver usually have WAAS.
• Real-Time Differential Correction An extension
  of the GPS system that uses land based radio
  beacons to transmit position corrections to GPS
  receivers enabled with DGPS typically a mapping
  grade receiver.
• Post-Processing Differential Correction GPS
  receivers create a log file which is than
  compared with a log file from a base station to
  make the correction. Processing is done with
  special software back in the office. This ability
  usually available in only the mapping grade
  receivers.

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GPS Accuracy

• Recreational grade receivers (Garmin, Magellan,
  etc) have about 15 meter accuracy (usually, it's
  much better than this)
• Mapping grade receivers (Trimble, Leica, etc)
  generally have sub-meter accuracy (post
  processing is available)
• Survey grade receivers can have between 3 or 3
  millimeter accuracy because they use an
  additional carrier-phase code

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Coordinate Systems

• A coordinate system is a reference system used to
  locate features on a two or three dimensional
  surface.
• A coordinate system is made up of a datum (which
  includes a spheroid), projection and units.
• Common coordinate systems are geographic
  coordinates measured in latitude and longitude or
  planar systems where the earths surface is
  projected on a 2D plane and locations are
  measured in feet or meters.
• GPS collects coordinates in WGS84

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GPS Applications

• Field data collection
• Collecting point, line and polygons in the GPS
  then downloading and importing into GIS
• Using GPS enabled GIS software to digitize
  feature in the field in native GIS formats
• Control points (transformations)
• Tracking (vehicles, wildlife, etc)

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Next Week

• Tuesday
• Review for quiz
• Lab on data sources
• Thursday
• Continue working on labs
• Quiz
• Projections / coordinate systems
• Data sources
• GPS