Geographic Information Systems

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Geographic Information Systems

• SGO 1910, SGO 4030
• October 18, 2005

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Quizzes

• Class average 24,8
• Two problem questions I concede!

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• 19. Efforts to promote an international standard
  for ellipsoids has led to the wide acceptance of
  the North American Datum of 1927 (NAD27) (False,
  WGS84 or NAD83) but many older data still
  adhere to earlier standards, such as NAD 27 (p.
  116)
• 24. The modern history of GIS dates from the
  early 1950s, when computers were developed.
  (False the modern history of GIS dates from
  the early 1950s, when the price of sufficiently
  powerful computers fell below a critical
  threshold p. 18)

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• 20. Geographic techniques can be applied to
  non-geographic spaces. True
• But many of the methods used in GIS are also
  applicable to other non-geographic spaces,
  including the surfaces of other planets, the
  space of the cosmos, and the space of the human
  body that is captured by medical images (p. 8)

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• 27. If you were going hiking in the forest, it
  would be most useful to take along a map with a
  small representative fraction. FALSE
• 150000 gt 11000000
• 30. Digital representations of geographic
  phenomena are formalized through photographic
  models. FALSE

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Revised Schedule

• Week 42 (October 18) GIS Data Collection Chapter
  9
• GPS
• Week 43 (October 25) Geographic
  Databases Chapters 10
• Week 44 (Nov. 1) Geographic Analysis Chapters 14,
  15
• Week 45 (Nov. 8) Mid-term Quiz II
• Map Production Chapter 12
• Week 46 (Nov. 15) GIS and Society Chapter 18
• Week 47 (November 22) NO CLASS
• Week 48 Final Exam Dec 1

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Uncertainty
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The more scientific knowledge we gain, the more
uncertain we are likely to be Richness of
representation and computational power only make
us more aware of the range and variety of
established uncertainties, and challenge us to
integrate new ones (Longley et al. 2005, p.
152).
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Data AcquisitionGetting the Map into the
Computer
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Data capture

• Primary (direct measurement, e.g. remote sensing
  and surveying)
• Secondary (derivation from other sources
  digitizing, scanning, etc.)

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Data transfer

• Input of data from other systems (via Internet,
  CD ROMs, tapes, etc.)

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GIS maps are digital

• Real maps traditional paper maps that can be
  touched
• Virtual maps an arrangement of information
  inside the computer the GIS can be used to
  generate the map however and whenever necessary.

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GIS Data Conversion

• Traditionally the most time-consuming and
  expensive part of a GIS project
• Involves a one-time cost
• Digital maps can be reused and shared.
• Requires maintenance (eg. updating)

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GIS data can be

• Purchased.
• Found from existing sources in digital form.
• Captured from analog maps by GEOCODING.

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Finding Existing Map Data

• Map libraries
• Reference books
• State and local agencies
• Federal agencies
• Commercial data suppliers

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Existing Map Data

• Existing map data can be found through a map
  library, via network searches, or on media such
  as CD-ROM and disk.
• Many major data providers make their data
  available via the Internet.

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Statenskartverkhttp//ngis.statkart.no/katalog/ja
va/katalog.asp

• Rasterdata
• Temakart
• Vektordata
• Primærdata
• Prosjekter

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1. Accessing GIS Data

• Example Costa Rica

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Data Collection

• One of most expensive GIS activities
• Many diverse sources
• Two broad types of collection
• Data capture (direct collection)
• Data transfer
• Two broad capture methods
• Primary (direct measurement)
• Secondary (indirect derivation)

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Data Collection Techniques
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GEOCODING

• Geocoding is the conversion of spatial
  information into digital form.
• Geocoding involves capturing the map, and
  sometimes also capturing the attributes.

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Primary Data Capture

• Capture specifically for GIS use
• Raster remote sensing
• e.g. SPOT and IKONOS satellites and aerial
  photography
• Passive and active sensors
• Resolution is key consideration
• Spatial
• Spectral
• Temporal

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Secondary Geographic Data Capture

• Data collected for other purposes can be
  converted for use in GIS
• Raster conversion
• Scanning of maps, aerial photographs, documents,
  etc
• Important scanning parameters are spatial and
  spectral (bit depth) resolution

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Vector Primary Data Capture

• Surveying
• Locations of objects determines by angle and
  distance measurements from known locations
• Uses expensive field equipment and crews
• Most accurate method for large scale, small areas
• GPS
• Collection of satellites used to fix locations on
  Earths surface
• Differential GPS used to improve accuracy

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Vector Secondary Data Capture

• Collection of vector objects from maps,
  photographs, plans, etc.
• Digitizing
• Manual (table)
• Heads-up and vectorization
• Photogrammetry the science and technology of
  making measurements from photographs, etc.
• COGO Coordinate Geometry

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Managing Data Capture Projects

• Key principles
• Clear plan, adequate resources, appropriate
  funding, and sufficient time
• Fundamental tradeoff between
• Quality, speed and price
• Two strategies
• Incremental
• Blitzkrieg (all at once)
• Alternative resource options
• In house
• Specialist external agency

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Summary

• Data collection is very expensive,
  time-consuming, tedious and error prone
• Good procedures required for large scale
  collection projects
• Main techniques
• Primary
• Raster e.g. remote sensing
• Vector e.g. field survey
• Secondary
• Raster e.g. scanning
• Vector e.g. table digitizing

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Digitizing

• Captures map data by tracing lines from a map by
  hand
• Uses a cursor and an electronically-sensitive
  tablet
• Result is a string of points with (x, y) values

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Digitizer
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The Digitizing Tablet
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Digitizing

• Stable base map
• Fix to tablet
• Digitize control
• Determine coordinate transformation
• Trace features
• Proof plot
• Edit
• Clean and build

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Selecting points to digitize
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Scanner
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Scanning

• Places a map on a glass plate, and passes a light
  beam over it
• Measures the reflected light intensity
• Result is a grid of pixels
• Image size and resolution are important
• Features can drop out

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Scanning example
This section of map was scanned, resulting in a
file in TIF format that was bytes in size. This
was a file of color intensities between 0 and
255 for red, green, and blue in each of three
layers spaced on a grid 0.25 millimeter apart.
How much data would be necessary to capture the
features on your map as vectors? Would it be
more or less than the grid (raster) file?
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Field data collection
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Pen Portable PC and GPS
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Data Transfer

• Buy vs build is an important question
• Many widely distributed sources of GI
• Key catalogs include
• US NSDI Clearinghouse network
• Geography Network
• Access technologies
• Translation
• Direct read

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Attribute data

• Logically can be thought of as in a flat file
• Table with rows and columns
• Attributes by records
• Entries called values.

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Database Management Systems

• Data definition module sets constraints on the
  attribute values
• Data entry module to enter and correct values
• Data management system for storage and retrieval
• Data definitions can be listed as a data
  dictionary
• Database manager checks values with this
  dictionary, enforcing data validation.

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The Role of Error

• Map and attribute data errors are the data
  producer's responsibility, but the GIS user must
  understand error.
• Accuracy and precision of map and attribute data
  in a GIS affect all other operations, especially
  when maps are compared across scales.

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Part II Global Positioning Systems (GPS)

• Sources of information
• http//www.trimble.com/gps/
• http//www.colorado.edu/geography/gcraft/notes/gps
  /gps.htmlDODSystem

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GPS is a Satellite Navigation System

• GPS is funded by and controlled by the U. S.
  Department of Defense (DOD). While there are many
  thousands of civil users of GPS world-wide, the
  system was designed for and is operated by the U.
  S. military.
• GPS provides specially coded satellite signals
  that can be processed in a GPS receiver, enabling
  the receiver to compute position, velocity and
  time.
• Four GPS satellite signals are used to compute
  positions in three dimensions and the time offset
  in the receiver clock.

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Space Segment

• The Space Segment of the system consists of the
  GPS satellites. These space vehicles (SVs) send
  radio signals from space.

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Space Segment (cont)

• The nominal GPS Operational Constellation
  consists of 24 satellites that orbit the earth in
  12 hours.
• The satellite orbits repeat almost the same
  ground track (as the earth turns beneath them)
  once each day. The orbit altitude is such that
  the satellites repeat the same track and
  configuration over any point approximately each
  24 hours (4 minutes earlier each day).
• There are six orbital planes (with nominally four
  SVs in each), equally spaced (60 degrees apart),
  and inclined at about fifty-five degrees with
  respect to the equatorial plane.
• This constellation provides the user with between
  five and eight SVs visible from any point on the
  earth.

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GPS Satellites Name NAVSTAR Manufacturer
Rockwell International Altitude 10,900 nautical
miles Weight 1900 lbs (in orbit) Size17 ft
with solar panels extended Orbital Period 12
hours Orbital Plane 55 degrees to equitorial
plane Planned Lifespan 7.5 years Current
constellation 24 Block II production satellites
Future satellites 21 Block IIrs developed by
Martin Marietta
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Latest Development

• Galileo, Europe's contribution to the Global
  Navigation Satellite System (GNSS), is creating a
  buzz in the Global Positioning Systems (GPS)
  applications market. With its advantages of
  signal reliability and integrity, it is poised to
  drive European GPS applications markets. Unlike
  its US counterpart, Galileo is envisioned as
  being independent of military control and is
  expected to be harnessed for widespread
  commercial and civilian purposes. (Space Daily,
  Dec. 18, 2003)

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Control Segment

• The Control Segment consists of a system of
  tracking stations located around the world.

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The Master Control facility is located at
Schriever Air Force Base (formerly Falcon AFB) in
Colorado. These monitor stations measure signals
from the SVs which are incorporated into orbital
models for each satellites. The models compute
precise orbital data (ephemeris) and SV clock
corrections for each satellite. The Master
Control station uploads ephemeris and clock data
to the SVs. The SVs then send subsets of the
orbital ephemeris data to GPS receivers over
radio signals.
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User Segment

• The GPS User Segment consists of the GPS
  receivers and the user community. GPS receivers
  convert SV signals into position, velocity, and
  time estimates. Four satellites are required to
  compute the four dimensions of X, Y, Z (position)
  and Time. GPS receivers are used for navigation,
  positioning, time dissemination, and other
  research.
• Navigation in three dimensions is the primary
  function of GPS. Navigation receivers are made
  for aircraft, ships, ground vehicles, and for
  hand carrying by individuals.

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• Precise positioning is possible using GPS
  receivers at reference locations providing
  corrections and relative positioning data for
  remote receivers. Surveying, geodetic control,
  and plate tectonic studies are examples.

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Here's how GPS works in five logical steps

• The basis of GPS is "triangulation" from
  satellites.
• To "triangulate," a GPS receiver measures
  distance using the travel time of radio signals.
• To measure travel time, GPS needs very accurate
  timing which it achieves with some tricks.
• Along with distance, you need to know exactly
  where the satellites are in space. High orbits
  and careful monitoring are the secret.
• Finally you must correct for any delays the
  signal experiences as it travels through the
  atmosphere.

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Triangulating

• Position is calculated from distance measurements
  (ranges) to satellites. 
• Mathematically we need four satellite ranges to
  determine exact position. 
• Three ranges are enough if we reject ridiculous
  answers or use other tricks. 
• Another range is required for technical reasons
  to be discussed later.

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Measuring Distance

• Distance to a satellite is determined by
  measuring how long a radio signal takes to reach
  us from that satellite. 
• 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. 
• 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. 
• Multiply that travel time by the speed of light
  and you've got distance.

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Getting Perfect Timing

• Accurate timing is the key to measuring distance
  to satellites. 
• Satellites are accurate because they have atomic
  clocks on board. 
• Receiver clocks don't have to be too accurate
  because an extra satellite range measurement can
  remove errors.

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2005 Nobel Prize in Physics

• Two physicists (Hall and Haensch) shared the
  Nobel Prize in Physics for advancing the
  developmetn of laser-based precision
  spectroscopy, a field htat opens the way to the
  next generation of global positioning system
  (GPS) navigation and ultra-precise atomic clocks.

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Satellite Positions

• To use the satellites as references for range
  measurements we need to know exactly where they
  are. 
• GPS satellites are so high up their orbits are
  very predictable. 
• Minor variations in their orbits are measured by
  the U.S. Department of Defense. 
• The error information is sent to the satellites,
  to be transmitted along with the timing signals.

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• Three satellites could be used determine three
  position dimensions with a perfect receiver
  clock. In practice this is rarely possible and
  three SVs are used to compute a two-dimensional,
  horizontal fix (in latitude and longitude) given
  an assumed height. This is often possible at sea
  or in altimeter equipped aircraft.
• Five or more satellites can provide position,
  time and redundancy. More SVs can provide extra
  position fix certainty and can allow detection of
  out-of-tolerance signals under certain
  circumstances.

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• Position in XYZ is converted within the receiver
  to geodetic latitude, longitude and height above
  the ellipsoid.
• Latitude and longitude are usually provided in
  the geodetic datum on which GPS is based
  (WGS-84). Receivers can often be set to convert
  to other user-required datums. Position offsets
  of hundreds of meters can result from using the
  wrong datum.

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GPS errors are a combination of noise, bias,
blunders.
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Selective Availability (SA)

• SA is the intentional degradation of the SPS
  signals by a time varying bias. SA is controlled
  by the DOD to limit accuracy for non-U. S.
  military and government users.
• SA was turned off in May, 2000!

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Bias Error sources

• SV clock errors uncorrected by Control Segment
  1 meter
• Ephemeris data errors 1 meter
• Tropospheric delays 1 meter. The troposphere is
  the lower part (ground level to from 8 to 13 km)
  of the atmosphere that experiences the changes in
  temperature, pressure, and humidity associated
  with weather changes. Complex models of
  tropospheric delay require estimates or
  measurements of these parameters.
• Unmodeled ionosphere delays 10 meters. The
  ionosphere is the layer of the atmosphere from 50
  to 500 km that consists of ionized air. The
  transmitted model can only remove about half of
  the possible 70 ns of delay leaving a ten meter
  un-modeled residual.
• Multipath 0.5 meters. Multipath is caused by
  reflected signals from surfaces near the receiver
  that can either interfere with or be mistaken for
  the signal that follows the straight line path
  from the satellite. Multipath is difficult to
  detect and sometime hard to avoid.

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Blunders can result in errors of hundred of
kilometers.

• Control segment mistakes due to computer or human
  error can cause errors from one meter to hundreds
  of kilometers.
• User mistakes, including incorrect geodetic datum
  selection, can cause errors from 1 to hundreds of
  meters.
• Receiver errors from software or hardware
  failures can cause blunder errors of any size.

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Correcting Errors

• The earth's ionosphere and atmosphere cause
  delays in the GPS signal that translate into
  position errors.
•  
• Some errors can be factored out using mathematics
  and modeling. 
• The configuration of the satellites in the sky
  can magnify other errors. 
• Differential GPS can eliminate almost all error.

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• GPS technology has matured into a resource that
  goes far beyond its original design goals. These
  days scientists, sportsmen, farmers, soldiers,
  pilots, surveyors, hikers, delivery drivers,
  sailors, dispatchers, lumberjacks, fire-fighters,
  and people from many other walks of life are
  using GPS in ways that make their work more
  productive, safer, and sometimes even easier.

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Location Where am I?

• The first and most obvious application of GPS is
  the simple determination of a "position" or
  location. GPS is the first positioning system to
  offer highly precise location data for any point
  on the planet, in any weather. That alone would
  be enough to qualify it as a major utility, but
  the accuracy of GPS and the creativity of its
  users is pushing it into some surprising realms.

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Navigation Where am I going?

• GPS helps you determine exactly where you are,
  but sometimes important to know how to get
  somewhere else. GPS was originally designed to
  provide navigation information for ships and
  planes. So it's no surprise that while this
  technology is appropriate for navigating on
  water, it's also very useful in the air and on
  the land.
• The sea, one of our oldest channels of
  transportation, has been revolutionized by GPS,
  the newest navigation technology.

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• By providing more precise navigation tools and
  accurate landing systems, GPS not only makes
  flying safer, but also more efficient. With
  precise point-to-point navigation, GPS saves fuel
  and extends an aircraft's range by ensuring
  pilots don't stray from the most direct routes to
  their destinations.
• GPS accuracy will also allow closer aircraft
  separations on more direct routes, which in turn
  means more planes can occupy our limited
  airspace. This is especially helpful when you're
  landing a plane in the middle of mountains. And
  small medical evac helicopters benefit from the
  extra minutes saved by the accuracy of GPS
  navigation.

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• Finding your way across the land is an ancient
  art and science. The stars, the compass, and good
  memory for landmarks helped you get from here to
  there. Even advice from someone along the way
  came into play. But, landmarks change, stars
  shift position, and compasses are affected by
  magnets and weather. And if you've ever sought
  directions from a local, you know it can just add
  to the confusion. The situation has never been
  perfect.
• Today hikers, bikers, skiers, and drivers apply
  GPS to the age-old challenge of finding their
  way.

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• In 1994 Norwegian Borge Ousland reached the
  North Pole after skiing 1000 kilometers from
  Siberia alone and unsupported. For this
  incredible challenge Børge carried a bible to
  read, some Jimi Hendrix to listen to, and a
  Trimble Scout GPS receiver to help find his way.
  

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Tracking

• Commerce relies on fleets of vehicles to deliver
  goods and services either across a crowded city
  or through nationwide corridors. So, effective
  fleet management has direct bottom-line
  implications, such as telling a customer when a
  package will arrive, spacing buses for the best
  scheduled service, directing the nearest
  ambulance to an accident, or helping tankers
  avoid hazards.
• GPS used in conjunction with communication links
  and computers can benefit applications in
  agriculture, mass transit, urban delivery, public
  safety, and vessel and vehicle tracking. So it's
  no surprise that police, ambulance, and fire
  departments are adopting GPS-based AVL (Automatic
  Vehicle Location) Manager to pinpoint both the
  location of the emergency and the location of the
  nearest response vehicle on a computer map. With
  this kind of clear visual picture of the
  situation, dispatchers can react immediately and
  confidently.

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Timing

• Although GPS is well-known for locating,
  navigation, and tracking, it's also used to
  disseminate precise time, time intervals, and
  frequency. Time is a powerful commodity, and
  exact time is more powerful still. Knowing that a
  group of timed events is perfectly synchronized
  is often very important. GPS makes the job of
  "synchronizing our watches" easy and reliable.
• There are three fundamental ways we use time. As
  a universal marker, time tells us when things
  happened or when they will. As a way to
  synchronize people, events, even other types of
  signals, time helps keep the world on schedule.
  And as a way to tell how long things last, time
  provides and accurate, unambiguous sense of
  duration.
• GPS satellites carry highly accurate atomic
  clocks. And in order for the system to work, our
  GPS receivers here on the ground synchronize
  themselves to these clocks. That means that every
  GPS receiver is, in essence, an atomic accuracy
  clock.

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Mapping

• Using GPS to survey and map it precisely saves
  time and money in this most stringent of all
  applications. Today, Trimble GPS makes it
  possible for a single surveyor to accomplish in a
  day what used to take weeks with an entire team.
  And they can do their work with a higher level of
  accuracy than ever before.
• GPS technology is now the method of choice for
  performing control surveys, and the effect on
  surveying in general has been considerable. GPS
  pinpoints a position, a route, and a fleet of
  vehicles. Mapping is the art and science of using
  GPS to locate items, then create maps and models
  of everything in the world. Mountains, rivers,
  forests and other landforms. Roads, routes, and
  city streets. Endangered animals, precious
  minerals and all sorts of resources. Damage and
  disasters, trash and archeological treasures. GPS
  is mapping the world.