Geodesy and Datums

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Geodesy and Datums

• Michael A. McAdamsGeography Dept.Fatih
  UniversityIstanbul, Turkey

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Is the earth round?

• Well...all the globes are round..so it must be...
• It must be flat because..I have never seen the
  earth curve
• It is not flat, but more like an elispse
• It depends on what you mean by round. It is a
  sphere, but not a perfect one.

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A View from Space (Google Earth)
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Another view of the earth
Türkiye
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Istanbul from Space(looks flat, but is it??)
You are here
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Fatih University
You are here
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Another image of Istanbul (infra-red)
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Some questions?

• Geodesy is the science of measuring and
  monitoring the size and shape of the Earth and
  the location of points on its surface. (NOAA
  website http//www.oceanservice.noaa.gov/educati
  on/kits/geodesy/welcome.html )
• Why is it used?
• To obtain accurate measures on the surface of
  the earth
• Who uses it?
• Surveyers, remote sensing specialists, GIS
  technicians, geographers, archeologists, anyone
  concerned with spatial analysis
• What is the difference between a geod and a
  datum?
• A datum is the specifications to construct a
  geod or at least a part of one

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The Elements of Geodesy The Figure of the Earth

• The Earth's shape is nearly spherical, with a
  radius of about 3,963 miles (6,378 km), and its
  surface is very irregular.
• Mountains and valleys make actually measuring
  this surface impossible because an infinite
  amount of data would be needed.
• It would take you many lifetimes to measure every
  crevice, valley, and rise. You could never
  complete the project because it would take too
  long.
• http//www.oceanservice.noaa.gov/education/kits/ge
  odesy/geo03_figure.html

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A Round Earth...not a new idea

• In 6th Century B.C.E. Pythagoras stated that
  people must live on a body of perfect shape
• Aristotle (4th Century B.C.E.) noted that sailing
  ships disappear from view hull first and then
  mast instead of getting smaller and smaller
• Astronomical observation also evident of the
  circular appearence of the earth
• Eratosthenes (librarian at the library at
  Alexandria) calculated a slightly larger
  circumference of the earth in 250 B.C.E.see
  Figure 4.1 in textbook

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Members of a 1922 geodetic survey expedition.
Until recent advances in satellite technology,
namely the creation of the Global Positioning
System (GPS), geodetic surveying was an arduous
task best suited to individuals with strong
constitutions, and a sense of adventure.
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Elipsoid and Geod

• An elipsoid is probably the most similar to the
  earths surface..but not exactly
• Various types of official elipsoids (see Table
  4.2)most widely used is the WGS84
• A geod is the three-dimenisonal shape that would
  be approximated by mean sea level in the oceans
  and the surface of a series of hypothetical
  sea-level canals crisscrossing the continents.
  (Robinson, 44)

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Different Abstractions of the Earth

• http//www.oceanservice.noaa.gov/education/kits/ge
  odesy/media/supp_geo03a.html

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Simple Explainations of the Abstractions of the
Earths surface

• Flat Earth (planar)A flat earth without
  accounting for the curvature of the earth. This
  is used for surveying and small areas.
• SphereSimular to a ball or what we think of as a
  globe. It discounts the irregularities in the
  earths surface such as mountaing ranges and
  canyons
• ElipsoidLooks like an egg. The earth is more
  like an elipsoid than a sphere so it is more
  accurate than a sphere.
• GeodBased on the elevation and earths
  gravatational pull. It is the most accurate
  description of the surface of the earth.

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What is a datum?

• A datum is as a set of spatial information that
  acts as a foundation for other data, just like
  concrete acts as a foundation for the structure
  of a building, including all the furnishings and
  decorations inside.
• In other words, it is the necessary information
  (data or datum) that can re-create a plane,
  sphere, elipsoid or geod.
• Without a datum there would be nothing to
  securely support other spatial information, such
  as digital elevations, land use, or population.
• There are two types of datums, horizontal and
  vertical
• More on datums at http//www.colorado.edu/geograp
  hy/gcraft/notes/datum/datum_f.html

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Horizonal and Vertical datums

• the horizontal datum is a collection of specific
  points on the Earth that have been identified
  according to their precise northerly or southerly
  location (latitude) and easterly or westerly
  location (longitude).
• The vertical datum is a collection of specific
  points on the Earth with known heights either
  above or below mean sea level. Near coastal
  areas, mean sea level is determined with a tide
  gauge. In areas far away from the shore, mean sea
  level is determined by the shape of the geoid.

http//www.oceanservice.noaa.gov/education/kits/ge
odesy/
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Difference between Elipsoid, Geoid and
Topgrapahic Surfaces
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Global Positioning System (GPS)

• Using the Global Positioning System (GPS), every
  point on Earth can be given its own unique
  address -- its latitude, longitude, and height.
  The U.S. Department of Defense developed GPS
  satellites as a strategic system in 1978. But
  now, anyone can gather data from them.
• GPS is a constellation of satellites that orbit
  approximately 11,000 miles above the Earth and
  transmit radio wave signals to receivers across
  the planet. By determining the time that it takes
  for a GPS satellite signal to reach your
  receiver, you can calculate your distance to the
  satellite and figure out your exact location on
  the Earth. Sound easy? In fact it is a very
  complicated process. For the GPS system to work,
  you need to have incredibly precise clocks on the
  satellites and receivers, and you must be able to
  access and interpret the signals from several
  orbiting satellites simultaneously. Fortunately,
  the receivers take care of all the calculations.
• http//www.oceanservice.noaa.gov/education/kits/ge
  odesy/geo09_gps.html

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Conclusion

• Geodesy is an important for accurate measurements
  of the earth
• Geods, elipsoids and spheres are abstractions of
  the earth
• Datums are the measurements or data that directs
  the contruction of an elipzoid, sphere or geod
• If accuracy is important (i.e., GPS)
• The mixing or non-transformation of datums can be
  perhaps the difference between life or death if
  using it for navigation purposes.
• If you hire a surveyor to survey a property you
  have just bought and he does not know what datum
  is being used..it is time for another surveyor.
• Datums, projections and coordinate systems are
  related issues, as will be discussed in the next
  lecture

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Some Other References

• Datums, who needs em anyway?http//www.esri.com/
  news/arcuser/0401/datum.html
• Geodesy Overview by Peter Danahttp//www.colorad
  o.edu/geography/gcraft/notes/datum/datum_f.htmlG
  eodesy Tutorial by NOAAhttp//www.oceanservice.n
  oaa.gov/education/kits/geodesy/welcome.html