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
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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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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Different Abstractions of the Earth

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

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Abstractions of the Earths surface

• 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 with a bulge in the
  middle. 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.
• 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

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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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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, elipzoids 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
• 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