Coordinates, GDA, MGA

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Coordinates, GDA, MGA

• Cliff Ogleby
• Department of Geomatics

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Overview

• This lecture will give some background to the Map
  Grid of Australia
• As well as introduce some geodetic concepts
• And reinforce the importance of coordinates

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Planar Coordinates

• We like to compute on the plane!
• Basic trigonometry, Pythagoras
• Areas, volumes, distances
• Based on two perpendicular axes
• X and Y, or for us East and North
• Should by now be nothing new!

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But the Earth is not Flat

• The Earth is now generally accepted to be
  roundish
• In fact an oblate spheroid with mountains and
  valleys
• A very difficult surface to compute
• So we try to simplify
• http//www.alaska.net/clund/e_djublonskopf/Flatea
  rthsociety.htm

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The Geoid

• The geoid is a surface of equal gravity over the
  Earth
• This approximates sea level
• However mountains and oceans affect gravity

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The Spheroid

• This is a mathematically defined surface that
  approximates the geoid
• There have been many
• Datums, ellipsoidal parameters
• Now almost universally we use a model based on
  the centre of gravity of the planet (geocentric)

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Map Projections

• In order to show the spheroidal Earth on a planar
  piece of paper we need to re-project coordinates
• There are many ways this can be done
• Some preserve area
• Some preserve direction

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From the Sphere to a Plane

• We have developed models and algorithms to
  transform spherical coordinates to rectangular
  coordinates
• For most mapping, this is based on the Universal
  Transverse Mercator projection (UTM)
• At this stage, we will ignore the maths!

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GDA

• The coordinate datum for Australia has been
  changed
• It is now geocentric
• This means that all new maps will agree with GPS
  coordinates
• Coordinates based on GDA are called MGA

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Useful Links

• http//www.icsm.gov.au/icsm/gda/
• www.land.vic.gov.au