Goals for this week

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Goals for this week

• Understanding geographicacy
• Understanding cartographic communication
• Relationship of scale to cartography
• Map symbolism
• Know different families of map projections
• Familiarity with some geographic grid systems
• Understand the difference between cartographic
  and geographic data

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Geographic Grids
A geographic grid is a system for describing or
defining a position on the earth in terms of (a
pair of) numbers
Co
Ordinates
There are more than 20 grid systems supported in
almost any commercial GPS recievers
Most GIS have similar capabilities
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Major grid systems (from my GPS)

• Geographic (lat.-lon.)
• Universtal Transverse Mercator (UTM)
• State Plane
• Taiwan grid
• Swiss grid
• Swedish grid
• New zealand grid
• MGRS
• Maidenhead
• India (various)
• Irish grid
• German grid
• British grid

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Grid Systems
We are familiar with grid systems from our early
geometry classes

• Any point can be described in a plane (2D space)
  by a coordinate pair

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Grid systems

• A line segment can be describe by a pair of
  coordinate points

• A more complex line can be expressed by bunch of
  line segments
• A polygon is several lines closing an area

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Coordinate geometry

• We have known how to make calculations using
  coordinate geometry for centuries

• Pythagoran theorum
• Euclidean distance

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Polar vs. Planar coordinates
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Three-dimensional coordinates

• Polar

• Planar

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Coversion of planar to polar
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Earth-based coordinates

• Geographic grids are describing the locations of
  things on the surface of a somewhat round earth
• Some referencing systems describe the earth as a
  sphere
• Others take distortions into accounts, and use a
  spheroid or elipsoid

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Ellipsoidal parameters

• Semi-major axis
• Semi-minor axis
• Eccentricity

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Some reference ellipsoids
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Geodetic datum
Geodetic data (datums in some text books!!!!)
account for deviations from a spherical earth
caused by things other than the flattening at the
equator they are ways of modeling the size and
shape of the earth

• Topographic and sea-level models
• Topographic surface
• Sea level
• Gravity models and geoids

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More on geodetics

• Datum can be anything from a flat-earth model to
  a complex (bumpy) model of the earth's surface
• Different nations have different reference data
  and different responsible agencies
• (in the US, it is the US geodetic survey)
• Global systems can measure things over large
  distances, but are cumbersome or simply
  inaccurate at small distances
• Regional systems are highly precise for small
  areas, but useless for large distances
• Linking your GIS dataset to the wrong datum can
  cause humungous errors

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Geographic projection

• The most familiar geographic referencing system
• Developed in Greenwich Observatory, London
• Any point on the earth can be described as a
  combination of two angles

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Polar coordinates

• Location on 3D space
• Angle from equator
• Angle from greenwich
• Distance from center of earth
• Dependent on reference ellipsoid!

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More on geographic coordinates

• Highly accurate for describing location of single
  point
• Useful for calculating large distances
• Distances must be calculated via arc distances
• Cumbersome to not useful at all for calculating
  areas

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Universal transverse mercator UTM

• This subject is best broached after a discussion
  on projections
• The UTM coordinate system is essentially a
  projection that allows the surface of the earth
  to be described in planar coordinate geometry
  (flat earth model)
• This can be done for small sections of the earth
• By dealing with the earth as a flat surface,
  calculations such as distance and area are
  greatly simplified

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UTM zones

• Divides the earth into a number of zones
• Each zone is 6 wide
• Starts at international date line (180)

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UTM zone

• Each zone is a thin band, going from 82 S to 82
  N
• Vertical component of location is described as
  distance from equator (in northern hemisphere) or
  south pole (in southern hemisphere
• Called northing
• Horizontal component is desrcibed as distance
  from central meridian
• Horizontal component is shifted by 500,000, so
  that we always have positive numbers
• Called easting

• Distances are in meters

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UTM example
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State plane system

• Read the discussion in chapter 3!!!

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