Cartography and GPS

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Cartography and GPS
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Projections

• A projection is a method of portraying the curved
  surface of the earth on a flay surface.
• Distortions of distance, direction, scale, and
  area always occur.
• Some projections preserve one property but
  distort the others (usually badly)
• Other projections distort all properties but less
  strongly.
• Selection of a given projection depends on the
  use.
• An airplane pilot might use an azimuthal
  preserving projection.
• Someone in the polar regions might use a polar
  projection.

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Types of projections http//mac.usgs.gov/mac/isb/p
ubs/MapProjections/projections.html

• Cylindrical projections
• Wrap a cylinder around the earth
• Mercator, UTM
• State plane
• Conic projections
• A cone rather than a cylinder
• Albers
• Azimuthal
• Project onto a plane
• Lambert
• Polar
• Others
• Robinson
• Sinusoidal

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Universal Transverse Mercator (UTM)

• Conformal cylindrical projections
• Scale correct only at central meridian
  distortion increases with distance from meridian
• 60 different zones 6 degrees wide for both north
  and south hemispheres
• Projection differs (different cylinder) for each
  zone
• Low distortion near equator
• Cannot combine zones
• A living fossil poorly suited for current uses
  but still widely used.
• California in in UTM zones 10 (S. Cal) and 11 (N.
  Cal).

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State Plane

• Usually similar to UTM but uses a different
  projection for each state/zone
• Used only in US
• A mix of NAD27 or NAD83
• Must be careful
• Errors of 10s feet possible
• Coordinates denoted in feet (or meters) as false
  northing and false easting from point of
  origin.

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DEM

• Often in a geographical (longitude,latitude)
  projections.
• Can be displayed as a variety of different
  projections.

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Remote sensing data

• Usually an image from an airplane or satellite.
• Original data is distorted in some way
  (especially for radar data).
• Most useful if registered (image-to-map) to some
  projection.

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GPS

• Started in 1978
• Now composed of at least 24 satellites
• Funded and controlled by US Dept. of Defense
• 12 hour orbit
• Sends signals at two wavelengths, L1 and L2 to
  compensate for ionospheric error.
• Sends location of satellite and satellite health
  information.
• Distance (range) from receiver to each satellite
  is measured.
• This can be used to solve (with least squares)
  the location of the receiver.
• Horizontal accuracy is much better than vertical
  accuracy.

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Sources of error

• Poor visibility of satellites due to trees,
  buildings or cliffs.
• Poor configuration of satellites.
• Multipath (reflected signals)
• Dithering by the US DOD.
• Typical error is 5 to 15 m horizontal for newer
  handheld GPS at least 10 m vertical.
• Check PDOP for estimate of error

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Other measurements

• Velocity (try a car)
• Bearing or azimuth
• Store waypoints along a route.
• Can be downloaded to a PC.
• Maps created in mapping program such as Arcview.
• Can get better accuracy with differential GPS.

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GPS and geology

• Field mapping
• Normal unit is not really quite good enough for
  detailed mapping.
• Differential GPS can solve this but requires
  extra effort afterwards.
• Use a total station or laser rangefinder for the
  most detailed mapping.
• Use GPS for absolute location
• Use total station for the relative locations.

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GPS and geology

• Differential and phase measurement GPS can be
  used to measure plate motions.
• Takes expensive equipment and considerable
  post-processing.

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S. California plate motions