Datum

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Datum Coordinate Systems

• Lecture 6

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Representing GeographyMaps

• Maps are a 2-D representation of Reality
• Legend
• Scale (ratio of map distance real world
  distance)
• Directional arrow
• As a 3-D world is represented in 2-D, the
  transformation Always involves distortion

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Datum

• A mathematical model that represents an
  approximation of the earth.

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• Once you have a mathematical model of the size
  and shape of the earth, you can apply a
  coordinate system to in.
• Coordinates without a datum are vague
• Datum provides an origin that gives meaning to
  the coordinates
• Datum are errorless in Abstract
• When a datum is applied to reality, error is
  introduced

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• Alexander Clarke - 1866
• Calculated the shape of the earth based on
  surveying measurements
• Established the Clarke 1866 Ellipsoid
• Standard reference for mapping
• Used to create NAD27
• North American Datum of 1927
• NAD83
• North American Datum of 1983
• Uses GRS 80 Ellipsoid as its Base Model

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• GPS has contributed to Datum Development over
  time
• WGS 84 - World Geodetic System 1984
• the reference frame upon which all
  geospatial-intelligence is based
• Latest update 2004
• NGS - National Geodetic Service
• Brass markers are placed to pinpoint locations
  based on Datum

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Coordinate Systems

• Systems used to pinpoint a location as related to
  an origin
• Cartesian Coordinate System
• X, Y axis
• Based on an origin
• Divides an area into 4 Quadrants
• Upper right (, )
• Lower left (-, -)

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UTM State Plane

• UTM
• The National Imagery and Mapping Agency (NIMA)
  (formerly the Defense Mapping Agency) adopted a
  special grid for military use throughout the
  world called the Universal Transverse Mercator
  (UTM) grid. In this grid, the world is divided
  into 60 north-south zones, each covering a strip
  6 degrees wide in longitude. These zones are
  numbered consecutively beginning with Zone 1. In
  each zone, coordinates are measured north and
  east in meters.

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• State Plane
• The state plane coordinate system (SPCS) is
  similar to the UTM system in many respects, but
  it does have some important differences. In the
  SPCS, each state has its own zone(s).The number
  of zones in a state is determined by the area the
  state covers and ranges from one for a small
  state such as Rhode Island to as many as five.

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Geographic Grid Coordinate System

• Degrees, Minutes, Seconds
• DD MM SSS
• Decimal Degrees
• DD.MMMMMMMM
• Origin
• Equator
• Prime Meridian

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Latitude

• Measured as the internal angle from an origin in
  the center of the earth.
• 0 degrees - Equator
• 90 degrees - Poles
• North is Positive
• South is Negative

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Longitude

• Measured as a portion of the circumference of the
  earth.
• Longitude lines are all Great Circles
• Range 0 180 degrees
• East is Positive
• West is Negative

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Degrees, Minutes, Seconds

• 1 Degree Latitude - 69 Miles
• 1 Degree Longitude - 69 Miles at the Equator
• Converges at the Poles
• In San Diego, 1 Degree Longitude 58.5 Miles
• 1 Degree 69 Miles
• 1 Minute (1/60 of a Degree) 1.15 Miles
• 1 Second (1/60 of a Minute) 0.01917 Miles
  (101.2 Feet)

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Where is San Diego?

• 32o 28 18 N (Latitude)
• 117o 48 53 W (Longitude)
• Also represented as
• 32o 28 18
• -117o 48 53

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Decimal Degrees

• Increment between lines of latitude or longitude
  are represented as the decimal fraction between
  them.
• lat 32.720939 N long 117.151969 W
• or
• lat 32.720939 long -117.151969

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Converting from DMS to DDD

• The latitude of London expressed in DMS is 51
  29' 16" North. To convert this location to DD,
  follow these steps
• 1. Divide each value by the number of minutes
  (60) or seconds (3600) in a degree
• 29 minutes 29/60 0.4833 degrees
• 16 seconds 16/3600 0.0044 degrees
• 2. Add up the degrees to get the answer
• 51 0.4833 0.0044 51.4877 DD