SC120 Map Interpretation, Air Photos, and GPS

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Module 3 - Map Coordinate Systems: Latitude and Longitude

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Reading Assignment: Chapter 6 of Sebert text.

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Coordinate Systems

• Coordinate systems are also known as Grid Systems or Locational systems.

• Maps portray the points or locations of features on the earth's surface.

• To find a place exactly, you need crossing lines that create intersections. This is a grid or coordinate system.

• Points or locations on the surface of the earth or on a map are defined by "X" (east-west) and "Y" (north-south) axes.

• Location of points on a map is called map referencing. A good map reference system must fulfil the following conditions:
  • It must be applicable to any point on the earth.
  • It must be applicable to all map scales.
  • It must give a point reference to any required precision.
  • It must be easy to understand and to use.

• Two important coordinate systems are:
  • Latitude/Longitude - Angular distance (degrees/minutes/seconds).
  • Universal Transverse Mercator (UTM) - Linear distance (meters).

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Latitude/Longitude

• System of latitude and longitude defines the location or geographic coordinates.

  

• It is one of the oldest systematic methods of a point location.

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Latitude:

• Angular distance (in degrees, minutes, seconds) of a point north or south of the equator.
  

• Equator:
  • A great circle.

  • An imaginary line halfway between the North and South Poles.

  • Goes around the middle of the earth, running east-west.

  • Divides the earth into Northerna and Southern Hemispheres.

• Lines of Latitude or Parallels -
  • Series of small circles parallel to the Equator.

  • Parallel lines define locations north and south of the equator.

  • Parallel lines are numbered in degrees north and south from the equator
    • Equator is assigned a value of 0 degree.
    • Poles are assigned a value of 90 degrees. Distance from equator to North or South Pole is 1/4 of the distance around the earth (360 degrees)

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Longitude:

• Angular distance (in degrees, minutes, seconds) of a point east or west of the Prime Meridian (that passes through Greenwich, near London, England).
  

• Lines of longitude are known as meridians. They are all great circles passing through North and South Poles.

• Lines of longitude converge at the poles. Distance between two meridians decrease towards poles.

• Distance between two meridians greatest at the equator. At equator, one degree east-west is 69 miles.

• Prime Meridian is the starting and stopping point for east and west directions.

• Prime Meridian is given a value of 0^o longitude.

• You can travel west or east halfway around the earth to the 180^o west or east (half of 360 degrees).

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Addressing Lat/Long Locations

Intersection of a parallel (latitude line) and a meridian (longitude line) gives a latitude and longitude reference.

• Latitude N (Northern hemisphere, north of equator)
• Latitude S (Southern hemisphere, south of equator)
• Longitude W (Western hemisphere, west of Prime Meridian)
• Longitude E (Eastern hemisphere, east of Prime Meridian

Example: 53^o,15^',15^"N Litutude, 110^o,15^',15^" W Longitude.

Measure your latitude by using night sky in Northern Hemisphere Find the North Star (Polaris). Extend one arm toward the star. Extend your other arm toward horizon. Use a protractor to measure the number of degrees between your outstretched arms. If the angle is 53 degrees, you are located 53o N latitude. At the equator, Polaris appears right at the horizon. Equator is at 0o. At the North Pole, Polaris is almost exactly overhead. North Pole is at 90o N Latitude.

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Graticule

Pattern of meridians and parallels on the earth is referred to as the graticule.

Following characteristics of the graticule are based on the assumption/fact that the earth is a sphere:

• Parallels are true east-west lines.

• Parallels are equally spaced between the equator and the poles.

• Parallels are always parallel to one another, so any two parallels are always the same distance apart all the way around the globe.

• Meridians are spaced farthest apart on the equator and converge to a single point at the poles.

• Parallels and meridians cross one another at right (90^o angles).

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