Geodesy The study of the size and shape of the Earth.
The Earth is… • 3000 BC Babalonians An oyster • 600 BC Greeks Flat • 500 BC a perfect shape A sphere • 300 BC Circumference ~ 25,000 miles, • Dark ages Flat again • 1492 Columbus A pear • 1753 French Oblate ellipsoid • Spheriod • Geoid to describe the deviations from a spheriod
Geographic Coordinate System • Parallels • Meridians • Great and Small Circles
Geographic Coordinate System GCS uses a 3-D spherical surface to define locations on Earth. GCS includes an angular unit of measure, a prime meridian and a datum.
Locating yourself on a Sphere • You need a frame of reference • That is the purpose of Latitude and Longitude • Defining these parameters: • Earth rotates on an imaginary axis ~ North and South Poles • Equator: is a great circle that lies equidistant between them.
Great Circles • ..are imaginary circles of the surface of the earth who's plane passes through the center of the earth. • The circumference of the earth is 25,000 miles or 40,000 km • "Great" because it is the largest possible circle
Great Circles: • Cut the earth in half and each half is known as a hemisphere • Are the circumference of the earth • Provide the shortest routes of travel on the earth's surface. • ** Planes travel in great circles. • ** We were always taught a line is the shortest distance between two points - Not True. • Small circles: circles whose planes do not pass through the center of the earth.
Latitude • Latitude: is the angular distance north or south of the equator. (0 –90 degrees N or S) • 1° of latitude = 40,000 km/ 360° • 1 degree = 60 minutes • 1 minute = 60 seconds 36°49'52" N • ArcView 3.x uses Decimal Degrees only • Sextant measures the angular distance between 2 points (sun & horizon) • **So it easy to determine latitude.
Longitude: • Longitude: no natural reference point • In 1884 by International Agreement Greenwich England was the chosen starting point. • This is called the prime meridian or zero degrees and everything is east or west of that. • Longitude is the angular distance east or west from Greenwich, England • (0 – 180 degrees E or W)
Geographic Coordinate System • Longitude and Latitude • Degrees, minutes, seconds • 1o latitude ~110.5 km (equator) • 1o longitude = cosine of the latitude • 1 minute of latitude ~1852 m
How to convert from DMS to DD • Example: 37°36’30” • Divide each value by the number of minutes or seconds in a degree • 36 minutes = .60 degrees (36/60) • 30 seconds = .00833 degree (30/3600) • Add it all up • 37° + .60 + .00833 = 37.60833 DD
The global grid: • Parallels: lines of latitude, only the equator is a great circle all other parallels are small circles (they never meet) • Meridians: these are line of longitude and when joined with its mate half way around the globe form great circles • * the distance between meridians will vary with latitude
How the Earth is Divided • Hemispheres: Northern, Southern, Eastern, Western
Time Zones • Solar noon: most towns used this, defined as when a vertical stake cast the shortest shadow. • By the 19th century transportation and communications (namely railroads and telegraph) connected towns and cities, the adopt of a standard time was necessary.
Time Zones (continued) • 1884 at the International Meridian Conference 24 time zones were established. • Greenwich Mean Time (GMT) = Universal time = Zulu time • 360°/24 = 15° for each time zone, however for convenience many time zones follow state and country lines. • International Date Line: where each new day begins 180th meridian • Chronometer