Location, Location, Location

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

The Earth as a Geoid

Geographic Coordinate System

•Parallels •Meridians•Great and Small Circles

Geographic Coordinate System

GCS uses a 3-D sphericalsurface to define locationson Earth.

GCS includes an angular unitof measure, a prime meridianand 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 

Time Zones