Question and Answers on Earth Shape and Geographic Grid

Chapter 4

 

Q1:   List and define the three more accurate approximations to the earth’s shape?

 

1. The earth is  a sphere: The earth is considered as spherical shape

 

Pythagoras (in 6th Century BC) said that ‘humans must live on a perfect sphere’? Aristotle also argues in the favor of a spherical earth.

For small scale mapping (continents, countries, large areas), the earth is still considered as sphere “authalic sphere” with 6,371 Kilometers as radius and 40,030 KM as a circumference.

 

2. The earth is  an ellipsoid: The earth is considered as an ellipse

 

In 1670, Issac Newton proposed that earth is an ellipsoid based on his theory of gravity. Earth bulged at equator and flattened near the poles

Based on satellite data World Geodetic System (WGS) 72 and 84 ellipsoid gives nearly best fit to the earth shape (Draw Figure 4.2 on Page 43).

 

3. The earth is  a geoid:

The earth is considered as 3 Dimension shape that approximate the Mean Sea Level (MSL) and it is a surface of equal gravity (equi-potential surface)

 

Q2: What are the cartographic use of  sphere, ellipsoid, and geoid?

Sphere     :  For small scale mapping (continents, countries, large areas)

Ellipsoid  : For large scale mapping (topographic maps and cadastral plans)

Geoid     : For horizontal and vertical positions of control points (Elevation related to Mean Sea Level, Triangulation)

 

Q3: List and define the most common coordinate systems used in cartography?

 

1. Geographical Coordinates: Based on equator and prime meridian (latitude, longitude)

Draw Figure 4.4 and 4.5  on page 47.

2. Plane coordinates (Cartesian system): Based on geocenteric approach (the earth’s center) and (X, Y, Z)

Draw Figure 6.7 on page 98.

 

Q4:Define the followings:

 

Latitude  :     North-South angular distance from the equator

Longitude:   East-West angular distance from prime meridian (Greenwich)

Authalic Latitude: North-South distance on sphere between each degree of latitude (identical value 111.2 KM).

Geodetic Latitude: Distance between geodetic latitude is not quite same (see authalic latitude). It is greater in polar areas        (111.69 KM)  and less near the equator (110.57).

Parallel:     Line joining points with the same latitude

Meridian:  Line joining points with the same longitude

Graticule: Imaginary  network of parallels and meridians

Great Circle: the shortest path between any two points on sphere

Loxodrome: Line of constant azimuth