Geodesy

Question 1

What is geodesy?

Answer 1

Science of measuring and understanding

• Geometric shape
• Gravity field
• Orientation

Question 2

Why is geodesy important?

Answer 2

Allows us to establish location

Question 3

What is the shape of the earth?

Answer 3

Biaxial ellipsoid
12,714 km - polar diameter
12,756 km - equatorial diameter

Question 4

What is the coordinate system?

Answer 4

Latitude and longitude
0° longitude - prime meridian

Latitude - -90 to 90° (N/S)
Longitude - -180 to 180° (W/E)

Question 5

What is the definition of longitude?

Answer 5

Rotational angle from the prime meridian

Question 6

What is the definition of latitude?

Answer 6

Angle from the equator

Question 7

What are the associated parameters?

Answer 7

Semi major axis - Rea - 6,378,147.0 m
Flattening factor - f - 1/298.257223563
Semi minor axis - Reb (Rea (1-f))

Question 8

What is the ellipsoidal horizontal spatial position?

Answer 8

Earth centred

Earth fixed coordinates

Question 9

What are ellipsoids used for?

Answer 9

Give vertical position but not in a useful sense as the earth doesn’t have a smooth surface

• used for horizontal measurements
• used as a reference point for ground level

Question 10

What is the definition of an ellipsoid?

Answer 10

Approximation of the universal best fit of the earth

• universal best fit (GRS80 - gps)
• local best fit (Airy 1839 - UK coordinate system)

Question 11

What is a Geoid?

Answer 11

Model that follows mean sea level
Used as a vertical reference plane
Geoids and ellipsoids aren’t always aligned

Question 12

What are Elevations?

Answer 12

H
Orthometric heights
Determined by calculating the height of the land surface above the geoid

Question 13

What are geometric heights?

Answer 13

h
Established relative to a conventional ellipsoid
- calculated from coordinates

Question 14

What is vertical distance?

Answer 14

Geoid height (N)

H = h + N

Question 15

What is a datum?

Answer 15

Reference point for the elevation of surfaces and features on the earth
Defined relative to mean sea level

Question 16

What is Mean sea level?

Answer 16

Described as the arithmetic mean of the hourly water elevation of a 19 year cycle

Question 17

How do plate tectonics affect things?

Answer 17

Cause changes in the shape of the earth
Move around
Coordinate systems aren’t static