Geodesy Flashcards
What is geodesy?
Science of measuring and understanding
- Geometric shape
- Gravity field
- Orientation
Why is geodesy important?
Allows us to establish location
What is the shape of the earth?
Biaxial ellipsoid
12,714 km - polar diameter
12,756 km - equatorial diameter
What is the coordinate system?
Latitude and longitude
0° longitude - prime meridian
Latitude - -90 to 90° (N/S)
Longitude - -180 to 180° (W/E)
What is the definition of longitude?
Rotational angle from the prime meridian
What is the definition of latitude?
Angle from the equator
What are the associated parameters?
Semi major axis - Rea - 6,378,147.0 m
Flattening factor - f - 1/298.257223563
Semi minor axis - Reb (Rea (1-f))
What is the ellipsoidal horizontal spatial position?
Earth centred
Earth fixed coordinates
What are ellipsoids used for?
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
What is the definition of an ellipsoid?
Approximation of the universal best fit of the earth
- universal best fit (GRS80 - gps)
- local best fit (Airy 1839 - UK coordinate system)
What is a Geoid?
Model that follows mean sea level
Used as a vertical reference plane
Geoids and ellipsoids aren’t always aligned
What are Elevations?
H
Orthometric heights
Determined by calculating the height of the land surface above the geoid
What are geometric heights?
h
Established relative to a conventional ellipsoid
- calculated from coordinates
What is vertical distance?
Geoid height (N)
H = h + N
What is a datum?
Reference point for the elevation of surfaces and features on the earth
Defined relative to mean sea level
What is Mean sea level?
Described as the arithmetic mean of the hourly water elevation of a 19 year cycle
How do plate tectonics affect things?
Cause changes in the shape of the earth
Move around
Coordinate systems aren’t static
