Geodesy Projections Flashcards
What is Geodesy? (3)
Geodesy is the science of accurately measuring and understanding three fundamental properties of the Earth:
… it’s geometric shape
… it’s gravity field
… it’s orientation in space
Shape of the Earth
Biaxial Ellipsoid
Why is an ellipsoid useful?
It allows us to overlay a horizontal spatial location on earth
Horizontal spatial location uses what measurements (2)
Latitude and Longitude
What is the prime meridian?
Central Vertical Line bisecting the earth
What does 1” equate to in meters along the equator
~31m
What happens to the value of 1” as we move north/south of equator?
Decreases
What is a Geoid?
A model of earth which closely follows mean sea levels
What forces is a Geoid mainly affected by?
Irregularities of gravitational fields.
What is the term used for elevation?
Orthometric height
How do we calculate Orthometric Height?
(H) Height of the lands surface above the geoid
What is Geometric height?
(h) height of the land surface above the conventional ellipsoid
Geoid height =
(N) Distance between Geoid and Ellipsoid
H = h - N
Orthometric height = Geometric height - Geoid Height
What is a datum?
A reference point
What is a significant complication in choosing datums?
Plate tectonics constantly shifting
Conformal projections
Preserves local shape/angles
Equal area projections
Preserve the areas of displayed features
Equidistant projections
Preserve the distances between certain points
Name two key projections used in UK (2)
(OSNG) Ordinance Survey National Grid and (UTM) Universal Transverse Mercator
Scale factor of distortion
F = 1 (e^2/2R^2)
What is used to reduce distortion in a transverse Mercator projection?
2 lines of intersection
