Lecture 4: Geodetic reference systems and projections
What do we need to describe a location on earth?
A geodetic reference system
What is a meridian?
Halves of great circles that all come together at the poles
What is the longitude?
the angle between the plane of the prime meridian and that of the meridian through a point (E/W direction)
What are parallels?
imaginary lines parallel to the equator, perpendicular to the Earth's rotation axis
What is a latitude?
The angle between the plane of the equator and a line from the point to the center
What values can a latitude have?
0-90 N and 0-90 S
Does longitude or latitude come first in GIS?
Longitude
What are the units of longitudes and latitudes?
Degrees, minutes, seconds
How do we define the shape of the earth?
Use the rotational ellipsoid model
What is the definition of the rotational ellispoid model?
the ellips (2d) rotates around one of its axes
What do you call a rotation around the short axis?
Oblate spheriod
What is datum?
ellipsoid + where to place the ellipsoid in relation to earth
What is earth centered datum?
the centre of the ellipsoid is the centre of earth's mass and this is suited everywhere
What is local centered datum?
Surface of ellipsoid is the geoid for a certain location and very suitable in some locations
What is geoid
equipotential surface in the Earth gravity field. Mean sea surface and its imaginary extension
Geoid seperation/height =
geoid - ellipsoid
Orthometric height =
height above sea level/geoid
Ellipsoidal height =
height above the ellipsoid
What defines a geodetic reference system?
ellipsoid model, location of ellipsoid, coordinate system
Name three tangent projections
azimuthal, cylindric, conic
Azimuthal projection =
plane touches the earth at one point. Good for small areas
Cylindric normal =
it touches the equator
Cylindric transverse =
it touches a meridian
Normal Conic projection =
cone touches the globe along a parallel
Name the three properties of map projections
Equal area, conformal, equidistant
Equal area projection =
Preserves the relative size of geographic features and distorts the shape of features
Conformal =
Preserves the local shapes and only changes the relative size
what projection to use for N-S direction?
Transverse cylindrical projection
What is the standard meridian?
Where the projection is correct - defines where the cylinder touches the earth model and the origin of the plain coordinate system
Standard parallel is for....
Cone projections