3 - Coordinate systems Flashcards
xy reading also known as
geographic location
Longitude / meridians
The angle subtended from the centre of the earth E/W of Greenwich (Prime meridian, 0degrees)
Range of longitudinal values
180E - 180W
Latitude/ parallels
The angle subtended from the centre of the earth north (N) and south (S) from the equator (0degrees)
Range of latitude values
90N - 90S
(North pole to South pole)
Graticule
A planar grid system of intersecting latitude and longitude lines
NE
+ long
+ lat
SE
+ long
- lat
SW
- long
- lat
NW
- long
+ lat
Lat south of equator
NEGATIVE
Long west of Greenwich
NEGATIVE
On maps / analog coordinates
DMS
GPS coordinates
DDM
GIS coordinates
DD
Oblate spheroid
Earth is wider at equator than at poles
Model of Earth considerations
- True surface of Earth
- Mean sea level (geoid)
- Reference ellipsoid
Major axis A
Equatorial radius
Minor axis B
Poles to the centre
Simplest mathematical model of the earth
Ellipsoid
Datum def
Mathematical model of the Earth & serves as a reference for calculating geographic coordinates.
Components of a datum
- A reference ellipsoid
- A reference point/ location
When is geometric distortion of geographic coordinates largest?
When the distance of a location is further away from the reference point of a datum on an ellipsoid.
Cape reference ellipsoid
Modified Clarke 1880
Cape initial point
Buffelsfontein - trigonometrical beacon, near Port Elizabeth
Hartebeesthoek 94 reference ellipsoid
WGS84
Hartebeesthoek 94 initial point
Hartebeesthoek Radio Astronomy telescope, near Pretoria
International reference ellipsoid
WGS84
International reference point
Centre of earth mass
What to use when measuring distance and area?
Planar (GIS and maps) - coordinate systems which allow for calculations
Ellipsoid - DMS so not appropriate for measurements
