Lecture 2 Georeferencing Flashcards
How does georeferencing help us?
1) Georeferencing helps us locate features in geographic space
2) Georeferencing provides us accurate geographic location and extent information
Georeferencing includes information on (4)
Datums
Ellipsoids
Geoids
Coordinate systems (x,y,z)
What are datums?
a datum is a mathematical model that approximates the shape of the earth to enable accurate positions, length and area calculations
What is Geodesy?
Geodesy is a study of the Earth’s shape, orientation, and positions for the past, present, and future.
What is an ellipsoid
ellipsoids are primarily used as a surface to specify point coordinates such as latitudes (north/south), longitudes (east/west), and elevations (height).
Different 6 ellipsoids used in GIS
- Australian National 1966
- Clarke 1866
- International 1924
- GRS 80
- WGS 84
- Nominal Radius of the Earth
What are Geoids?
The geoid is a model of global mean sea level that is used to measure precise surface elevations.
Horizontal Datums
A horizontal datum provides a reference frame for coordinates on Earth
Difference between NAD 27 and NAD 83?
NAD 83 is more accurate than NAD 27
NAD 27 is based on the longitude and longitude, and the direction between points while NAD 83 is an earth centered datum which does not have beginning point or direction.
What is a vertical datum used for?
A vertical datum is used for measuring the elevations of points on the earth’s surface.
What are the differences between the two coordinate systems? name and differentiate
Global coordinate systems
A geographic coordinate system (GCS) is used to define locations on a model of the surface of the earth.
projected coordinate systems
A projected coordinate system (PCS) is a GCS that has been flattened using a map projection. Never 100% real due to distortion
Longitude and latitude, which one runs vertically and horizontally? which one has the prime meridian/ equator?
Longitude run LONG and vertically, prime meridian
Latitude run across and horizontally equator
1(degree)=56’=3500”… what is degrees, minutes and seconds?
1(degree)=56’(mins)=3500”(secs)
What is the rule of thumb involving 1(degree) aprox. equal to 111km?
If longitude is the same and if there is a degree of difference between latitudes is almost certain that they are aprox. 111km apart.
Eg. (a(24(degree)N, 105(degree)E)
(a(25.5(degree)N, 105(degree)E)
=1.5*111=answer..
What is conformal projection?
A projection that preserves the correct shapes of small areas.
