# 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