9 Locational reference systems and Map projections

Why is it difficult to locate features on the earth’s surface?

no standard reference system, each has their own purpose

What does LRS stand for?

Locational Reference System(s)

Which model for the Earth surface is used for LRS?

(Which model is the datum?)

What are characteristics of a LRS?

• geometry (spherical / planar = “projected”)

• coverage (global / continental / local)

• units (metric / imperial / angular)

Name 3 examples for LRS

Example 1: What is the CH1903+ / LV95?

Swiss local reference system

based on Bessel-Ellipsoid, datum at Zimmerwald

planar, uses 2 dimensions, third dimension is used for height

works because CH is small

Example 2: What is the UTM?

Global reference system for military

planar, easting and northing

world is sectioned in grid, coordinates for cell in the grid

Example 3: What is the WGS 84?

Global reference system for GPS

geocentric, datum at the center of the earth

geographic, latitude and longitude

How can height be measured?

vertical datum usually based on ellipsoid, because it’s easier

Principle of projection (of the curved earth to planar 2D)

Earth’s surface is flattened

1. use approximation (3D)

2. do transformation (3D → 2D)

How is this projection done for the Swisstopo maps?

Datum at Zimmerwald (Bern)

3D approximation: ellipsoid

2D transformation: oblique, tangential cylinder projection; conformal (anges are preserved)

Which steps should you take to find the ideal map projection?

1. set scale of reference globe

2. choose developable surface (cone / cylinder / plane)

3. project globe to surface

4. rock an unroll

Does the projection matter for the analysis of a map?

Yes

sometimes it has to be adjusted

What aspects and cases can a projection have?

Projection uses in cartography / GIS:

• What is (Web) Mercator used for?

• Advantages

• Disadvantages

online mapping

Advantages:

• always up north

• seamless, works for the entire globe

Disadvantages:

• not equivalent (not area-preserving), e.g. Africa is smaller than reality

Projection uses in cartography / GIS:

• What is Azimuthal Equal Area used for?

• Advantages

• Disadvantages

global scale thematic maps

Advantages:

• area preserving

Disadvantages:

• angles distorted

Projection uses in cartography / GIS:

• What are 3 angle-preserving projections

• What are they used for?

• Advantages

• Disadvantages

Mercator, Gall-Peters (Cylindrical Equal Area), Conic Conformal

Navigation, large map scales

Advantage: angle-preserving

Disadvantage: area distorted

Projection uses in cartography / GIS:

What is the rhumb line?

What is it used for?

Rhumb line or loxodrome (Schieflaufende crosses all meridians at the same angle

Angle-preserving

→ shortest connection between 2 points

→ good for navigation, large map scales

Projection uses in cartography / GIS:

• What is Gnomonic projection used for?

• Advantages

• Disadvantages

shows shortest distance between two points on sphere

Advantage: direction preserving

Disadvantage: direction preserving only at central touching point

Projection uses in cartography / GIS:

• What is Azimuthal Equidistant projection used for?

• Advantages

• Disadvantages

distances (flow maps), shortest travel directions

Advantage: distance-preserving

Fazit?

No best projection!

Each projection can preserve one or more, never all characteristics

But, what are compromise projections?

Balance between distortion properties

Klicker:

How do we call the form expressed by the dashed line?

Ellipsoid (dotted line)

Geoid (white circle)

Klicker:

This locational reference system is?

Local

LV95

Cartesian

Klicker:

The dashed line is called?

Loxodrome (Schieflaufende)

Klicker:

Is this an appropriate map projection to show the literacy rates of countries on a global scale?

Klicker:

Is this a useful map projection for showing the population density of Russia?

Klicker:

This map projection is?

Conformal (angles preserved - see circles, ellipses would mean angle distortion)

Azimuthal

Klicker:

This map projection is?

Equidistant

Klicker:

This map projection is?

Conformal (distorted areas, preserved angles)