4: Map Projections

Question 1

EXPLAIN WHY WE NEED LOCATIONAL REFERENCE SYSTEMS

Answer 1

We need a locational reference system so we can accurately describe where something is located.
Might be x y coordinates, a grid reference on a map, or latitude and longitude.

Question 2

LIST TYPES OF LRS

Answer 2

Planar - assumes a 2D surface
Spherical - assumes a curved 2D surface
Most well know: latitude and longitude (spherical example of a geographic coordinate system)

Question 3

EXPLAIN WHY MAP PROJECTIONS MATTER FOR GIS

Answer 3

Maps are flat, the planet is not, projections are used to project the globe to a 2D plane. However, this introduces distortions!

Data layers can only be aligned in GIS given the same locational reference system AND projection!

It can also cause distances and areas to be wrong if you use a projection that distorts either of these properties.

Question 4

GIVE EXAMPLES OF MAP PROJECTION TYPES

Answer 4

Planar (azimuthal), Cylindrical, Conic

Question 5

LIST DISTORTION CHARACTERISTICS OF MAP PROJECTIONS

Answer 5

Three important properties:
Conformal (shape)
Equal Area (area)
Equal Distance/Direction (angle/direction and distance)

Question 6

Longitude

Answer 6

meridians
cut through the poles
always divide globe in half
each is equal in length
Prime Meridian (0) is Greenwich
widest apart at equator
closest at poles
measured in degrees relative to the prime meridian (-180 west to +180 east)

Question 7

Longitude

Answer 7

also called meridians
cut through the poles
always divide globe in half
each is equal in length
Prime Meridian (0) is Greenwich
widest apart at equator
closest at poles
measured in degrees relative to the prime meridian (-180 west to +180 east)

Question 8

Latitude

Answer 8

also called parallels
lie at right angles with longitude
concentric circles
each circle has a different circumference
0 at equator
are represented at the poles as a point
measured relative to equator -90 at south pole, +90 at north pole

Question 9

How is a projection different than an LRS?

Answer 9

A map projection uses mathematical formulas to relate spherical coordinates (x, y) on the globe to planar 2D Cartesian coordinates

Question 10

What is a projected coordinate system?

Answer 10

A geographical (spherical) coordinate system (LRS) projected onto a flat 2D plane.

Question 11

How is a map projection different than an LRS?

Answer 11

A map projection uses mathematical formulas to relate spherical coordinates (x, y) on the globe to planar 2D Cartesian coordinates

Question 12

Peters vs Mercator projections

Answer 12

Mercator - used for navigation, any course is a straight line
Peters - equal area

Question 13

Peters vs Mercator projections

Answer 13

Mercator - used for navigation, any course is a straight line, distorts massively at the poles

Peters - equal area, most accurate at equator

Question 14

3 types of map projections

Answer 14

Plane (azimuthal or planar)
resulting map is a circle, often used for mapping poles, distortion biggest at extremes and accurate in the middle

Cylinder (cylindrical)

Cone (conic)
wrap around one end of the globe, most accurate at the latitude where it touches

in all examples… most accurate where the ‘paper’ touches the globe (tangent) is the line of minimum distortion, centre map on this location

Question 15

Conformal

Answer 15

Distortion characteristics in which shapes of small features are preserved - anywhere on the projection the distortion is the same in all directions
(navigation, straight line has a constant bearing)

Question 16

Equal Area

Answer 16

Distortion characteristics in which shapes are distorted but preserve areal relationships (calculation of property areas, thematic mapping)

Question 17

Equal Distance

Answer 17

Distortion characteristics in which shapes are distorted but great circle distances/directions are true from one to all other points
(calc travel distance)

Question 18

Cylindrical projection

Answer 18

Wrap a cylinder around the globe
Mercator is one of the best known (cylinder wrapped around an equator)
Conformal distortion property
-at any point scale is the same in both directions
-shape of small features is preserved
-features in high latitudes are significantly enlarged

Question 19

Planar (azimuthal) projection

Answer 19

Touch paper with the globe
Azimuthal equidistant projection is one of the best known planar projections
Tangent location
-at a point on the sphere (standard point)
-or cut through the sphere (standard line)
Tangency can be varied: projection aspect
-polar aspect (tangent at pole)
-oblique aspect (at mid-latitude)
-equatorial aspect (at equator)
Distance/direction distortion property
-true from centre point along great circles

Question 20

Conic projection

Answer 20

Wrap a cone around the globe
Lambert Conformal Conic projection commonly used to map North America
-standard parallels occur where cone intersects the earth
-lines of latitude appear as arcs of circles
-lines of longitude are straight lines radiating from the north pole