4 - Map projections

Question 1

Projection def

Answer 1

A mathematical transformation of the earths 3D spheroid
surface into 2D format, so that distances can be measured.
Geographical coordinates -> Cartesian (planar) coordinates

Question 2

Map projections can be described in terms of their:

Answer 2

1. Class (cylindrical, conical or azimuthal),
2. Point of secancy (tangent or secant),
3. Aspect (normal, transverse or oblique),
4. Distortion property (equivalent, equidistant or conformal)

Question 3

Map projection meaning

Answer 3

surfaces on which we project the shadows of the graticules… (plane, cylinder & cone)

Question 4

Tangent & secant def

Answer 4

The line of contact between the earth and the projection surface/plane is called a tangent.
If there are two such lines, they are called secants

Question 5

Tangent cylindrical projection def

Answer 5

Circumference touches globe; one standard parallel

Question 6

Where do you have 0 distortion?

Answer 6

Tangents or secants; the contact point btwn the spheroidal earth’s surface and the plane of the map projection is the only location where the properties of the projection are true.

Question 7

Secant cylindrical projection def

Answer 7

Intersects the globe; therefore you have two standard parallels

Question 8

Where are the 2 standard parallels selected in a secant projection?

Answer 8

Selected at one-sixth below and above the limit of the mapping area

Question 9

Aspect def

Answer 9

The orientation of the projection plane/surface (normal, transverse or oblique)

Question 10

Conformal (orthomorphic) projection

Answer 10

Angles btwn lines in the
map are identical to the angles between the original lines on the curved reference surface.
- angles (with short sides)
- shapes (of small areas) are shown correctly on the map.

Question 11

Equal-area (equivalent) projection

Answer 11

Areas in the map are
identical to the areas on the curved reference surface

Question 12

Equidistant projection

Answer 12

Length of particular lines in the map are the same as the length of the original lines on the curved reference surface.

Question 13

Conformal and equivalent are…?

Answer 13

Mutually exclusive (cannot happen at the same time)

Question 14

Tissot’s Indicatrix

Answer 14

Tissot’s Indicatrix is a method to
visualize the distortions of a map
projection.

Question 15

Conformal distortions

Answer 15

• Preserves shape
• a=b
  -DISTORTS AREA (original size changes)
• Parallels and meridians intersect at 90

Question 16

Conformal examples

Answer 16

• Mercator (cylindrical & conformal)
• Lambert (conic & conformal)

Question 17

Equal area distortions

Answer 17

• PRESERVES AREA
• correct relative area of infinitesimally small area (Used for theory)
• a X b = 1
• DISTORTS shape

Question 18

Equal- area examples

Answer 18

• Mollweide (cylindrical, equal area)
• Albers equal area (conic, equal area)

Question 19

The act involved in South African coordinate reference system

Answer 19

Land Survey Act (Act 8 of 1997)

Question 20

SA Projection for Orthophoto maps

Answer 20

Gauss Conform (LO system)
1 : 10 000

Question 21

SA projection for topographical maps

Answer 21

Gauss Conform (LO system)
1 : 50 000

Question 22

SA projection for aeronautical maps

Answer 22

Lambert Conformal Conic
1 : 1 000 000

Question 23

SA projection for military maps

Answer 23

Albers Equal area
1 : 250 000
1 : 500 000
Topographical & topo-cadastral maps

Question 24

South African Projections

Answer 24

• The South African Coordinate Reference System
• Gauss Conform (LO – system)
• Lambert Conformal Conic
• UTM (Universal Transverse Mercator)
• Military maps

Question 25

Topographical vs topo-cadastral maps

Answer 25

Topographical maps - depict the physical characteristics of the terrain
Topo-cadastral maps -integrate topographical data with data showing property boundaries, subdivision lines, buildings, and related details.
- provide a comprehensive view of both the natural landscape and land ownership boundaries and parcels.

Question 26

Datum shows us

Answer 26

How the earth’s surface was generalized

Question 27

Map projection shows us

Answer 27

The nature of distortion on the
map

Question 28

Coordinate system shows us

Answer 28

How distances are measured

Question 29

RECAP

Answer 29

Geoid —> Datum —> Map

Question 30

Geographic coordinate system (GCS)

Answer 30

Defines where the data is located on the earth’s surface.

Question 31

Projected coordinate system (PCS)

Answer 31

Tells me how to draw on a flat surface, like on a paper map or a
computer screen.