Map projection selection Flashcards
Why is selecting a map projection difficult?
Many variables involved when creating a map. Like the map scale, the size of the area being mapped, the level of generalisation and the thematic symbology.
There are many projections to select from, one will always have to compromise.
Snyder’s heirarcal map selection guideline
Organised to:
1. The region of the world to be mapped
2. The projection property
3. The projection case
Snyder’s geographical areas.
- World
- Hemisphere
- Continent, ocean and smaller region
World map projections (conformal)
If conformality and constant scale along the standard line is required
The Mercator projection is a good choice
Constant scale along the Equator
Other aspects:
Transverse = Constant scale along the chosen standard meridian
Oblique = Constant scale along the chosen standard great circle
World map projections
(equivalent)
If equivalency (true to area) is required, there are three types of projections to choose from:
1. Noninterrupted (mollweide)
2. Interrupted (goode homolosine)
3. Oblique aspect (Mollweide oblique)
World map projections
(equidistant)
If an equidistant map projection is needed, then the azimuthal projection is preferred as all points on the map has the correct direction (azimuth) from the centre point.
World map projections
(compromise)
If both directional and aerial distortion should be minimized, then a compromise projection such as the Robinson projection is a good choice as the distortion is only severe at high latitudes.
Hemisphere map projections
(conformal)
If conformality is required for a map covering a hemisphere, then the stereographic projection is a good choice as the direction is also preserved from the centre of the map to any other point on the map.
e.g Stereographic projection
Hemisphere map
projections
(equivalent)
If equivalency (true to area) is required for a map covering a hemisphere, then the Lambert azimuthal equal area projection is a good choice. Note that angles are not accurately represented and scale only true at the standard point (centre of map).
e.g Lambert azimuthal projection
Hemisphere map
projections
(global view)
The Orthographic map projection provides a realistic view of the earth as it would appear from space.
One sixth rule
The “one-sixth rule” places the first standard parallel at one-sixth the range above the southern boundary and the second standard parallel minus one-sixth the range below the northern limit.
Effect of map projections at large mapping scales (geographic small scales)
as area becomes smaller the effect of chosing conformal or equdistant map projection is less NB.
for smaller areas the standard lines can be chosen so that most of the area being mapped is very close to the standard lines.
SF deviations are very small.
Choosing the correct standard line(s), and limiting the projection
extent becomes very important
UTM
Universal Transverse Mercator.
Projection is a popular projection for mapping regions of 6° width or less.
The UTM projection applies northings and eastings (shifts) so that all the coordinates within the mapped area is positive.
Lo Coordinate System
South African co-ordinate system
This coordinate system is based on the Gauss Conform projection, which is for all practical purposes identical to the Transverse Mercator projection (south orientated)
SF increases away from central meridian.
