2.4.1.3 Aspect
Aspect refers to the orientation of the developable surface to the reference globe. Aspect plays an important role in the position of standard lines, which means it also impacts where distortion is on the map.
There are three types of “aspect” that can be used on a developable surface:
1. Normal Aspect
A map projection with a developable surface that is oriented according to the Earth’s axis of rotation (with north at top and south at bottom by default). On normal tangent planar projections at the poles, the standard point is at the North or South Pole.
Notice that the developable surface is aligned with the poles.
2. Transverse Aspect
Transverse map projections turn the developable surface 90 degrees from the normal orientation. A transverse aspect results in standard lines that run along meridians (north and south). Think of transverse aspect as the Uranus projection – the planet that spins north-to-south.
The Transverse Mercator projection is simply a cylindrical developable surface turned 90 degrees, so the standard line runs from the North to the South Pole.
3. Oblique Aspect
Oblique map projections tilt the developable surface at an angle that is neither normal nor transverse. (These are often the most novel and fun projections!) Oblique aspects are generally used to 1. show great circle routes (with the Mercator), 2. perspective (e.g., how the Earth would look from a specific point in the atmosphere/from space), and 3. for aesthetic purposes.
Figure 21 shows an example of Great Circle Route.
Figure 22 shows an example of Perspective. Notice that it shows the Earth from space or the atmosphere.
Figure 23 shows an example of Aesthetic and just plain coolness!
In Figure 24, each of the developable surfaces represented using all three aspects.
Guidelines for Matching Aspect with Mapped Phenomena
- Apply a normal aspect for geographic features that are oriented primarily east-west (i.e., that are best depicted with a landscape page layout). This is particularly true for cylindrical and conic projections.
- Apply a transverse aspect for geographic features that are oriented primarily north-south (i.e., that are best depicted with a portrait layout). This is particularly true for cylindrical projections.
- Use oblique aspects when you are emphasizing the relationship between two locations or a particular perspective on a single location.
2.4.1.4 Centering
All projections need to be centered on the Earth. The center of your projection simply refers to the middle point of the projection and overall extent of your map. (You may have noticed that some projections cannot show the entire Earth at once!
The only rule of thumb for centering is this:
Always put the region of interest in the center and make it as large as possible on the map page (i.e., scale your map to fill the page or screen with your area of emphasis).
2.5 Projection Distortions
Projections are murderous. They disfigure, destroy, and maim our Earth. Never forget: any time you use a projection, you’re disfiguring Mother Earth! Do so with care.
How much do maps distort our Earth? Well, the Mollweide Projection is a commonly used world projection. One that we take for granted as looking “normal.” This is what it does to the Earth, though. The above is a room projected in a Mollweide Projection. You can see that different areas of the image have different types, and levels, of distortion. Imagine if we thought of images like this as accurate representations of our living quarters. It’s truly amazing just how warped our minds have become by looking at maps!
With that caveat…
To determine the amount and extent of distortion across a map, cartographers use the Tissot Indicatrix. This is a technique of graphically representing the distortions imposed by a projection (see image below).
Tissot’s Indicatrix works by placing perfectly round and scaled circles along standard lines or at standard points. The circles get distorted by shape, area, and angle the same way the Earth does.
2.5.1 Classifying Projections Based on the Properties Preserved
Projections are typically classified based on which properties are preserved. There are five types of projections based on property preservation.
2.5.1.1 Conformal Projections
If a map projection preserves angular relationships at very small points (i.e., form or shape). (Sticklers insist conformal projections don’t preserve shape, but you come off as nerdy if you don’t say these preserve shape, so feel free to. ;-) )
What a line would look like on the Earth, using a conformal projection, and using a different projection that distorts angles.
The Mercator is the most famous conformal projection. All of the above, however, are conformal projections. You can see that shape does get tweaked slightly among all of these. Notice also that area is not the same. That’s because no conformal projection can preserve area relationships too. Form and area are mutually exclusive!
Area always gets distorted on conformal maps, with areas further away from the standard line(s) getting larger.
2.5.1.2 Equivalent / Equal-Area Projections
These projections preserve areas across the map. Equivalent projections should always be used for thematic mapping.
Angle gets distorted, but not area. All of these circles have the same area.
Cartographic Controversy!
There has been a debate for decades about the need to only use equivalent projections to represent the world so that people do not get a biased view that mid-latitude countries are more important than equatorial ones (i.e., the developing world). This controversy flairs up about every ten years with regularity, and it makes most academic cartographers yawn. There is no right or wrong projection. There are projections better for different purposes. If you really want people to not have a distorted view of the world, use a globe or a digital globe atlas.
About every 10 years, proponents try to push the Peters Cylindrical Equal Area Projection into schools, saying it will make people realize how large the less-developed world is compared to Europe, North America, and Asia. It also makes those continents look liked warped mirror figures. The truth is that neither projection is right or wrong, better or worse. Both distort.
2.5.2 Equidistant
These projections preserve distance. BUT THERE IS A CATCH! They can only do so from a single point or two individual points on any given map. Thus, they’re only equidistant from very limited places. All maps are equidistant along the standard lines.
An example of an equidistant conic projection.
2.5.3 Azimuthal
These projections preserve the direction from a single point, with all straight lines drawn from the center of the map representing a great circle route. Key point: all azimuthal map projections are planar, but not all planar projections are azimuthal.
Azimuthal projections preserve angles from a standard point across the entire globe. They differ from conformal projections, which preserve angle across the whole map. Another key point: azimuthal maps can be equidistant. The two types of preservation are not mutually exclusive, as area and form are.
This is an example of an azimuthal equidistant projection with Madison as the standard point.
2.5.4 Compromise
Compromise projections are inventions by cartographers that preserve no map property perfectly, but maintain shape, area, direction, and distance across the entire map reasonably well. They compromise! They are frequently used for reference maps. Examples included below.
Arthur Robinson invented this projection (that was later used by National Geographic). It just “looks right.”
The Winkel Tripel is a frequently used compromise projection. Notice how there is distortion but overall it looks pretty good.
Conclusion
So we’ve come full circle. You now know a lot about projections – how they’re created, what they do differently, and how to think about them creatively to better highlight the information you’re mapping. But talk is cheap. The best way to become a projection aficionado is to get out and play with them.
There are some stellar tools created by Bernhard Jenny and his former graduate students to test projections on your own terms.
The first is the Projection Wizard by Bojan Savric (now at Esri). You can draw a box over the Earth and see how the map changes depending on the type of spatial preservation you select. Loads of fun!
Visit the site here: http://projectionwizard.org/
The second tool is the Adaptive Composite Map Projections Visualizer
Video : Interactive equal-area projections. Source: CartoGroup
Play with this cool too yourself here: http://cartography.oregonstate.edu/demos/AdaptiveCompositeMapProjections/