Hello! Welcome to the seventh and final lesson of our module on the Foundations of Cartography.
In our last lesson, we established the fundamental principle of map projections: it's impossible to represent the 3D spherical Earth on a 2D flat map without some form of distortion. We learned that every projection is a compromise, distorting at least one of four key properties: Shape, Area, Distance, or Direction (SADD). We also introduced the main categories of projections—conformal, equal-area, and compromise—using the classic comparison of the Mercator and Gall-Peters projections.
Today, we will build directly on that foundation to meet our learning outcome: Identify properties (e.g., shape, area, distance) preserved by different map projection types. We'll move beyond general categories to examine specific, named projections, learning to identify what each one does well, what it distorts, and for what purpose it is best suited. This will complete your foundational knowledge of maps before we transition to our next major topic: the atmosphere.
1. A Systematic Look at Projection Properties
In the previous lesson, we saw that cartographers must choose which properties to preserve at the expense of others. Let's formalize the main categories of projections based on the single property they manage to preserve perfectly.
To start, let's read a clear and concise guide from Axis Maps that defines the four main projection properties. This will give us a solid framework for the rest of the lesson.
Read the subsections under 'Projection Properties': 'Area', 'Form', 'Distance', and 'Directions'. Pay special attention to the crucial caveats mentioned for distance and direction—that they can only be preserved relative to specific points, not everywhere on the map.
As you've just read, we can classify projections based on what they preserve. It's also helpful to know some of the technical synonyms for these types, as you will encounter them in geography literature.
| Property Preserved | Projection Class Name | Alternative Names | Key Characteristic |
|---|---|---|---|
| Shape & Angles | Conformal | Orthomorphic | Landmasses look correct locally, and angles are true. |
| Area (Size) | Equal-Area | Equivalent, Homolographic | The relative size of landmasses is accurate. |
| Distance | Equidistant | - | Scale is correct from one or two points to all others. |
| Direction | Azimuthal | True-Direction | Directions (azimuths) are correct from a central point. |
A key takeaway is that the first two properties, conformal and equal-area, are mutually exclusive. A map can preserve shape or it can preserve area, but it cannot do both. The other properties are also subject to significant limitations.
2. A Tour of Common Map Projections
Now, let's explore specific projections within these families. We'll look at their visual characteristics, what they preserve, what they distort, and their primary uses.
This video from Geography King provides an excellent tour of many important projections, organized by the property they preserve. We will watch it in segments.
Map Projections Overview and How They Distort the Earth
First, let's learn about projections that prioritize preserving shape.
Watch the segment on conformal projections from 02:42 to 05:52. Pay close attention to the discussion of the Mercator projection, its historical use for navigation, and its modern use in web mapping (like Google Maps).
Conformal (Shape-Preserving) Projections
As the video explained, the primary goal of a conformal projection is to preserve local shapes and angles.
- Mercator Projection: This is the most famous conformal projection. All lines of constant direction (rhumb lines) are straight, making it ideal for sea navigation. However, it suffers from extreme area distortion near the poles. This is why Greenland appears larger than Africa on a Mercator map, when in reality Africa is 14 times larger.
- Lambert Conformal Conic: This projection is often used for aeronautical charts because a great circle route (the shortest path on a sphere) is nearly a straight line. It's well-suited for mapping mid-latitude countries with a larger east-west extent.
Map Projections Overview and How They Distort the Earth
Next, let's look at the opposite approach: projections that prioritize preserving area, even if it means distorting shape.
Now watch the segment on equal-area projections from 05:52 to 08:04. Note the variety of shapes and focus on the Albers and Gall-Peters projections.
Equal-Area (Size-Preserving) Projections
These are essential for thematic maps where you need to compare the size of different regions accurately (e.g., maps of population density, climate zones, or land use).
- Gall-Peters Projection: This projection is famous for being a direct response to the area distortion of the Mercator. It preserves area correctly but visibly distorts shape, making landmasses appear vertically stretched.
- Albers Equal-Area Conic: Like the Lambert Conformal, this is a conic projection. It is a standard for thematic maps of mid-latitude countries. For example, the U.S. Census Bureau and the U.S. Geological Survey often use this projection for maps of the United States.
Map Projections Overview and How They Distort the Earth
What about preserving distance? Let's see how equidistant projections work.
Watch this short segment on equidistant projections from 08:04 to 09:06. The key is to remember the limitation we read about earlier: distance is only preserved from a central point.
Equidistant Projections
An Equidistant projection maintains true scale from one or two points to any other point on the map.
- Azimuthal Equidistant: This is a common example, often centered on a pole or a specific city. The flag of the United Nations uses this projection to show all nations centered on the North Pole. It's useful for showing the range of aircraft or missiles from a central point.
Map Projections Overview and How They Distort the Earth
Finally, what if you don't need to preserve any single property perfectly? This is where compromise projections come in.
Watch this final segment on composite and compromise projections from 09:06 to 11:40. Focus on the Robinson and Winkel Tripel projections, and notice the interrupted 'orange peel' style of the Goode's Homolosine.
Compromise Projections
These projections don't preserve any single property perfectly. Instead, they offer a compromise by moderately distorting all properties (shape, area, distance, and direction) to create a visually appealing world map with less dramatic distortion at the edges.
- Robinson Projection: For a long time, this was the standard for the National Geographic Society. It presents a pleasing, balanced view of the world, though it has noticeable distortion near the poles.
- Winkel Tripel Projection: In 1998, National Geographic adopted this projection as its new standard for world maps. It has even lower overall distortion than the Robinson, particularly in the shapes and sizes of polar regions.
- Goode's Homolosine: This is an "interrupted" projection that looks like a flattened orange peel. It preserves area very well and greatly reduces shape distortion for the continents by creating interruptions in the oceans.
3. Visual Summary
Let's consolidate what we've learned with a visual comparison.
Take a moment to look at each map and see if you can identify the characteristics we've discussed. For instance, compare the shape of Greenland in the conformal Mercator versus the equivalent Sinusoidal projection.
Test your understanding!
You are tasked with creating three different world maps for a digital atlas:
- A map to show the global distribution of tropical rainforests, where accurately comparing the area of the Amazon to the Congo Basin is critical.
- A map for a historical section about 16th-century sea exploration routes.
- A general-purpose political map for the atlas's main landing page, intended to look familiar and balanced.
For each map, which specific projection (or type of projection) would you choose, and why?
Show answer
-
Rainforest Map: You should choose an equal-area projection, such as the Gall-Peters or Albers Equal-Area. This is because the primary goal is to accurately compare areas. Using a conformal projection like the Mercator would be highly misleading.
-
Exploration Routes Map: You should choose the Mercator projection. Its key property is that lines of constant compass bearing (rhumb lines) are straight, which is exactly how sailors navigated during that era. Preserving correct angles and directions for navigation is the top priority here.
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General Political Map: You should choose a compromise projection like the Winkel Tripel or Robinson. These projections do not preserve any one property perfectly but offer the best balance of low distortion for shape, area, and distance, making them visually appealing and suitable for general reference.
Conclusion
This lesson concludes our module on the foundations of cartography. You've moved from understanding Earth's coordinate system to interpreting contours on a topographic map, and finally, to grappling with the complex trade-offs involved in representing the entire globe on a flat surface.
Key Takeaways:
- Projections are classified by the properties they preserve: Conformal (shape), Equal-Area (size), Equidistant (distance from a point), and Azimuthal (direction from a point).
- The choice of projection is always dictated by the purpose of the map. There is no single "best" projection.
- Mercator (conformal) is for navigation; Albers (equal-area) is for thematic data; and Winkel Tripel (compromise) is for general reference world maps.
- "Interrupted" projections like Goode's Homolosine minimize distortion of landmasses by placing cuts in the oceans.
Preview of the Next Module:
We now shift our focus from the ground beneath our feet to the air above our heads. Our next module, "The Atmosphere: Composition and Energy Balance," marks our entry into the fascinating field of Climatology, a topic you expressed a keen interest in. In the first lesson, we will start from the basics, aiming to describe the composition and vertical structure (layers) of the atmosphere. We will explore the mixture of gases that makes life possible and see how temperature changes create distinct layers from the surface to the edge of space.