Projection (cartography) - Applications and Resources

Suitability of Projections for Different Applications Understanding which map projection to use requires knowing what qualities matter most for your specific purpose. Different applications prioritize different properties—some need accurate areas, others need accurate directions, and still others need aesthetic appeal. This section explores how to match projections to real-world mapping needs. Large-Scale Mapping Large-scale maps show detailed areas like countries or states, where the Earth's curvature is not a major problem. National mapping systems typically use the transverse Mercator projection or similar variants. Why this choice? At large scales, you're covering a relatively small area of the Earth, so maintaining conformality (preserving angles and shapes locally) is important. The transverse Mercator works well because: It maintains angles and shapes accurately, which is essential for detailed navigation and planning within a country Scale variation remains low across the mapped area—meaning distances don't get distorted too much It's practical to use and well-established in official mapping standards For example, the United States Geological Survey uses variations of the transverse Mercator for official topographic maps. Small-Scale (Continental or Global) Mapping When you're mapping the entire world or large continents, you face a fundamental trade-off: you cannot preserve all desirable properties at once. Conformal projections like Mercator make land near the poles impossibly large. Equal-area projections must distort shapes. For world maps intended for reference or education, cartographers typically choose projections that balance compromises: Winkel Tripel: A composite projection that minimizes all three types of distortion (area, distance, and angle) equally. It's widely used for world maps in atlases. Robinson: A pseudo-cylindrical projection designed specifically to be visually appealing, with moderate distortion across all dimensions. Mollweide: An equal-area projection with relatively balanced shape distortion. These projections exist because at small scales, no projection can be perfect. Cartographers must decide what kind of distortion is most acceptable—these three options spread distortion around rather than concentrating it in extreme areas. Thematic Maps A thematic map displays data that refers to area—population density, crop production per square kilometer, disease prevalence, climate zones, etc. If the projection distorts area, the map becomes misleading. Critical principle: Always use an equal-area projection for thematic maps. Here's why: Imagine showing population density on a Mercator projection. Greenland appears vastly larger than it actually is, making its population seem more significant proportionally. Viewers unconsciously interpret visual area as actual area. An equal-area projection prevents this misrepresentation. Common equal-area projections used for thematic mapping include: Cylindrical Equal-Area Sinusoidal projection Mollweide projection Navigational Maps Marine navigation has a unique requirement: ships need to follow constant-bearing routes (known as rhumb lines, which are straight lines of constant compass direction). The Mercator projection is specifically designed for this purpose: Rhumb lines appear as straight lines on a Mercator map, making navigation straightforward—a navigator can draw a straight line between two points and read the compass bearing directly While landmass shapes are distorted (especially toward the poles), this doesn't matter for ocean navigation Scale increases toward the poles, but that's acceptable since there's little navigation near the poles anyway This is why Google Maps and maritime charts historically used Mercator—not necessarily because it's the "best" world map, but because it's functionally superior for navigation. Digital mapping has complicated this somewhat, but Mercator remains standard for nautical charts. <extrainfo> Interestingly, there's an important distinction between rhumb lines and great circles (the shortest distance between two points on a sphere). Mercator preserves rhumb lines but not great circles. For very long ocean voyages, the shortest path follows a great circle, not a rhumb line. Modern navigation uses both concepts, but the straight-line bearing property of Mercator is still valuable for practical navigation. </extrainfo> Aesthetic Considerations Beyond technical requirements, aesthetic preferences matter—especially for maps meant to educate or inform general audiences rather than serve specific technical purposes. Different projections create different visual impressions: Mercator makes high-latitude regions (Canada, Russia, Greenland) look large and imposing Robinson creates a familiar, balanced appearance that looks "right" to most viewers Mollweide emphasizes the continuity of oceans and creates a distinctive oval shape Winkel Tripel minimizes overall visual distortion There's no objectively "correct" choice here. Cartographers consider what visual distortions their audience will find most acceptable. Some argue that equal-area projections are more "honest," while others believe projections like Robinson are more useful because they create less dramatic shape distortion, even if areas aren't perfectly preserved. This reflects a broader principle: map design involves communicating geographic information clearly to a specific audience. Your choice of projection should support that communication goal. Common Rectangular World Map Projections Two rectangular projections are particularly common and often discussed in contrast: Mercator projection: Creates a familiar rectangular shape with curved lines of latitude and longitude. Widely recognized but criticized for exaggerating polar regions. Gall-Peters projection: A rectangular projection designed to correct what its advocates saw as Mercator's distortion of area. The Gall-Peters stretches the world vertically, compressing it horizontally to preserve equal areas. It's sometimes promoted as more "fair" or "accurate," though its unusual proportions feel strange to viewers accustomed to Mercator. This comparison illustrates an important lesson: there's no universally superior projection. Different projections solve different problems and create different trade-offs. The choice depends on the map's purpose. <extrainfo> The Mercator vs. Gall-Peters debate has become somewhat political and philosophical, with advocates of each arguing their choice is more "honest" or "fair." In reality, both are legitimate tools for different purposes—Mercator for navigation and small-scale reference maps, Gall-Peters for emphasizing equal-area representation. The debate itself is a useful reminder that projection choice involves both technical and values-based decisions. </extrainfo> Summary: Matching Projections to Applications Here's a practical decision tree: | Application | Key Property Needed | Example Projections | |---|---|---| | Large-scale national maps | Conformality + low scale variation | Transverse Mercator | | World reference maps | Balanced distortion | Robinson, Winkel Tripel | | Thematic/data maps | Equal area | Mollweide, Cylindrical Equal-Area | | Navigation charts | Rhumb lines as straight lines | Mercator | The fundamental principle: choose the projection whose preserved properties best serve your map's purpose.