Why projections are necessary
A globe is the most faithful model of Earth’s surface, but it is not very practical for printing, hanging on a wall, fitting into a textbook, or loading quickly on a phone. The moment you try to represent a curved surface on a flat sheet, you face a geometric problem: you cannot flatten a sphere (or near-sphere) without stretching, tearing, or compressing it. A map projection is the set of mathematical rules that converts locations on Earth’s curved surface into positions on a flat map.
Every projection introduces distortion. The key idea is not “Which projection is correct?” but “Which projection is best for this purpose?” Different maps look different because they choose different ways to manage unavoidable distortion. Some preserve angles for navigation, some preserve area for fair comparisons, some preserve distances from a point, and some aim for a balanced compromise that looks visually pleasing.
Four common kinds of distortion
Area distortion: The size of regions changes. A country may appear much larger or smaller than it is relative to others.
Shape distortion: Coastlines and outlines may look stretched, squashed, or skewed, especially toward the edges of the map.
Distance distortion: The length between two places on the map may not match the real-world distance, and the error can vary depending on direction and location.
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Direction (bearing) distortion: A straight line on the map may not correspond to a constant compass bearing on Earth, or bearings may be accurate only in limited areas.
Because these distortions trade off against one another, a projection usually prioritizes one property and sacrifices others. Understanding what a projection preserves helps you interpret what you see and choose the right map for a task.
How projection families shape the look of the world
Many projections can be grouped by the geometric surface they resemble when “wrapped” around the globe: cylindrical, conic, and azimuthal (planar). These are not physical steps you must perform, but helpful mental models for why graticules (the network of meridians and parallels) and continents appear the way they do.
Cylindrical projections
In a cylindrical family, the map looks like a rectangle. Meridians often appear as vertical lines and parallels as horizontal lines. Distortion typically increases away from the equator (or away from the line of tangency/standard lines, depending on the specific projection).
Mercator (conformal): Preserves local angles and shapes, making it useful for navigation because a constant compass bearing (a rhumb line) plots as a straight line. The tradeoff is strong area inflation toward the poles, making high-latitude regions look much larger than they are.
Gall–Peters (equal-area): Preserves area, so regions are sized proportionally. The tradeoff is noticeable shape distortion, especially near the equator and toward higher latitudes, where landmasses look stretched vertically or horizontally.
Web Mercator (common in online maps): A variant optimized for tiled web mapping. It keeps the familiar Mercator look and conformal behavior locally, but it cannot show the poles and greatly enlarges high latitudes. It is popular because it supports smooth zooming and consistent tile grids.
Conic projections
Conic projections often look like a fan or a rounded wedge. They are frequently used for mid-latitude regions that extend more east–west than north–south. Distortion is minimized along one or two “standard parallels,” which are latitudes where the map surface conceptually touches or cuts the globe.
Lambert Conformal Conic (conformal): Preserves local angles and shapes, making it common for aeronautical charts and regional maps in mid-latitudes. Area is not preserved, but distortion is controlled across the zone of interest.
Albers Equal-Area Conic (equal-area): Preserves area and is widely used for thematic maps (for example, population density) of countries or continents in mid-latitudes.
Azimuthal (planar) projections
Azimuthal projections are often circular. They are built around a central point, and properties are usually best at or near that center. Many azimuthal projections preserve direction from the center, which can be valuable for route planning from a hub location.
Azimuthal Equidistant: Preserves distances from the center point to any other point on the map. It is useful for showing airline distances from a city or for radio/telecommunications range maps.
Stereographic (conformal): Preserves angles locally and is often used for polar maps because it can show polar regions with relatively good shape near the pole.
Orthographic: Looks like a photograph of Earth from space. It is visually intuitive but distorts area and distance away from the center and shows only one hemisphere at a time.
What “preserving” really means: the main projection properties
Projection descriptions often use technical terms. Here are the most practical ones to understand when reading a map or choosing one for a project.
Conformal projections (preserve local angles)
A conformal projection preserves angles at small scales, meaning local shapes are maintained (small features look “right”). This is helpful when you care about bearings and local geometry, such as marine navigation, aviation, or detailed street mapping. The cost is that areas can be badly distorted, especially far from the projection’s best-fit region.
Practical implication: if you compare the apparent size of Greenland and Africa on a conformal world map like Mercator, you may be misled. Greenland can look comparable to Africa even though Africa is far larger in reality.
Equal-area projections (preserve area)
An equal-area projection ensures that any region on the map has the correct proportional area relative to any other region. This is essential for thematic maps where the message depends on fair size comparison: land cover, climate zones, agricultural output, deforestation, or population density.
Practical implication: shapes may look unfamiliar, but area-based comparisons become trustworthy. If you are teaching or analyzing “how much” of something exists across regions, equal-area is usually the safer choice.
Equidistant projections (preserve certain distances)
No flat map can preserve all distances everywhere, but an equidistant projection preserves distance accurately along specific lines or from a specific point. For example, an azimuthal equidistant map centered on your city can show true straight-line distances from your city to any other place.
Practical implication: if you measure distance between two places that are not in the preserved set (for example, two cities neither of which is the center), the distance may be wrong.
Azimuthal (true-direction) properties
Some projections preserve direction (azimuth) from a central point. On these maps, a straight line from the center to another location indicates the correct initial bearing.
Practical implication: this is useful for planning routes from a hub (a shipping port, an airport, or a communications station), but direction accuracy is not guaranteed between arbitrary pairs of points.
Compromise projections (balance distortions)
Compromise projections do not perfectly preserve a single property; instead, they aim to reduce overall distortion and create a visually balanced world map. Examples include Robinson and Winkel Tripel, often used in educational and reference contexts.
Practical implication: these maps can be good for general-purpose world views, but you should avoid using them for precise area comparisons or navigation.
Why the same places “move” or “change size” across maps
When you switch projections, you are changing the rules for how latitude-like and longitude-like positions are transformed into x–y coordinates. This can change:
Relative size: High-latitude regions often inflate on cylindrical conformal maps. Equal-area maps correct that but may stretch shapes.
Relative shape: Continents can look wider, taller, or more curved depending on the projection’s geometry and where distortion is minimized.
Relative position: The “center” of the map and the chosen aspect (normal, transverse, oblique) can shift what appears central and what is pushed toward edges.
Edge effects: Many projections have severe distortion near the outer boundary. A world map must “cut” somewhere, often along an ocean, which affects how you perceive adjacency (for example, the Pacific-centered view versus the Atlantic-centered view).
Map aspect: normal, transverse, and oblique
A projection can be oriented in different ways. A cylindrical projection in its “normal” aspect is typically aligned with the equator. In a “transverse” aspect, the cylinder is rotated so distortion is minimized along a chosen meridian instead. This is useful for regions that extend north–south. An “oblique” aspect tilts the projection to best fit a diagonal region.
Practical implication: two maps can use the same named projection but look different if their aspect or central meridian changes.
Step-by-step: choosing a projection for a real task
Use this practical workflow whenever you need to create, select, or evaluate a map.
Step 1: Define the map’s purpose in one sentence
Examples: “Show the distribution of rainfall by country,” “Plan flight routes from Dubai,” “Compare the size of climate regions,” “Provide an intuitive world overview for a classroom poster.” Your purpose determines what must be preserved.
Step 2: Decide what property matters most
If you need fair size comparisons: choose equal-area.
If you need accurate local shapes and bearings: choose conformal.
If you need distances from a hub: choose azimuthal equidistant or another equidistant option.
If you need a general world view: choose a compromise projection.
Step 3: Match the projection family to the region’s shape
World map: compromise (Robinson, Winkel Tripel) or a carefully chosen equal-area world projection if area is central.
Mid-latitude continent/country (wide east–west): conic (Albers equal-area conic for thematic; Lambert conformal conic for shape/bearing).
Polar region: azimuthal (stereographic for conformal; azimuthal equal-area for area fairness).
Equatorial belt/global tropics emphasis: cylindrical equal-area variants can work well, depending on the message.
Step 4: Check where distortion will be smallest
Look for the projection’s standard lines or central point. Ask: “Is my area of interest near those lines/point?” If not, distortion may be unnecessarily large.
Step 5: Test with known references
Before finalizing, compare a few known facts visually:
Do high-latitude regions look implausibly huge? That suggests strong area distortion.
Do familiar coastlines look excessively stretched? That suggests strong shape distortion.
Do routes that should be near-straight look oddly curved (or vice versa)? That suggests a property mismatch for navigation or route display.
Step-by-step: a quick projection “diagnostic” when reading a map
You can often infer what kind of projection you are looking at by inspecting the graticule and the map outline, even if the projection name is not stated.
Step 1: Look at the overall boundary
Rectangle: likely cylindrical (Mercator, equirectangular/plate carrée, cylindrical equal-area).
Circle: likely azimuthal (orthographic, azimuthal equidistant, azimuthal equal-area).
Rounded “oval” world: often a compromise projection (Robinson, Winkel Tripel) or pseudocylindrical (like Mollweide).
Step 2: Inspect meridians and parallels
If meridians are straight, parallel vertical lines and parallels are straight, evenly spaced horizontals, you may be seeing a simple cylindrical like equirectangular (not great for area/shape globally).
If meridians are straight vertical lines but parallels get farther apart toward the poles, that is characteristic of Mercator-like behavior.
If meridians curve and meet at poles while parallels are arcs, that suggests a conic projection.
If meridians radiate from a center and parallels are concentric circles, that suggests an azimuthal projection.
Step 3: Identify what looks “most correct”
On many maps, there is a zone where shapes look least distorted. That zone often reveals the projection’s intent: a mid-latitude band for conic maps, a central point for azimuthal maps, or the equator for many cylindrical maps.
Practical examples: how projection choice changes interpretation
Example 1: Comparing country sizes on a world map
Task: You want students to compare the land area of several countries across latitudes. If you use a conformal cylindrical world map, high-latitude countries will appear larger than they are. This can lead to incorrect conclusions about relative size. An equal-area world projection (such as Mollweide or an equal-area cylindrical chosen appropriately) will preserve area relationships, making the comparison meaningful.
Example 2: Displaying global airline routes
Task: You want to show flight paths between major hubs. On many world maps, the shortest path between two points (a great-circle route) appears curved. This is not an error; it is a result of projecting a curved surface onto a plane. If your goal is to show shortest routes, consider an azimuthal projection centered on a hub (or a set of regional maps), where great-circle paths from the center can be represented more intuitively. If your goal is to show constant compass headings, a Mercator-type map will make rhumb lines straight, but those are not usually the shortest routes over long distances.
Example 3: Thematic mapping of climate-related variables
Task: You want to map carbon emissions per square kilometer or forest loss rates. Because the variable is tied to area, an equal-area projection is strongly preferred. Otherwise, high-latitude regions may visually dominate the map simply because the projection inflates them, not because the phenomenon is larger there.
Example 4: A national map for a mid-latitude country
Task: You want a map of a country that stretches far east–west in the mid-latitudes. A conic projection with standard parallels chosen to bracket the country can keep distortion low across the whole territory. If the map is for thematic comparisons, an equal-area conic is a good fit; if it is for navigation-like uses, a conformal conic may be better.
Understanding “projection vs. datum” (and why it matters)
Projection is only one part of how geographic data is represented. A datum is a reference model of Earth’s shape and how coordinates relate to that model. Different datums can shift positions slightly because they fit Earth differently. In many everyday mapping contexts, you will see common global datums used consistently, but when combining datasets (for example, layers from different sources), mismatched datums can cause misalignment even if the projection name looks similar.
Practical implication: if two layers do not line up in a GIS or mapping app, the issue may be a datum mismatch, a projection mismatch, or both. Always check the coordinate reference system information for each dataset before assuming the data is wrong.
Step-by-step: selecting a projection in a GIS or mapping tool (generic workflow)
Different software uses different menus, but the decision process is similar.
Step 1: Identify your area of interest
Is it global, continental, national, or local? Is it near the equator, mid-latitudes, or polar? The broader the extent, the more important projection choice becomes.
Step 2: Identify your map type
Reference map (general viewing): consider a compromise projection for world maps or a region-appropriate conformal/conic for regional reference.
Thematic map (data-driven shading/symbolization): prefer equal-area for quantities tied to area.
Route/distance map: consider equidistant or azimuthal options depending on whether distances are from a center or along certain lines.
Step 3: Pick a candidate projection and set its parameters
Many projections require choices such as central meridian, standard parallels, or a center point. Choose these to place the least distortion over your area of interest. For a conic projection of a mid-latitude country, standard parallels are often set near the northern and southern edges of the country to distribute distortion evenly.
Step 4: Validate with a quick visual and numeric check
Overlay a trusted boundary layer (coastlines or administrative boundaries) and check for unexpected warping. If your software provides distortion indicators (such as Tissot’s indicatrix or scale factor), use them to see where distortion grows.
Step 5: Document the choice
When you export or publish the map, include the projection name and key parameters in the metadata or map notes (not as a large headline). This helps others interpret the map correctly and reproduce your results.
A simple mental model: Tissot’s indicatrix
Cartographers often use a tool called Tissot’s indicatrix: imagine placing tiny circles on the globe and projecting them onto the map. If the projection preserves shape locally (conformal), the circles remain circles but change size. If the projection preserves area (equal-area), the circles may become ellipses but keep the same area. Where the circles become very stretched or very large, distortion is high.
Even if you never draw these circles, the concept is useful: any projection is “stretching” the surface in some places more than others. The different looks of world maps are different patterns of stretching.
Common misunderstandings to avoid
“A map projection is biased, so it is wrong”
A projection can influence perception, especially on world maps, but distortion is unavoidable. The practical question is whether the projection matches the map’s purpose and whether the map reader is informed about what is preserved and what is distorted.
“If a route looks curved, it must be longer”
On many projections, the shortest path on Earth appears curved. Curvature on the map does not automatically mean a longer route; it often reflects how the projection transforms geometry.
“One projection should work for everything”
A projection that is excellent for local navigation may be misleading for global area comparisons. A projection that is perfect for comparing areas may look unfamiliar for street-level reference. Switching projections is not inconsistency; it is good practice when the task changes.
