The projection on a globe in the shape of a cylinder is known as a cylindrical map projection. This is a fundamental concept in cartography used to represent the spherical Earth on a flat surface.
Understanding Cylindrical Projection
A cylindrical projection is created by conceptually surrounding the globe with a cylinder. Points from the globe's surface are then projected onto the surface of this cylinder. Once the projection is complete, the cylinder is "unrolled" or flattened into a two-dimensional plane, which becomes the map.
According to the definition of cylindrical projection, it is "any of numerous map projections of the terrestrial sphere on the surface of a cylinder that is then unrolled as a plane." Historically, early mapmakers also achieved projections by "a systematic method of drawing the Earth's meridians and latitudes on the flat surface," essentially simulating this projection process.
How it Works (Conceptually)
Imagine a light source at the center of a translucent globe. Now, wrap a sheet of paper into a cylinder around this globe. As the light shines outwards, it casts shadows of the globe's features (like continents, meridians, and parallels) onto the inner surface of the cylinder. When you unroll the cylinder, these shadows form the map projection.
- Meridians (lines of longitude): These are typically projected as equally spaced vertical lines on the cylindrical map.
- Parallels (lines of latitude): These are projected as horizontal lines. In many cylindrical projections, they are parallel and their spacing changes with latitude.
Characteristics and Considerations
Cylindrical projections are popular for world maps because they show the entire Earth and generally represent directions (especially East-West and North-South) accurately. However, projecting a sphere onto a cylinder introduces distortion.
Here are some key characteristics:
- Shape Distortion: Areas near the poles are significantly stretched and appear much larger than they are in reality. Areas near the equator are represented with less shape distortion.
- Area Distortion: Consequently, areas are distorted. Regions further from the equator have their areas greatly exaggerated.
- Straight Meridians and Parallels: Lines of latitude and longitude appear as straight, intersecting lines, often at right angles.
- Direction: Some cylindrical projections, like the Mercator projection, are conformal, meaning they preserve angles and shapes locally, making them useful for navigation where maintaining direction is crucial.
Common Examples
While the term "cylindrical projection" covers a broad category, some specific types are well-known:
- Mercator Projection: Famous for its use in navigation charts due to its preservation of angles. However, it severely distorts areas near the poles (which cannot be shown).
- Plate Carrée (Equirectangular) Projection: Simple projection where parallels are equally spaced. Distortion increases towards the poles.
- Gall-Peters Projection: An equal-area cylindrical projection, meaning it preserves the relative size of areas, but distorts shapes.
| Feature | Description |
|---|---|
| Projection Surface | Cylinder |
| Output Shape | Rectangle |
| Meridians | Straight, parallel lines |
| Parallels | Straight, parallel lines (spacing varies) |
| Primary Use | World maps, navigation (depending on type) |
Understanding cylindrical projections helps explain why different maps of the world look so different, illustrating the challenges of accurately representing a 3D sphere on a 2D plane.
