CARTOGRAPHIC PROJECTIONS: WHAT THEY ARE FOR AND TYPES - GEOGRAPHY - 2025

The cartographic projections are representations on a plane of the points that are on the surface of the Earth, which is curved. In this way, the coordinates of a location: latitude and longitude, are converted into a point with Cartesian coordinates x and y.

Of course, this is not easy, as this means "flattening" the earth's sphere, which cannot be done without deformation. In fact, this is what happens on most maps.

Figure 1. In cartographic projections there is always distortion of some kind. Source: Pixabay.

The reader can try to remove the peel in the middle of an orange and then try to make it completely flat. The only way to achieve this is by breaking it in some parts, but this way it is clear that the surface deforms quite a bit.

For this reason, neither the lengths, the directions or the shapes represented on the maps exactly coincide with the real ones, or at least not all those characteristics at once. When one of them is preserved, in one way or another the others are lost, at least in part. However, depending on the purpose of the map, such losses may be acceptable.

Despite this, flat representations of the earth's surface have many advantages. For starters, maps are portable and can be taken to many places without taking up too much space.

They can also be specially made for certain regions and magnify the details that are considered important, minimizing deformation. This is not feasible with a more realistic representation, which is necessarily smaller: a globe to scale.

Globes are built in the shape of the Earth, but for size reasons, they cannot contain much information.

Types of map projections

- According to the conserved characteristics of the projection

Depending on the characteristics that are preserved in the projection, there are the following types of cartographic projections:

- Conformal: it maintains the existing angles between two lines on the earth's surface, therefore it is an appropriate projection for navigation charts

- Equivalent (equal-area): this projection keeps the terrain surfaces correct, although deformation may occur and the shapes are no longer similar. It is the appropriate projection for parcel maps.

- Equidistant: as its name indicates, in this projection the distances between two points are kept identical, joined by an arc on the earth's surface and by a straight line on the map.

- Aphylactic: in this projection no angles, surfaces or distances are preserved, but the distortion of the shape is minimal.

- According to the figure on which it is projected

Figure 2. The types of map projections most used. Source: Wikimedia Commons.

There are so many ways to make projections. Another widely used criterion is to classify according to the plane figure on which it is projected, which can be a plane, a cylinder or a cone, for example.

When a plane is used, the projection is called a plane or azimuthal projection, while when a geometric figure is used, then it is a development, because the geometric figure can later develop to become a plane, as we will see below.

Plane or azimuth projections

They are built from the projection of the Earth's surface from a point known as the projection vertex, towards a plane tangent to the Earth's surface. The point of tangency is called the projection center.

This type of projection has several variants, depending on the location of both the vertex of the projection and the point of tangency of the plane.

Conical projection

The cone and the cylinder are the geometric figures most used as auxiliaries to develop the projection. In the first case, the conical projection, the sphere is covered with a cone, whose axis of symmetry passes through the poles.

Now, curved lines are drawn on the earth's surface to position each point: the parallels and the meridians. When projecting onto the cone, the parallels appear as concentric circles, while the meridians appear as concurrent lines at the vertex of the cone.

Cylindrical projection

In the cylindrical projection, the earth's surface is covered with a cylinder tangent to the sphere, the axis of the cylinder being parallel to the axis that passes through the poles. Then the cylinder is extended, on which the meridians and parallels will remain as straight lines.

The lines corresponding to the meridians will be equidistant, but not the lines corresponding to the parallels, whose distance increases as the length increases.

However, the cylinder can be positioned in another way, not necessarily touching the earth's surface on the equator, as shown in the figure. The cylinder may be smaller in diameter than Earth.

Most popular cartographic projections

The types of projections described above can be combined to create new projections. The best known are briefly described below.

Mercator projection

It is among the most used projections to represent world maps. It was invented by the geographer Gerard Kremer, also known as Gerardus Mercator (1512-1594) in the year 1569.

It is a conforming cylindrical projection, that is, it respects the angles, which is why it is a projection highly appreciated by sailors. However, it does not preserve areas, as this type of projection is appropriate for equatorial areas. Outside these latitudes, regions look much larger than they really are.

Despite these disadvantages, it is the projection used in the most popular map applications on the internet.

Lambert conic projection

This projection was created by the Swiss mathematician Johann Lambert (1728-1777), who also showed that the number π is irrational. This projection preserves distances very well and is very suitable for representing mid-latitude regions, but it is not suitable for equatorial latitudes due to deformation.

Modified map projections

This group of projections is used to represent the earth's surface, trying to minimize deformations. Among the best known are:

Sinusoidal projection

This projection allows the parallels to be horizontal and equidistant, unlike the cylindrical projection. The central meridian is a line perpendicular to the parallels, but the other meridians curve around it.

The distances between the parallels, as well as between the parallels and the central meridian are true and also preserve the areas.

Mollweide projection

This projection seeks to preserve the areas. Here the equator is twice as long as the central meridian. The meridians take the shape of ellipses and the parallels are horizontal lines parallel to the equator, whose separation depends on the faithful conservation of the areas, being very appropriate for mid-latitudes.

Goode projection

Figure 3. Goode projection. Source: Wikimedia Commons.

This is a projection that, unlike the previous ones, is discontinuous. In it, the earth's surface is represented in the form of irregular and united areas, minimizing the distortion in the continents. Not so in oceanic surfaces, which are divided, as can be seen in the figure.

However, the Goode projection has the advantage of preserving the shape of the continents and also the areas, which is why it is widely used in economic maps, to represent the distribution of products worldwide.

References

1. Aguilar, A. 2004. General Geography. 2nd. Edition. Pearson Education.
2. Gisgeography. What are Map Projections? Recovered from: gisgeography.com
3. Snaider, P. Cartographic Projections and Reference Systems. Recovered from: hum.unne.edu.ar.
4. USGS. Map projections. Recovered from: icsm.gov.au
5. Wikipedia. List of map projections. Recovered from: en.wikipedia.com