Man has been using geographical maps since ancient times. The first attempts to depict the surface of the Earth were made back in ancient Greece by such scientists as Eratosthenes and Hipparchus. Naturally, since then cartography as a science has advanced far. Modern maps are created using satellite imagery and using computer technology, which, of course, helps to increase their accuracy. And yet, on each geographic map there are some distortions with respect to the natural forms, angles or distances on the earth's surface. The nature of these distortions, and therefore the accuracy of the map, depends on the types of cartographic projections used to create the particular map.
Concept map projection
Let us examine in more detail what a cartographic projection is and what their types are used in modern cartography.
Cartographic projection is an image of the Earth's surface on a plane. A deeper definition from a scientific point of view is as follows: cartographic projection is a way of displaying points of the Earth’s surface on a certain plane, in which some analytical relationship is established between the coordinates of the corresponding points of the displayed and displayed surfaces.
How is a cartographic projection built?
The construction of any kind of cartographic projections takes place in two stages.
- First, the geometrically irregular surface of the Earth is mapped onto some mathematically correct surface, which is called the relativity surface. For the most accurate approximation in this capacity, a geoid is most often used - a geometric body bounded by the water surface of all the seas and oceans interconnected (sea level) and having a single water mass. At each point on the surface of the geoid, gravity is applied normally. However, the geoid, like the physical surface of the planet, also cannot be expressed by a single mathematical law. Therefore, instead of a geoid, an ellipsoid of revolution is adopted as the surface of relevance, giving it the maximum similarity to the geoid using the degree of compression and orientation in the Earth’s body. This body is called the terrestrial ellipsoid or reference ellipsoid, and in different countries for them take different parameters.
- Secondly, the adopted relativity surface (reference-ellipsoid) is transferred to the plane using one or another analytical dependence. As a result, we get a flat cartographic projection of the earth's surface.
Projection distortion
Have you ever wondered why on different maps the outlines of the continents are slightly different? On some cartographic projections, some parts of the world look more or less relative to some landmarks than on others. The thing is the distortion with which the projections of the Earth are transferred to a flat surface.
But why do map projections appear distorted? The answer is pretty simple. The spherical surface is not possible to deploy on a plane, avoiding wrinkles or tears. Therefore, the image from it cannot be displayed, avoiding distortion.
Projection Methods
Studying cartographic projections, their types and properties, it is necessary to mention the methods of their construction. So, cartographic projections are obtained using two main methods:
The geometric method is based on the laws of linear perspective. Our planet is conventionally adopted by a sphere of a certain radius and is projected onto a cylindrical or conical surface, which can either touch or dissect it.
Projections obtained in this way are called promising. Depending on the position of the observation point relative to the surface of the Earth, perspective projections are divided into types:
- gnomonic or central (when the point of view is aligned with the center of the earth's sphere);
- stereographic (in this case, the observation point is located on the surface of relevance);
- orthographic (when the surface is observed from any point located outside the Earth’s sphere; the projection is constructed by transferring the points of the sphere using parallel lines perpendicular to the imaging surface).
The analytical method for constructing cartographic projections is based on mathematical expressions connecting points on the sphere of relevance and the display plane. This method is more universal and flexible, allowing you to create arbitrary projections according to a predetermined nature of the distortion.
Types of cartographic projections in geography
To create geographic maps using many types of projections of the Earth. They are classified according to various criteria. In Russia, the Kavraisk classification is used, which uses four criteria that determine the main types of cartographic projections. As characteristic classifying parameters use:
- nature of the distortion;
- display form of coordinate lines of a normal grid;
- the location of the pole point in the normal coordinate system;
- mode of application.
So, what are the types of cartographic projections according to this classification?
Projection Classification
The following is a classification of the types of cartographic projections with examples based on the above main criteria.
By the nature of the distortion
As mentioned above, distortion, in essence, is an integral property of any projection of the Earth. Any surface characteristic may be distorted: length, area or angle. By the type of distortion distinguish:
- Equal or conformal projections in which azimuths and angles are transferred without distortion. The coordinate grid in conformal projections is orthogonal. Maps obtained in this way are recommended for determining distances in any direction.
- Equal or equivalent projections where the area scale is maintained, which is assumed to be equal to unity, i.e., the areas are displayed without distortion. Such maps are used to compare areas.
- Equidistant or equidistant projections , the construction of which preserves the scale in one of the main directions, which is assumed to be single.
- Arbitrary projections on which all varieties of distortion may be present.
According to the form of displaying coordinate lines of a normal grid
Such a classification is the most obvious and, therefore, the easiest to perceive. Note, however, that this criterion applies only to projections oriented normally to the observation point. So, on the basis of this characteristic feature, the following types of cartographic projections are distinguished:
Circular , where the parallels and meridians are represented by circles, and the equator and the middle meridian of the grid in the form of straight lines. Similar projections are used to image the surface of the Earth as a whole. Examples of circular projections are the Lagrangian equiangular projection, as well as the arbitrary Grinten projection.
Azimuthal . In this case, the parallels are represented in the form of concentric circles, and the meridians in the form of a beam diverging straight from the center of the parallels. A similar type of projection is used in the upright position to display the poles of the Earth with adjacent territories, and in the transverse as a map of the western and eastern hemispheres familiar to everyone from geography lessons.
Cylindrical , where the meridians and parallels are represented by straight lines intersecting normally. With minimal distortion, territories adjacent to the equator or stretched along some standard latitude are displayed here.
Conical , which is a scan of the lateral surface of the cone, where the lines of parallels are arcs of circles centered at the top of the cone, and the meridians are guides diverging from the top of the cone. Such projections most accurately depict territories lying in mid-latitudes.
Pseudo- conic projections are similar to conical ones, only the meridians in this case are represented by curved lines symmetrical with respect to the rectilinear axial meridian of the grid.
Pseudocylindrical projections resemble cylindrical ones, only, just as in pseudoconical, meridians are represented by curved lines symmetrical to the axial rectilinear meridian. They are used to represent the Earth as a whole (for example, the elliptical projection of Molweide, the isometric sinusoidal Sanson, etc.).
Polyconic , where the parallels are depicted in the form of circles whose centers are located on the middle meridian of the grid or its extension, meridians in the form of curves located symmetrically to the rectilinear axial meridian.
By the position of the pole point in the normal coordinate system
- Polar or normal - the pole of the coordinate system coincides with the geographic pole.
- Transverse or transversal - the pole of the normal system is combined with the equator.
- Oblique or inclined - the pole of the normal coordinate grid can be at any point between the equator and the geographic pole.
According to the method of application
According to the method of use, the following types of cartographic projections are distinguished:
- Solid - the projection of the entire territory on a plane is made according to a single law.
- Multiband - the terrain being mapped is conventionally divided into several latitudinal zones that project onto the display plane according to a single law, but with a change in the parameters for each zone. An example of such a projection is the Mufling trapezoidal projection, which was used in the USSR for large-scale maps until 1928.
- Multifaceted - the territory is conditionally divided into a number of zones by longitude, projection onto a plane is carried out according to a single law, but with different parameters for each of the zones (for example, the Gauss-Kruger projection).
- Compound , when some part of the territory is displayed on the plane using one regularity, and the rest of the territory with another.
The advantage of both multi-band and multi-faceted projections is the high accuracy of the display within each zone. However, a significant drawback in this case is the inability to obtain a solid image.
Of course, each map projection can be classified using each of the above criteria. So, the famous Mercator Earth projection is conformal (equiangular) and transverse (transverse); Gauss-Krueger projection - conformal transverse cylindrical, etc.