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Definition types and how to construct map projection full topic

 Definition of Map Projection:

A map projection is the conversion of the ellipsoidal figure of Earth into a planar surface. Basically, a map projection displays the locations of the features on a two-dimensional surface. It is done by plotting the geographical coordinates from the 3D model of earth to the planar surface.

Advantages of Map Projection:

 Advantages of map projection are;

1.      It permits us to utilize two dimensional maps.

2.      It permits the use of planar coordinates in place of geographical coordinates.

Limitations of Map Projection:

We cannot transform a 3D object into 2D perfectly. Therefore, distortion will always be present in the map projection. This is why we have lots of map projections. That is why certain properties are always preserved whereas leftover is compromised.

Types of Map Projection:

Map projections are generally categorized by conserved property and the object used in map projection

Classification on the basis of preserved properties:

Map projections are classified into four general types according to maintained properties which are as follows;

Conformal Projection: This one maintains local angles and forms.

Equal area Projection: this one shows exactly size of areas.

Equidistant projection: this one retains the consistency of scale along some lines.

Azimuthal Projection: maintains some accurate directions.

The maintained property is sometimes incorporated in the projection’s name. For example; Lambert Conformal Conic Projection.

A projection may have several preserved property except for conformal and equal area. Conformal and equal area properties cannot be maintained in a single map projection.

The preserved property helps in the selection of projection. For example; an accurate impression of population densities on the world map will be produced by choosing Equal area Projection.

How to Construct a Map Projection:

The geometric object used in projection to map the Earth on the planar surface is termed projection Surfaces. Generally used objects are a cylinder, cone, and plane. For example, we can create a map projection utilizing an object and a globe (ellipsoidal shape of the World). This is done by putting the object tangent to the globe or the object intersects the globe. Then a map projection is produced by locating the geographical coordinates from the ellipsoidal shape of the World onto the cube.

Classification on the basis of projection surfaces:

Hence, we can construct a map projection by employing the three projection objects. Thus, On the basis of projection objects, a map projection is divided into three types. Which are as follows;

Cylindrical projection: Projection that is constructed using the cylinder tangent to or intersecting the globe.

Conic projection: Projection produced by putting the cone tangent to or intersecting the globe.

Azimuthal Projection: Projection created by putting the cone tangent to or intersecting the globe.

Like the preserved property, The name of the geometric object is also occasionally included in its name. For example; Lambert Conformal Conic Projection.

Case and Aspect:

The concept of case and aspect can be explained by the help of geometric objects. As mentioned earlier, a map projection can be created by putting the geometric object to the world model such that it is tangent to or intersects the globe. When the geometric object is tangent to the globe, this is the simple case. This results in one line of tangency for the other two projection surfaces except for plane projection for which it results in a point of tangency. When an object intersects with the globe which is the secant case, it results in two lines of tangency for conic and cylindrical projections. When the geometric object used is a plane, it results in a line of tangency.

The position of a geometric object on the globe is described by the aspect. For example, a globe can have a plane at any point. The tangency at the pole is mentioned as a polar aspect while at the equator as an equatorial aspect. The tangency anywhere between the pole and the equator is stated as an oblique aspect.

Map Projection Parameter:

A map projection is based on some parameters in which few parameters are as follows;

1.      The first parameter is the lines of tangency called standard lines. As discussed above, the simple case has a single tangency line which means that in a simple case a map projection has only one standard line. Similarly, in the second case, a map projection has two standard lines. If the line of tangency goes along latitude. The standard line is called a standard parallel. Also, if it follows a longitude/meridian it is called a standard meridian. Likewise, the simple case has one standard parallel or meridian while the secant case has two standard parallels or meridians.

2.      A map projection has no distortion along the standard line. The tearing, shearing, and compression can cause distortion if we move away from standard line or lines. The scale is a measure of distortion, and “It is the ratio of the distance on the map to its corresponding ground distance”.

The principal scale, or the size of the reference globe, can accordingly be gotten from the proportion of the globe's radius to the Earth's radius. The principal scale applies just to the line of tangency in a map projection. That is why standard meridian and standard parallel are often called longitude of the true scale and latitude of true scale respectively. The local scale applies to the other parts of the map projection. The scale factor is the standardized local scale, characterized as the proportion of the local scale to the principal scale. The scale factor is 1 along the line of tangency and turns out to be either under 1 or more than 1 away from the line of tangency.

3.      The other parameter of map projection is the central lines. Central lines are central parallel and central meridian. It represents the center of a map projection. it is different from the standard lines which show the pattern of distortion in a map projection. Likewise, the standard meridian and standard parallel, central parallel, and central meridian are often called latitude of origin and longitude of origin respectively.

The difference between the central lines and standard lines can be seen through the transverse Mercator projection. it is defined by the central meridian and two standard meridians. The scale factor at the standard meridians is one and is less than one at the central meridian.

1.      The parameter of map projection is the false origin. False origin is the X and Y assigned coordinate values in other to avoid negative coordinates. When a map projection is used as the basis of a coordinate system, central meridian and central parallel become the origin of the coordinate system then the system is divided into four quadrants. So, the locations on the map can have negative coordinates depending on the quadrant in which the location lies.  That is why false origin is used. Where X assigned coordinate is called false easting and the Y coordinate is called false northing.


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