Step 1 in Map Projection: Choosing a Projection Surface (Especially for GATE-Geospatial 2022)

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A surface that can be unfolded into a plane or sheet without stretching, tearing, or shrinking is called a ‘developable surface.’ One way of describing a projection is first to project from the Earth՚s surface to a developable surface such as a cylinder or cone, and then to unroll the surface into a plane.

Surface for Reference Globe

In this stage the very first thing is to choosing the model for the shape of the earth. Projection construction is also affected by how the shape of the earth is approximated. Shape of the earth is assumed as sphere. The earth is not exactly spherical but is closer in shape to an ellipsoid. Spherical model is useful for small-scale maps such as world atlases and globes. The ellipsoid model is commonly used to construct topographic maps and other large-scale and medium-scale maps. A third model of the shape of the earth is geoid, a complex and more accurate representation of the globe.

This Image Shows in Red Areas Are the Idealized Ellipsoid

In above figure, red areas are the idealized ellipsoid. Blue areas are below:

This Image Shows Two Globes in Blue Areas Are Ellipsoid

The sphere or ellipsoid is not applicable with a plane surface so any projection that attempts to project them on a flat sheet will have to distort the image.

Surface for MAP

A surface that can be unfolded or unrolled into a plane or sheet without stretching, slitting, or shrinking is called a developable surface. The cylinder, cone and plane are all developable surfaces. The sphere and ellipsoid are not developable surfaces.

The three basic Map Projection family members are:

  • Cylindrical
  • Conical
  • Azimuthal (Plane)
Figure Shows Types of Map Projections

Cylindrical

The term “normal cylindrical projection” is used to refer to any projection in which meridians are mapped to equally space vertical lines and circles of latitude are mapped to horizontal lines.

Conical

The term “normal conical projection” is used to refer to any projection in which meridians are mapped to equally spaced line radiating out from the height and circles of latitude are mapped to circular curves centered on the height.

Azimuthal (Plane)

In Azimuthal (Zenithal) Projection a flat paper is supposed to touch the globe at one point and the light may be kept at another point so as to reflect or project the lines of latitude and longitude on the plane.

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