1. What is globe?
A globe is the spherical model of the whole earth reduced to a scale. In other words globe is the replica of earth in reduced form.
As our earth is spherical and the globe is also spherical, Globe is the true representation of the earth. Globe represents all aspects of the earth such as, shape, area of the continents and oceans, direction, distance etc. truly on scale. Globe is essentially a 3-Dimensional figure. A globe is not a map.
2. What is map?
A map is the graphical representation of the whole earth’s surface or part of it on to a plane surface. Maps are drawn to scale and projection. Projections are used to maintain certain properties of the 3-Dimensional spherical earth on to the 2-Dimensional map. Besides another feature of map is use of cartographic symbols. Earth’s surface features are not truly represented on map, rather different conventional cartographic symbols are use to represent the features on map.
3. Characteristics of map:
Maps are 2-Dimentional.
Maps are drawn to predetermined scale.
Maps are drawn on specific projections.
Features shown on map are selective i.e., only relevant features which are important for the purpose of map making are shown, not all.
Features are symbolized i.e. standardized symbols (points, lines, area) and colours are used to represent.
Features are generalized i.e. intricate detail is simplified.
Details of features depend on scale.
All maps concerned with two elements- location and attributes i.e. characteristics of features.
4. What is projection?
The process of representing a point on to a plane surface by means of light from a fixed view point is called projection.
5. What is map projection?
Map projection is the systematic representation/transformation of the three dimensional earth surface on to a two dimensional plane surface either geometrically or mathematically in such a way that each points on the globe corresponds on the graticule.
Or,
map projection is the systematic drawing of the mutually intersecting lines of latitudes and longitudes (graticule) on a plane surface from a three dimensional surface in order to prepare the map.
6. What is the necessity of map projection?
Geography is a spatial science. The interpretation of geographic features is incomplete without map. Map describes the spatial distribution and arrangements of geographic phenomena and their patterns of arrangements on the earth’s surface. Geographers deal with the map.
Map is two dimensional feature and earth surface is a three dimensional feature. So there is a necessity of the true transformation of three dimensional surface into two dimensional surface. In order to do this transformation map projection is must because map projection helps in transforming the three dimensional globe in to a two dimensional plane.
7. Scale and scale factor.
Map projection is actually a two step process:
Reduction of the earth in to a generating globe of desired size and,
The transformation of the globe on to a plane surface.
The scale of the generating globe (reduced earth or, replica of earth) is called principal scale which is the ratio between the distance on globe and corresponding distance on earth surface.
When the transformation takes place by means of projection, map is produced. The scale of the map projection is called actual scale or real scale which is the ratio between the distance on map projection and corresponding distance on earth surface.
There are two types of scale factor:
Radial scale factor: Ratio between the two scales along the meridians.
Tangential scale factor: Ratio between the two scales along the parallels.
8. Some measurements on the globe.
9. Some Terminologies related to map projection.
Geographical Coordinates:
Graticule:
Generating globe
Plane of Projection
Developable Surface
Central Meridian
Standard Parallel
Constant of Cone
10. What are the properties of map projection?
It is practically impossible to maintain all the properties of the globe (i.e. Area, shape, distance and direction) on the projection. A particular projection maintains some of the above four properties.
Equal Area/homolography: when the area on globe equals to the corresponding area on map projection. (Product of tangential scale and radial scale factor is unity)
Equal shape/orthomorphism: when the shape on globe preserves to the map projection. (Tangential scale factor equals to radial scale factor)
Equidistant: when the distance between two points on globe (along parallels and meridians) equals to the corresponding two points on map projection.
Azimuthal/Zenithal: when the direction (bearing) between two points on globe preserves on to the corresponding two points on map projection.
Aphylactic: when none of the above properties are truly maintained.
11. Classification of map projection.
12. International map projection.
This is a kind of map projection usually drawn for the whole earth surface with internationally accepted standards such as, modified polyconic projection.
13. Conventional projection.
This is another group of projection apart from the all other categories of map projection and usually drawn on a set of conventions purely based on mathematical operation for the whole world to meet specific purpose.
14. Use of Map projections:
A. Polar Zenithal Group:
1. Polar Zenithal Gnomonic Projection:
In Gnomonic projection source of light lies at the centre and hence, the any straight line represents the part of a great circle. So it is used to draw great circle route in Mercator’s projection for navigation purposes.
It is used in navigation purposes in the polar areas because Mercator’s projection ca not be drawn for polar region.
B. Conical Group:
1. Simple Conical Projection (one Std parallel)
Conical group of projection is most suitable for drawing mid latitude regions. Simple conical projection is widely used for drawing areas in the mid latitude regions with narrow north south extension along standard parallel. This projection, though does not preserve either area or shape, the distortion is minimum along the standard parallel.
Railways, roads, international boundaries, long river valleys running parallel to the standard parallel are well represented by this projection.
This projection also maintains true distance along meridian. Thus narrow east-west area like Chile, Rocky, and Andes can be drawn in this projection.
2. Bonne’s Projection:
As this projection is an equal area projection for smaller areas, it is used for representing small compact countries in mid latitude regions like, Australia, India, Spain, France, Germany etc.
For topographical maps in mid latitude countries this projection is also used.
3. Polyconic Projection:
Distortion away from central meridian is increased in this projection. So this projection is used in representing the small areas of mid latitude region because for smaller areas distortion of area and shape is minimum.
Topographical map is prepared in mid latitude countries by using narrow strips with north south extension and central meridian is used for each strips, thus distortion remain minimum. It should be noted that the sheets north and south of one another fit exactly but not the sheets east and west of one another.
A modified form of this projection forms the framework of international projection.
C. Conventional Map Projection:
1. Sinusoidal Projection
Sinusoidal projection is the modification of Bonne’s projection conventionally drawn for representing the whole world. Thus it called conventional projection.
It is suitable for small areas in equatorial regions because it preserves equal area and distortion of shape is minimum for small equatorial regions. Mid African countries, African continent, countries in South America can be represented well as the obliquity is reduced due to small east west extension.
D. cylindrical Group:
1. Cylindrical Equal Area Projection
This projection maintains equal area. But shape is distorted away from equator. So this projection is used to represent the tropical countries.
World distribution of tropical products like rubber, tea, rice, coconut, cotton, groundnut etc. are easily represented in this projection.
2. Mercator’s Projection:
This is also called cylindrical orthomorphic projection as it preserves the true shape. Another property is the bearing. Any straight line drawn on this projection is a line of constant bearing, which is called Rhumb Line or Loxodrome.
This projection is mostly used in air and sea navigation except the Polar Regions. Any directional items can be represented in this projection.
15. Choice of Map Projection:
Maps are drawn on certain projections. Maps serve many purposes.
To represent areal distribution of commodities we select equal areal projection.
To represent the political boundaries we select equal area map.
To represent any directional features like, drainage lines, road and rail lines, air and sea navigation routes we select orthomorphic or azimuthal projection.
Cylindrical projections are suitable for tropical regions, conical projections are suitable for mid-latitude regions and polar projections are suitable for polar areas.
While selecting any suitable projection we must consider the following:
Purpose of the map,
Size of the area,
Shape of the area,
Location of the area on earth etc.
No comments:
Post a Comment