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| 3) Properties of Map Projections 4) The classification of Map Projections 5) Selecting a suitable Map Projection | Matter and Photos are Credits of: |
References
Knippers, R.A. (1998). Coordinate systems and Map projections. Non-published notes, Enschede, ITC.
Knippers, R.A. (1999). Geometric Aspects of Mapping. Non-published notes, Enschede, ITC.
Stefanovic, P. (1996) Georeferencing and Coordinate Transformations. Non-published notes. Enschede, ITC.
Every map must begin, either consciously or unconsciously, with the choice of a map projection and its parameters. The cartographer's task is to ensure that the right type of projection is used for any particular map. A well choosen map projection takes care that scale distortions remain within certain limits and that map properties match to the purpose of the map.
The selection of a map projection has to be made on the basis of:
- shape and size of the area
- position of the area
- purpose of the map
The choice of the class of a map projection should be made on the basis of the shape and size of the geographical area to be mapped. Ideally, the general shape of a geographical area should match with the distortion pattern of a specific projection. For example, if an area is small and approximately circular it is possible to create a map that minimises distortion for that area on the basis of an Azimuthal projection. The Cylindrical projection should be the basis for a large rectangular area and a Conic projection for a triangular area.
The position of the geographical area determines the aspect of a projection. Optimal is when the projection centre coincides with centre of the area, or when the projection plane is located along the main axis of the area to be mapped (see example figure below).
Choice of position and orientation of the projection plane for a map of Alaska
Once the class and aspect of a map projection have been selected, the choice of the property of a map projection has to be made on the basis of the purpose of the map.
In the 15th, 16th and 17th centuries, during the time of great transoceanic voyaging, there was a need for conformal navigation charts. Mercator's projection -conformal cylindrical- met a real need, and is still in use today when a simple,straight course is needed for navigation.
Because conformal projections show angles correctly, they are suitable for sea, air, and meteorological charts. This is useful for displaying the flow of oceanic or atmospheric currents, for instance.
For topographic and large-scale maps, conformality and equidistance are important properties. The equidistant property, possible only in a limited sense, however, can be improved by using secant projection planes.
The Universal Transverse Mercator (UTM) projection is a conformal cylindrical projection using a secant cylinder so it meets conformality and reasonable equidistance for topographic mapping.
Other projections currently used for topographic and large-scale maps are the Transverse Mercator ( the countries of . Argentina, Colombia, Australia, Ghana, S-Africa, Egypt use it ) and the Lambert Conformal Conic (in use for France , Spain, Morocco, Algeria ). Also in use are the stereographic (the Netherlands ) and even non-conformal projections such as Cassini or the Polyconic (India).
Suitable equal-area projections for distribution maps include those developed by Lambert, whether azimuthal, cylindrical, or conical. These do, however, have rather noticeable shape distortions. A better projection is the Albers equal-area conic projection, which is nearly conformal. In the polar aspect, they are excellent for mid-latitude distribution maps and do not contain the noticeable distortions of the Lambert projections.
An equidistant map, in which the scale is correct along a certain direction, is seldom desired. However, an equidistant map is a useful compromise between the conformal and equal-area maps. Shape and area distortions are moderate.
The projection which best fits a given country is always the minimum-error projection of the selected class. The use of minimum-error projections is however exceptional. Their mathematical theory is difficult and the equidistant projections of the same class will provide a very similar map.
In conclusion, the ideal map projection for any country would either be an azimuthal, cylindrical, or conic projection, depending on the shape of the area, with a secant projection plane located along the main axis of the country or the area of interest.The selected property of the map projection depends on the map purpose.
Nevertheless for each country to use its own projection would make international co-operation in data exchange difficult. There are strong arguments in favour of using an international standard projection for mapping.
Activity You have been asked to produce a small-scale thematic map of your country showing the distribution of the population. Which projection class, aspect and property would you choose considering the location, size and shape of the country and the purpose of the map? Justify your answer!
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