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Geo-Informatics and Space Technology Development Agency (Public Organization):GISTDA

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Choice of property-based map projection

Choice of property-based map projection

Choice of property-based map projection
        When choosing a map projection, it is necessary to consider properties of map projections that match certain areas of the globe to be displayed on the map. The projections with different shapes are appropriate for displaying different areas. Their properties are classified into three types including equal-area, conformality and azimuthal properties. In order to make a map, we can choose map projections with the following properties:

1. Equal-area propert

        This type of map making may lose the true shape. The spatial scale on the map is the same as that on the globe. The equal-area property is found in conic projection, particularly around the standard parallels which is the area where the cone touches the globe. There are differences in each pattern of tangency. The central conical projection, for example, can display details of an area stretching in east-west direction, such as countries in the Scandinavian Peninsula, but distortion increases when it is far from the standard parallels. A secant conical projection can better display details of intersected areas which came from two sources: the intersection of two standard parallels and the polyconic projection. The latter provides more accuracy around the meridians and equatorial plane, but at the periphery, scale distortion grows.

2. Conformality property 

        Conformal mapping is mostly used with small areas as the shape can be better controlled than in a wide area. A large-scale topography is more distorted and its scales are not constant, but the direction is correct. In this type of map, the parallels are  perpendicular to the meridians. The conformal map projections include cylindrical and conic projections which maintain the shapes of areas tangent to a cylinder or a cone, such as the equatorial regions in the Mercator projection, and the meridians in the transverse Mercator projection. Thailand uses this type of projection for making the country’s base map because of its conformality property in the north-south direction. In addition, it is fundamental element of the Universal Transverse Mercator rectangular coordinate system, which is commonly used worldwide for its conformal and azimuthal properties.
 

3. Azimuthal property

        An azimuthal map has the true geographical azimuth. This type of map projection is typically used for navigation, aviation and astronomical maps. Although it does not have the equal-area and conformality properties, the projection can indicate the true azimuth of a object on the map. This type of map projections include the azimuthal projection, Gnomonic projection, stereographic projection and orthographic projection. In addition, there are also azimuthal cylindrical projections, which include the Gall’s projection that maintains the north-south direction, and the Mercator projection that maintains the east-west direction, which are also typical for making an aviation map, and the transverse Mercator that maintains the direction of the Central Meridian.