Skip to main content. You are here Home. Planar, Azimuthal or Zenithal projection This type of map projection allows a flat sheet to touch with the globe, with the light being cast from certain positions, including the centre of the Earth, opposite to the tangent area, and from infinite distance.
This group of map projections can be classified into three types: Gnomonic projection, Stereographic projection and Orthographic projection. Gnomonic projection The Gnomonic projection has its origin of light at the center of the globe. Stereographic projection The Stereographic projection has its origin of light on the globe surface opposite to the tangent point. Conic projection This type of projection uses a conic surface to touch the globe when light is cast.
When the cone is unrolled, the meridians will be in semicircle like the ribs of a fan. The tangent areas of conic projection can be classified as central conical projection or tangent cone, secant conical projection, and polyconic projection.
Central conical projection This simple map projection seats a cone over the globe then casts the light with the axis of the cone overlapping that of a globe at tangent points. Secant conical projection The projection uses a conical surface to intersect the surface of a globe, creating two tangent points and subsequently two parallels. Polyconic projection The projection seats a series of cones over a globe with the axis of each cone lapping over the axis of a globe, creating parallels in equal number to that of the tangent cones.
Cylindrical projection This type of projection uses a cylinder as a tangent surface that wraps around a globe, or to intersect the globe at certain positions. If the cylinder is unrolled into a flat sheet, the parallels and meridians will be straight lines that create the right angles where they intersect each other. The projection displays directions and shapes correctly. The area close to tangent points will be more accurate. The more distant it is from tangent points, the more distortion will be shown.
This type of projection is typically used to map the world in particular areas between 80 degrees north and 80 degrees south latitudes.
The cylindrical projection is classified into three types: 1. Cylindrical equal area projection The projection places a cylinder to touch a globe at normal positions.
Mercator projection Mercator invented this type of projection in the 16th Century and it has been commonly used ever since. Some projection parameters, called angular parameters , are set with these latitude-longitude values. Once the earth's back has been broken with a projection, locations are described in terms of constant units like meters or feet. Some projection parameters, called linear parameters , use these constant units or they use ratios, such as 0. Top: Round data is described with meridians, parallels, and latitude-longitude values.
Bottom: Flat data is described with x ,y units. Projection parameters use both kinds of descriptions. Angular parameters. Every projection has a central meridian , which is the middle longitude of the projection.
In most projections, it runs down the middle of the map and the map is symmetrical on either side of it. It may or may not be a line of true scale. True scale means no distance distortion. In ArcGIS, you can change the central meridian of any projection. Occasionally, it's the only angular parameter you can change. The central meridian is also called the longitude of origin or the longitude of center. Its intersection with the latitude of origin see below defines the starting point of the projected x ,y map coordinates.
Every projection also has a latitude of origin. The intersection of this line with the central meridian is the starting point of the projected coordinates. In ArcGIS, you can put the latitude of origin wherever you want for most conic and transverse cylindrical projections. In many world projections, on the other hand, it is defined to be the equator and can't be changed.
The latitude of origin may or may not be the middle latitude of the projection and may or may not be a line of true scale. More about the latitude and longitude of origin. The important thing to remember about the latitude and longitude of origin is that they don't affect the distortion pattern of the map. All they do is define where the map's x ,y units will originate.
When data is unprojected , it doesn't have x ,y units. Locations are measured in latitude and longitude, as you know from the previous module. But when you set a projection and flatten everything out, you also start using a new way to measure location. This new way is in terms of constant distance units like meters or feet measured along a horizontal x-axis and a vertical y-axis.
The place where the axes cross is the coordinate origin, or 0 ,0 point. Commonly, this is in the middle of the map but it doesn't have to be. In the top graphic below, the intersection of the central meridian longitude of origin and the latitude of origin is marked with a cross.
This point becomes the origin of the x ,y coordinates. Area is increasingly distorted toward the polar regions. For example, although Greenland is only one-eighth the size of South America, Greenland appears to be larger than South America in the Mercator projection. Distortion values are the same along a particular parallel and they are symmetric across the equator and the central meridian.
The projection is appropriate for large-scale mapping of the areas near the equator such as Indonesia and parts of the Pacific Ocean. Due to its property of straight rhumb lines, it is recommended for standard sea navigation charts. Its variant, the Web Mercator projection, is standard for web maps and online services.
The projection is often misused for world maps, wall charts, and thematic mapping on web maps. Mercator auxiliary sphere does not support the ellipsoid and uses sphere-based equations with a sphere specified by the Auxiliary Sphere Type parameter. The conformal and straight rhumb lines properties are not maintained when the ellipsoid is used in this variant.
Mercator variant A differs from variant B only in projection parameters. They share the same algorithm. Mercator variant C differs from variant B only in projection parameters. The poles cannot be represented on the Mercator projection. Large area distortion makes the Mercator projection unsuitable for general geographic world maps and thematic mapping. Also, the lines of longitude are evenly spaced apart. But the distance between the lines of latitude increase away from the Equator.
This relationship is what allows the direction between any two points on the map to be constant true direction. While this relationship between lines of lines of latitude and longitude correctly maintains direction, it allows for distortion to occur to areas, shapes and distances. Nearest the Equator there is little distortion. Distances along the Equator are always correct, but nowhere else on the map.
This is why, for uses other than marine navigation, the Mercator projection is recommended for use in the Equatorial region only. Despite these distortions the Mercator projection is generally regarded as being a conformal projection. This is because within small areas shapes are essentially true.
In the s Arthur H. Robinson, a Wisconsin geography professor, developed a projection which has become much more popular than the Mercator projection for world maps. In its time, the Robinson projection replaced the Mercator projection as the preferred projection for world maps. As it is a pseudo-cylindrical projection, the Equator is its Standard Parallel and it still has similar distortion problems to the Mercator projection.
Also, there is less distortion in the Polar regions. Unlike the Mercator projection, the Robinson projection has both the lines of altitude and longitude evenly spaced across the map. The other significant difference to the Mercator is that only the line of longitude in the centre of the map is straight Central Meridian , all others are curved, with the amount of curve increasing away from the Central Meridian. In he released both his Conformal Conic projection and the Transverse Mercator projection.
The Transverse Mercator projection is based on the highly successful Mercator projection. This set of virtues and vices meant that the Mercator projection is highly suitable for mapping places which have an east-west orientation near to the Equator but not suitable for mapping places which have are north-south orientation eg South America or Chile.
This touch point is called the Central Meridian of a map. This meant that accurate maps of places with north-south orientated places could now be produced.
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