Coordinate Systems and Projections 

Coordinate Systems In Maps:

One of the most important functions of a GIS is to allow users to analyze geographic data using accurate locations and measurements. To ensure that geographic information is placed accurately on a map, mapmakers created a system that could locate features on the earth's surface. The system of imaginary intersecting lines was created, creating the framework for a coordinate system.

Each coordinate represents a distance from a defined point of origin. In GIS, each location will have two or three coordinates associated to it. An X coordinate resembles the horizontal distance from the point of origin, the Y coordinate represents the vertical distance from the point of origin, and the Z coordinate represents the height of the point from the ground which is used for 3D visualizations. On a globe, distances are measured in angular degrees, also known as longitude and latitude. Projection types result from projecting coordinates from a spherical coordinate system onto a flat surface. These surfaces are used by cartographers as the basis for maps.

Figure 56: Longitude And Latitude Lines

Geographic coordinate systems

A geographic coordinate system uses a three-dimensional spherical model to show points or areas on the earths surface. The pairs of coordinate values that identify a feature on a map are relative to its geographic coordinate system. Each coordinate system is commonly illustrated with a network of intersecting lines of latitude and longitude called the graticule.

Projected coordinate systems

A projected coordinate system is based on a geographic coordinate system. Projected coordinate systems are used to convert feature locations from the spherical earth to a flat map. To do so, latitude and longitude coordinates from geographic coordinate systems are projected to planar coordinates.

Projection Types:

Figure 57: Demonstrate Projection

    Map projections are so called because of the concept behind their construction. If you could project light from a source through the earth's surface onto a two-dimensional surface, you could then trace the shapes of the surface features onto the two-dimensional surface. This two-dimensional surface would be the basis for your map. Each projected surface creates a different projection type. This type of surface is called a developable surface, and cartographers use three of them: the cylinder, the cone, and the plane.

    Converting a round map to a flat surface will result in distortion to the properties of the map. A good projection minimizes distortion in your area of interest. A general guideline is to choose a projection according to the latitude of your area of interest.

• To map tropical regions, use a cylindrical projection.
• To map middle latitudes, use a conic projection.
• To map a polar region, use a planar (azimuthal) projection.

    Projections can be classified according to the developable surface on which they are based, but they can also be classified according to the spatial property, or properties, that they preserve: 

• Shape
• Area
• Distance
• Direction

Conclusion:

    Some projections preserve more than one spatial property. These spatial properties are also used to categorize projections, and they are often found within a projection's name. In order to choose a projection for a map, you need to determine the purpose of the map and then decide which properties should be preserved to meet that purpose. When you know the type of properties to preserve, you can choose a projection that will be appropriate for the map.

    There is a significant difference between projecting your map and projecting your data. When you need your data to line up in your map display you can project your map by applying a projected coordinate system to your data frame. On the other hand, if you need to perform analyses with your data that will require the data to contain valid values for linear measurements, you must be sure that the data is in an appropriate projection.