About Global Coordinate Systems

You can define a new global coordinate system based on your own data or use a predefined coordinate system as a basis for creating your own. You can begin by defining a new datum or ellipsoid, or use existing elements in your coordinate system definition.

When you define a coordinate system you specify some or all of the following parameters:

• map projection — Used to convert points from latitude and longitude to Cartesian coordinates.

  Depending on the projection you use to define a custom coordinate system, you must provide certain information. For example, if you use the Transverse Mercator projection, you must specify a longitude value called the central meridian. For all projections you msut specify one of the following:

  • datum — Reference point, line, or surface for mapping.
  • ellipsoid — Geometric surface whose plane sections are all circles or ellipses.
• Geographic coordinate system — A coordinate system that uses a three-dimensional sphere to define locations on the earth. It includes an angular unit of measure, a prime meridian, and a datum (based on an ellipsoid). Locations are referenced by longitude and latitude values. You can equate longitude values with the X axis and the latitude values with the Y axis.

  Longitude and latitude are not uniform units of measure, except along the equator. Above and below the equator, the circles defining the parallels of latitude get gradually smaller until they become a single point at the North and South Poles where the meridians converge and the distance represented by one degree of longitude decreases to zero.

• Projected coordinate system — A coordinate system defined on a two-dimensional surface. Unlike a geographic coordinate system, a projected coordinate system has constant lengths, angles, and areas across the two dimensions.

  Locations are identified by x,y coordinates on a grid, with the origin at the center of the grid. Each location has two values (x, y coordinates) relative to the origin: these specify its horizontal position and its vertical position. The coordinates at the origin are x = 0 and y = 0. You can equate the longitude values with the X axis and the latitude values with the Y axis.

• Scaling — Paper maps must scale the mapped area down to fit on a piece of paper. GIS systems are not limited to a page size. GIS coordinate systems usually use a scale of one, which allows you to work in real world units. Length and area calculations produce real-world results in the units you specify.
• Projection origin — The origin of rectangular coordinates for a map projection. For example, the longitude origin of a conic projection is the line of longitude that is straight and perfectly vertical. This longitude value can be converted to an X-coordinate of zero, and is usually specified as the center of the map. The latitude of the projection origin is often the nearest round number that is south of the southernmost extent of the region to map.
• False origin — When the central meridian bisects the mapping region, half of the X coordinates are negative values. An offset called the false origin can be added to all coordinates to make them positive. The X coordinate of this offset is called the false origin easting. The Y coordinate of this offset is called the false origin northing.
• Scale reduction factor — To produce the smallest possible distance between the projection surface and any point in the region you are mapping, you must specify a scale reduction factor. This is particularly important when you are mapping large regions.
• Quadrant — One of the four regions defined by the axes of a Cartesian coordinate system. Quadrants are numbered counterclockwise, one through four. Numbering begins with the quadrant in which both the x- and y-coordinates are positive (usually the upper-right quadrant).
• Useful range parameters — The minimum and maximum latitude and longitude for which a coordinate system is considered valid.