**Avenza Projections Introduction**

A coordinate system within MAPublisher defines a mathematical model to represent a specific location on the Earth as a set of coordinates. A model is specified by the coordinate system parameters, including the Earth model (ellipsoid or datum), the units used to measure the coordinates, the projection type, and any parameters specific to the projection type.

Within MAPublisher and Geographic Imager, coordinate systems are organized in coordinate system categories (Geodetic or Projected). This structure allows grouping into a logical collection for convenience (per continent, country, or local sub-divisions).

In projected coordinate systems, the map projection provides the mathematical formulas used to convert a three-dimensional geodetic coordinate system (latitude, longitude) to a two-dimensional flat projected coordinate system (x,y). The transformation a projection will perform is determined by a set of parameters. Some projections have fixed parameters (e.g. Gall-Peters projection has a standard parallel fixed at 45°N). But, in general, projections have several variable parameters that will be described in this chapter.

In MAPublisher and Geographic Imager, the projection is indicated or selected in the Definition tab of the Projected Coordinate System Editor* (accessed by clicking the information button or when creating a new coordinate system). Depending on the selected projection, a different list of mandatory parameters will appear.

* For more information, refer to the MAPublisher (appendix 2) or Geographic Imager (chapter 4) User Guides.

For pre-defined coordinate systems provided by Avenza Systems, the envelope is a good indication of the geographical area of validity of the system (although it does not always have to be limited to it). For projected systems, the central meridian and/or latitude of origin values are also a good indication (e.g. a central meridian of 0° indicates a European and African centered map, while a central meridian of -100 would be centered on the Americas).

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