# Coordinate systems

In Analyst's Notebook Connector for Esri, coordinate systems determine how the coordinates and addresses that you specify in chart items are converted to positions on maps in the Map window. For a map to be displayed in the window, Analyst's Notebook must have a definition of a coordinate system that the map supports.

Analyst's Notebook Connector for Esri is supplied by default with a set of six coordinate systems (also known as spatial references). To support more maps in the Map window, you can add definitions of more coordinate systems. Any definition that you add must use one of the supported map projections.

Coordinate systems come in two types, geographical and projected. Any definitions that you add are likely to be for projected coordinate systems.

## Geographical coordinate systems

A geographical coordinate system is based on a spheroid that approximates the shape of the earth. Angular values, called latitude, and longitude, are measured in transverse planes from the center of the spheroid. They are extended out to give a precise location on its surface.

The Earth is not a perfect spheroid. No single spheroid can be used in a fixed location and orientation to accurately model all locations on the surface of the Earth. Different spheroids are used in various local regions. The location of the center of each of these spheroids, along with its orientation, is described relative to a reference spheroid known as WGS84. WGS84 is a spheroid that is the current best fit for the entire Earth, and its center is at Earth's center of gravity. It is commonly used for satellite navigation systems.

## Projected coordinate systems

A projected coordinate system is always based on an underlying geographical coordinate system. It defines a system for plotting locations from the associated geographical coordinate system on a two-dimensional map surface by using a pair of planar coordinates.

Various projections are used to visualize the surface of the Earth on a two-dimensional map, and each introduces its own distortions of scale, shape, size, and angles. The projected coordinate system specifies which projection to use. It also specifies parameters for defining the location of the origin, any offsets for the origin, the scale factor, and the coordinate units.