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## GIS Coordinate Systems

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### Coordinate Systems

The location of a pixel in a file or on a displayed or printed image is expressed using a coordinate system. In two-dimensional coordinate systems, locations are organized in a grid of columns and rows. Each location on the grid is expressed as a pair of coordinates known as X and Y. The X coordinate specifies the column of the grid, and the Y coordinate specifies the row. Image data organized into such a grid are known as Raster data.

There are two basic coordinate systems:

• File Coordinates-refer to the location of the pixels within the image.
• Map Coordinates-show the location of a pixel on a map.

### Latitude and Longitude

Latitude and Longitude is a spherical coordinate system that are not associated with a map projection. The Latitude and Longitude express locations in the terms of a spheroid, not a plane. Therefore, an image is not usually rectified to latitude and longitude, although it is possible to convert images to latitude and longitude.

### Coordinate System in GIS

Two types of coordinate systems used in GIS:

#### Geographic Coordinate Systems

A Geographic Coordinate System (GCS) is a reference framework that defines the locations of features on a model of the earth. It’s shaped like a globe-spherical. Its units are angular, usually degrees.

#### Projected Coordinate Systems

A Projected Coordinate System (PCS) is flat, it’s converts that GCS into a flat surface, using projection algorithm and other parameters. Its units are linear, most commonly in meters.

### Geographic vs Projected Coordinate System

• Geographic Coordinate System defines where the data is located on the earth’s surface. Projected Coordinate System tells the data how to draw on a flat surface.
• A GCS is necessary for data to know where exactly on earth’s surface it is located. A PCS is necessary to draw the data on a flat map.

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