Coordinate System in GIS: A Guide to Accurate Mapping

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

A Coordinate System in GIS defines how geographic data is spatially located on the Earth’s surface. It provides a framework to reference locations in space.

Key Components of a Coordinate System:

• Datum: Mathematical model of the Earth (e.g., WGS 84, NAD 27)
• Spheroid: Defines Earth’s size and shape
• Projection (for PCS): Method to transform 3D Earth to 2D map
• Units: Degrees (GCS) or meters/feet (PCS)

Two types of coordinate systems used in GIS:

• Geographic Coordinate Systems (GCS)
• Projected Coordinate Systems (PCS)

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.