In Geographic Information Systems (GIS), coordinate systems and map projections are fundamental concepts that define how geographic data is spatially located and represented on maps.

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1. Coordinate Systems

A coordinate system is a framework that uses numbers (coordinates) to uniquely determine the position of a point or geographic feature.

Types of Coordinate Systems

1. Geographic Coordinate System (GCS):
   • Based on a 3D spherical surface (the Earth).
   • Coordinates are given in latitude and longitude (angular units, usually degrees).
   • Example: WGS 84 (used in GPS).
   • Origin: Center of the Earth.
2. Projected Coordinate System (PCS):
   • A 2D representation of the Earth’s surface, derived from a GCS.
   • Uses linear units (meters, feet).
   • Requires a map projection.
   • Example: UTM (Universal Transverse Mercator).

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2. Map Projections

A map projection is a method used to transform the 3D surface of the Earth into a 2D map.

Projection Types (by distortion properties)

Every projection distorts at least one of the following:

• Area (equal-area)
• Shape (conformal)
• Distance (equidistant)
• Direction (azimuthal)

Common Map Projection Classes

1. Cylindrical Projections:
   • Wrap the globe onto a cylinder.
   • Example: Mercator Projection (conformal, distorts area at poles).
2. Conic Projections:
   • Project the globe onto a cone.
   • Best for mid-latitudes.
   • Example: Albers Equal-Area Conic.
3. Azimuthal Projections:
   • Project onto a flat plane.
   • Good for polar regions.
   • Example: Stereographic.

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3. Coordinate Reference Systems (CRS)

A CRS defines how a coordinate system is tied to the Earth’s surface.

• Includes both a datum (model of the Earth) and a projection (if projected).
• Examples:
  • EPSG:4326 – WGS 84 (lat/long)
  • EPSG:3857 – Web Mercator (used in web maps like Google Maps)

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4. Practical GIS Use

Common GIS tools (like QGIS or ArcGIS) support reprojection and coordinate transformation.

Always ensure all layers in a GIS project use the same CRS to avoid misalignment.

Reprojection is the process of converting data from one CRS to another.

Diagram showing how GCS and PCS