Geographic and Projected Coordinate System

In GIS, spatial referencing is essential to accurately locate and analyze geographic features. Two fundamental systems used for spatial referencing are the Geographic Coordinate System (GCS) and the Projected Coordinate System (PCS).

1. Geographic Coordinate System

A Geographic Coordinate System (GCS) is a system that defines locations on the Earth's surface using a three-dimensional spherical surface. It uses latitude and longitude as coordinates.

Components

Datum: A mathematical model representing the Earth's shape.
- Example: WGS84 (used in GPS), NAD83, and ETRS89.
Prime Meridian: The reference meridian (0° longitude), usually Greenwich Meridian.
Units of Measurement: Degrees (°), Minutes ('), and Seconds (") or Decimal Degrees (DD).
Latitude & Longitude:
- Latitude: Measures north-south position (0° at the equator, ±90° at poles).
- Longitude: Measures east-west position (0° at the Prime Meridian, ±180° east/west).
Ellipsoid: Defines the Earth's approximate shape.
- Example: WGS84, Clarke 1866, GRS80.

Example

New Delhi, India: (28.6139°N, 77.2090°E)
New York, USA: (40.7128°N, 74.0060°W)

Advantages of GCS

✔ Preserves true location on a global scale.
✔ Used in GPS and global datasets.

Limitations of GCS

✘ Not suitable for distance and area calculations due to Earth's curvature.
✘ Angular units (degrees) make it difficult to measure in linear units like meters.

2. Projected Coordinate System

A Projected Coordinate System (PCS) is a two-dimensional, planar coordinate system that represents the Earth on a flat surface using X (easting) and Y (northing) coordinates. PCS applies mathematical transformations (projections) to convert GCS (spherical coordinates) into a flat map.

Components

Projection: The method used to transform 3D Earth to 2D.
- Example: Mercator, UTM, Albers, Lambert Conformal Conic.
Datum: The same datum as in GCS but adapted for projection.
Units of Measurement: Typically in meters or feet.
Coordinate Axes:
- Easting (X-axis): Measures distance eastward.
- Northing (Y-axis): Measures distance northward.

Types of Map Projections in PCS

Cylindrical Projections (e.g., Mercator Projection)
- Best for navigation and equatorial regions.
- Example: Google Maps uses Web Mercator.
Conic Projections (e.g., Albers Equal-Area, Lambert Conformal Conic)
- Best for mid-latitude areas (e.g., USA, Europe).
- Used in climate mapping and land-use studies.
Planar (Azimuthal) Projections (e.g., Polar Stereographic)
- Best for polar regions.
- Used in Arctic and Antarctic studies.

Example of PCS Coordinates

New Delhi, India (UTM Zone 43N): (X: 722,567.89 m, Y: 3,168,234.56 m)

Advantages of PCS

✔ Maintains distance, area, and shape for regional/local mapping.
✔ Uses linear measurement units (meters, feet), making calculations easier.

Limitations of PCS

✘ Distortion increases with area size (No projection can preserve all properties at once).
✘ Not globally applicable—designed for specific regions.

Comparison

Feature	Geographic Coordinate System	Projected Coordinate System
Representation	Spherical (3D)	Planar (2D)
Coordinates	Latitude (φ), Longitude (λ)	X (Easting), Y (Northing)
Units	Degrees (°)	Meters, Feet
Best Use Case	Global navigation, GPS	Local/regional mapping
Example Systems	WGS84, NAD83	UTM, State Plane

Practical Example in GIS

Scenario: Mapping Flood-Prone Areas in Kerala, India

Step 1: Use GCS (WGS84) for Global Positioning
- Collect raw satellite data (Sentinel-2, Landsat) in WGS84.
Step 2: Convert to PCS (UTM Zone 43N)
- Convert coordinates for high-accuracy flood mapping.
- Use UTM projection to measure affected area in square kilometers.

GCS is essential for global-scale mapping and GPS navigation.
PCS is crucial for accurate distance and area calculations in local/regional studies.
Choosing the right coordinate system depends on the purpose, scale, and accuracy needed.

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