Coordinate Systems, Map Projections & Datums Explained
The Earth is a 3D sphere (roughly), but maps are 2D. To represent locations accurately, GIS relies on three interconnected concepts: **Coordinate Systems**, **Datums**, and **Map Projections**. Getting these right is critical — a wrong CRS can shift your entire map by kilometers. ---
📋 Prerequisites
- No prior GIS software required.
- Curiosity about spatial data and maps.
Step-by-Step Instructions
Geographic vs Projected Coordinate Systems
### Geographic Coordinate System (GCS) - Uses **Latitude and Longitude** in degrees - Represents locations on the curved surface of the Earth (a sphere/ellipsoid) - Example: WGS 84 (EPSG:4326) — the most common GCS, used by GPS ### Projected Coordinate System (PCS) - Converts the curved Earth onto a **flat 2D surface** - Uses linear units like meters or feet - Needed for accurate area, distance, and direction measurements - Example: UTM (Universal Transverse Mercator) **Rule of thumb:** Use a Geographic CRS for storing/sharing global data, and a Projected CRS when you need to measure area or distance. ---
Understand Datums
A **Datum** defines the reference surface (a mathematical model of the Earth's shape) used to calculate coordinates. - **WGS 84** – Global standard, used by GPS and most modern data - **Local Datums** – Some countries use their own datum for higher local accuracy (e.g., Everest 1830 in parts of South Asia) **Why it matters:** The same location can have slightly different coordinates depending on the datum used — mismatched datums cause misalignment between layers. ---
Understand Map Projections
Since you can't flatten a sphere without distortion, every projection sacrifices something — shape, area, distance, or direction. | Projection Type | Preserves | Common Use | |---|---|---| | **Conformal** (e.g., Mercator) | Shape/angles | Navigation | | **Equal-Area** (e.g., Albers) | Area | Thematic mapping | | **Equidistant** | Distance from center | Distance measurement | | **UTM** | Balance of all (in narrow zones) | Most GIS work | ---
What Is UTM (Universal Transverse Mercator)?
UTM divides the Earth into **60 zones**, each 6° wide in longitude. Each zone has its own coordinate system in meters, which keeps distortion very low. - Example: `UTM Zone 43N` covers a specific longitude band in the Northern Hemisphere (e.g., parts of India) - To find your UTM zone: `Zone = floor((longitude + 180) / 6) + 1` ---
How to Check/Set CRS in QGIS
1. Open QGIS → check the CRS shown in the bottom-right status bar 2. Right-click a layer → **Properties → Source** → view its current CRS 3. To reproject: **Layer → Save As** → choose the target CRS from the dropdown 4. Project-wide CRS: **Project → Properties → CRS** ---
🧠 Quick Recap & Practice Questions
## Quick Recap
- GCS = degrees (lat/long); PCS = meters (flat grid)
- Datum defines the Earth model used for coordinates
- Projections always distort something — pick based on your need
- UTM is the most commonly used projected system in GIS work
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## Practice Questions
1. Why can't you measure area accurately using a Geographic CRS?
2. What does "EPSG:4326" refer to?
3. How many UTM zones cover the globe?
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