Chapter 03

Coordinate Systems & Map Projections

Every map distorts reality — coordinate systems and projections control how Earth's curved surface is represented on flat displays.

QGISArcGIS Proproj4EPSG.iospatialreference.org

60UTM Zones↗ Each 6° wide

6,000+EPSG Registry↗ Defined CRS codes

±2cmWGS84 Accuracy↗ Current realization

Theory & Foundations

Geographic Coordinate Systems use angular measurements (latitude/longitude) on a 3D ellipsoid. Projected Coordinate Systems flatten this to 2D using mathematical transformations.

A datum defines the reference ellipsoid and its relationship to Earth's actual shape. WGS84 is the GPS standard. NAD83 is used in North America. ITRF2014 is the most precise global reference.

Map projections introduce distortion in area, shape, distance, or direction. Conformal projections (Mercator) preserve shape but distort area. Equal-area projections (Albers) preserve area but distort shape.

The Universal Transverse Mercator (UTM) system divides Earth into 60 zones, each 6° wide. Bangladesh falls in UTM Zone 45N and 46N. UTM provides meter-based coordinates ideal for distance/area calculations.

Web Mercator (EPSG:3857) is used by Google Maps, Mapbox, and all major web mapping platforms. It preserves shape near the equator but grossly distorts area at high latitudes — Greenland appears as large as Africa.

In-Depth Coverage

Geographic vs Projected CRS

The fundamental distinction in coordinate reference systems.

Geographic: Latitude/longitude in degrees on an ellipsoid
Projected: X/Y in meters (or feet) on a flat surface
Geographic CRS for storage, Projected CRS for analysis
Never calculate area/distance in geographic CRS

Common Map Projections

Each projection preserves different properties.

Mercator: Navigation, web maps (distorts area)
UTM: Surveying, military, engineering (6° zones)
Albers Equal Area: Thematic maps (preserves area)
Robinson: World maps (compromise projection)
Stereographic: Polar regions

💡 Pro Tip: For Bangladesh, use BTM (Bangladesh Transverse Mercator) or UTM Zone 46N for local analysis.

EPSG Codes Reference

Standard identifiers for coordinate systems worldwide.

EPSG:4326 — WGS84 Geographic (GPS default)
EPSG:3857 — Web Mercator (Google/Mapbox/OSM)
EPSG:32646 — UTM Zone 46N (Bangladesh)
EPSG:4269 — NAD83 (North America)
EPSG:32632 — UTM Zone 32N (Central Europe)

Key Techniques

Datum transformationOn-the-fly reprojectionUTM zone selectionDistortion analysisCRS metadata documentation

Data Visualization

Map Projection Usage in Practice (%)

Mercator: 35%
UTM: 25%
Albers: 15%
Robinson: 10%
Web Mercator: 15%

CRS Selection Guide

🌍Global StorageUse WGS84 (EPSG:4326)
🖥️Web DisplayUse Web Mercator (EPSG:3857)
📐Local AnalysisUse UTM zone for your area
📏Area CalculationEqual-area projection required

Practical Workflow

Identify your study area location
Determine appropriate datum (WGS84 for global)
Select projection based on purpose
Check EPSG code for your CRS
Reproject all layers to common CRS
Verify alignment visually
Document CRS in metadata
Handle datum transformations carefully

Software & Tools

PROJFreeLibrary — Coordinate transformation engine

EPSG.ioFreeWeb Tool — Look up CRS definitions and EPSG codes

Video Tutorials

▶ Map Projections ExplainedVox · YouTube · 5 min ▶ Coordinate Systems in GISGeoDelta Labs · YouTube · 28 min

Real-World Application

The Mars rovers use a custom coordinate system based on the Mars datum. Incorrect CRS handling once caused a $125M NASA satellite to crash because one team used metric and another used imperial units.

Real-World Projects

NASA Mars Climate Orbiter Loss📍 Space

Metric/imperial unit mismatch in navigation coordinates caused $125M satellite loss.

Impact: Established strict unit-checking protocols in space missions.

Case Study

Bangladesh Survey deploys CORS (Continuously Operating Reference Stations) using ITRF datum, transforming to local Bangladesh Transverse Mercator (BTM) for national mapping.

Problem-Based Learning

P1 Layers from different sources don't align when overlaid

Census boundaries in WGS84, drone orthomosaic in local UTM, and OpenStreetMap in Web Mercator.

Solution: Identify CRS of each layer using metadata/QGIS. Set project CRS to UTM Zone 46N. Reproject all layers to common CRS using QGIS Reproject tool.

QGIS Reproject LayerEPSG.io lookupproj4 transformations

✅ All three layers aligned perfectly, enabling accurate overlay analysis with sub-meter precision.

Knowledge Check

1. Which EPSG code is WGS84?

2. UTM zones are how wide?

3. Web maps typically use which projection?