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Coordinate Systems vs Projections

Goal: Walk away knowing exactly what a coordinate system is, what a projection is, and how they fit together.

→ Pair this with Coordinate Systems & Projections (roadmap).

The simplest explanation

A coordinate system says: "Where on Earth is this?"

A projection says: "How do we flatten the round Earth onto a flat map?"

Together, they make a spatial reference.

Two kinds of coordinate systems

Geographic Coordinate System (GCS)

  • Coordinates: latitude / longitude (degrees)
  • Position is on the 3D ellipsoid (the Earth model)
  • No flattening yet
  • Examples: WGS84 (EPSG:4326), NAD83 (EPSG:4269)

Projected Coordinate System (PCS)

  • Coordinates: X, Y in linear units (meters, feet)
  • Position is on a flat plane
  • Includes a projection formula that flattens the GCS
  • Examples: Web Mercator (EPSG:3857), UTM Zone 17N (EPSG:26917), State Plane

A projection is part of a PCS

PCS = GCS (datum) + projection method + parameters

NAD83 / Georgia State Plane West (EPSG:2240)
   = NAD83 datum
   + Lambert Conformal Conic projection
   + central meridian -84.166...
   + 2 standard parallels
   + false easting / northing
   + units: feet (US Survey)

So when someone says "what projection is this?" — they actually mean "what's the projected coordinate system?".

"But maps in lat/long are flat!"

Yes — but they're flat by plate carrée projection (just plotting x = longitude, y = latitude). It distorts a lot, especially near the poles. Pick a better projection for serious maps.

The 3 questions you should ask before any analysis

  1. What CRS is each layer in? (Right-click layer → Properties → Source.)
  2. Are they all the same? (If not, project them.)
  3. Is the CRS appropriate for what I'm doing?
    • Distance / area work → projected (UTM, State Plane)
    • Just storing / sharing → geographic (WGS84)
    • Web map → Web Mercator

"Define" vs "Project" — the killer distinction

Tool What it does When to use
Define Projection Just labels the data with a CRS. Doesn't change coordinates. Only when the data is missing a CRS, and you know what it should be
Project Recalculates every coordinate to the new CRS Almost always — when you need to change the CRS

If you accidentally use Define Projection to "fix" a CRS mismatch, you'll mislabel the data and it will appear in the wrong place forever.

Real-world story

A common stumble: you have a shapefile that draws in the wrong place. You "Define" it as WGS84 hoping it'll fix the offset. It doesn't move — because the coordinates were already correct, just labeled wrong with a different datum. Now it's labeled WGS84 but the coordinates are NAD27. Every downstream operation will be 100+ meters off.

The right fix: figure out the correct original CRS, Define it to that, then Project to the CRS you actually want.

EPSG codes (memorize these)

EPSG Name Use
4326 WGS84 (lat/long) Storing, GPS data
4269 NAD83 (lat/long) US Census, federal data
3857 Web Mercator Web maps
5070 NAD83 / Conus Albers Equal-area US thematic
26917 NAD83 / UTM 17N Eastern US analysis
2240 NAD83 / Georgia State Plane West Georgia analysis
32633 WGS84 / UTM 33N Northern Europe

→ Look up any EPSG: https://epsg.io

Quick decision tree

flowchart TD
    Q{What's the goal?}
    Q -->|Just store the data| WGS[Use WGS84<br/>EPSG:4326]
    Q -->|Web map| WM[Use Web Mercator<br/>EPSG:3857]
    Q -->|Distance / area<br/>regional analysis| Reg{Region?}
    Reg -->|US one state| StP[State Plane]
    Reg -->|Country / continent| UTM[UTM zone]
    Q -->|Continent thematic<br/>area-correct| Albers[Equal-area<br/>e.g., NAD83 Albers]

    classDef q fill:#fef3c7,stroke:#f59e0b,color:#92400e
    class Q,Reg q
    classDef ans fill:#dcfce7,stroke:#10b981,color:#065f46
    class WGS,WM,StP,UTM,Albers ans

Practice

Walk through the difference

  1. Add a US states shapefile in NAD83 (EPSG:4269).
  2. Right-click → Properties → Source. Note the CRS.
  3. Run Project to NAD83 / Albers Equal Area (EPSG:5070).
  4. Compute area on both. Compare.
  5. Now test: in the original NAD83, calculate area in km². Numbers will be tiny / weird because units are degrees².
  6. In the projected Albers, area in km² is correct.