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Coordinate Systems & Projections

Goal: Understand the difference between geographic and projected coordinate systems, why every map distorts the Earth, and how to pick the right one for your analysis.

Most GIS bugs are projection bugs

"My layers don't line up." → Projection mismatch. "My buffers are too small." → Geographic CRS used for distance. "My areas are wrong." → Wrong projection for that region.

Master this topic and you'll save yourself hundreds of hours.

The Earth is round. Maps are flat.

You can't flatten a 3D ball onto a 2D sheet without distortion. Every map projection trades off four properties:

flowchart TB
    P[Projection must distort<br/>at least one of these]
    P --> A[Area]
    P --> S[Shape]
    P --> D[Distance]
    P --> Dir[Direction]

    classDef box fill:#fef2f2,stroke:#ef4444,color:#991b1b
    class P box
    classDef prop fill:#eef2ff,stroke:#4338ca,color:#312e81
    class A,S,D,Dir prop

A projection can preserve at most two of these — never all four.

Two kinds of coordinate systems

Geographic Coordinate System (GCS)

  • Coordinates are latitude / longitude (degrees)
  • Describes positions on the 3D ellipsoid
  • Common: WGS84 (EPSG:4326), NAD83 (EPSG:4269)
  • ✅ Good for storage and worldwide data
  • ❌ Cannot measure distance or area accurately

Projected Coordinate System (PCS)

  • Coordinates are in meters or feet (X/Y on a flat plane)
  • Earth has been "flattened" using a projection formula
  • Common: Web Mercator (EPSG:3857), State Plane, UTM
  • ✅ Great for measuring distance and area within its zone
  • ❌ Distorts more as you move away from its center

Quick rule

  • Storing or sharing global data? Use a GCS like WGS84.
  • Doing analysis (buffer, area, distance)? Use a PCS.

Datum vs projection

These two terms confuse beginners.

Term What it is
Datum The mathematical model of Earth's shape (e.g., WGS84, NAD83)
Projection The math that flattens that model onto a plane (e.g., Mercator, UTM)

A complete CRS = Datum + Projection (+ units).

Common projections you'll see

  • WGS84 (EPSG:4326)

    Geographic, lat/long. The default for GPS and most web data.

  • Web Mercator (EPSG:3857)

    What Google Maps, OSM, and ArcGIS Online use. Distorts area badly near the poles — never use for area analysis.

  • UTM (Universal Transverse Mercator)

    60 zones worldwide. Excellent for regional analysis (one country / state).

  • State Plane (US)

    Optimized for individual US states. Used by surveyors, planners, and engineers.

  • Albers Equal Area

    Preserves area. Best for thematic maps of large regions like the contiguous US.

  • Lambert Conformal Conic

    Preserves shape locally. Used for aviation charts and many country maps.

How to pick the right CRS

flowchart TD
    Q{What are you doing?} --> Store[Just storing data?]
    Store --> WGS[Use WGS84<br/>EPSG:4326]
    Q --> Web[Web map?]
    Web --> WM[Use Web Mercator<br/>EPSG:3857]
    Q --> Region[Regional analysis<br/>distance, area?]
    Region --> UTM[Use UTM zone for that area<br/>or State Plane]
    Q --> Continent[Whole continent<br/>thematic map?]
    Continent --> Albers[Use an equal-area<br/>projection like Albers]

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

EPSG codes

Every CRS has a numeric EPSG code. Memorize these:

EPSG Name When to use
4326 WGS84 Storage, global lat/long
3857 Web Mercator Web maps
4269 NAD83 US Census, NHD, many federal datasets
26917 NAD83 / UTM 17N Eastern US (e.g., Atlanta)
2240 NAD83 / Georgia State Plane West Georgia analysis

Look up any EPSG: https://epsg.io

In ArcGIS Pro

  • Map → Properties → Coordinate Systems sets the map's projection (just for display).
  • Project (Data Management) tool changes the file's projection.
  • Define Projection tool labels a file with a CRS — use only when the data has no CRS yet (it doesn't reproject).

Define vs Project

  • Project = re-calculate every coordinate (changes the data).
  • Define Projection = just attach a label (use only for unlabeled data).

Confusing them is the #1 beginner mistake.

Practice

Three exercises

  1. Open ArcGIS Pro and check your map's projection (bottom right of the map view).
  2. Add a US states shapefile and a city shapefile in WGS84. Are the cities in the right place?
  3. Run the Project tool on the cities to convert to NAD83 / Albers (EPSG:5070). Check the area of each state — does it stay the same? Why or why not?

→ Deeper dive: Coordinate Systems vs Projections tutorial

Next up

→ Vector Data — points, lines, and polygons.