Guides

Hand-written articles on the coordinate-systems questions that actually come up in GIS, surveying and mapping. Plain language, technically correct, opinionated where it matters.

• Datum vs Projection: What's the Difference and Why It Matters

  Datum and projection are the two halves of every Coordinate Reference System. Confusing them is the most common cause of metre-scale GIS errors. Here's how each works, in plain language.

  6 min read

• UTM Zones Explained: How to Pick the Right Zone for Your Data

  UTM divides the world into 60 north-south zones, each 6° wide, projected separately to keep distortion low. Here's how to identify the right zone, what UTM is and isn't good for, and the difference between WGS 84 UTM and ETRS89 UTM.

  7 min read

• WGS 84 vs NAD83: Why They're Not the Same Datum

  WGS 84 and NAD83 were designed to be near-identical in 1986 but have diverged by 1-2 m due to plate motion. Here's exactly why, when it matters, and how to convert between them.

  5 min read

• Web Mercator vs True Mercator (EPSG:3857 vs EPSG:3395)

  Web Mercator and True (ellipsoidal) Mercator look the same but differ by up to 20 km at high latitudes. Here's why both exist, when each is used, and the gotchas.

  5 min read

• State Plane Coordinate System: A Plain-Language Guide

  What State Plane is, how its zones are organised, why each state has its own projection, and how to pick the right EPSG code for your project.

  6 min read

• Lat/Lon or Lon/Lat? The EPSG:4326 Axis Order Problem, Solved

  EPSG:4326 officially puts latitude first, but GeoJSON, proj4 and most software put longitude first. This mismatch is the most common coordinate bug in GIS — here is how every major tool handles it.

  6 min read

• ITRF, Epochs and Plate Motion: Why Modern Coordinates Drift

  ITRF2020, ETRS89, GDA2020, epoch 2010.0 — modern geodesy attaches dates to coordinates because the ground itself moves. Here is what reference frames and epochs actually mean, and when you must care.

  7 min read

• Multiple EPSG Transformations Exist — Which One Should You Use?

  NAD27 to WGS 84 has dozens of published transformations; ED50 has even more. Area of use, accuracy and method tell you which one is right for your data — here is how to read them.

  6 min read