Coordinate Reference Systems

Lesson 3 of 8 · GIS Software Fundamentals · Beginner

One of the most common sources of spatial errors in GIS work is a mismatched or incorrectly assigned coordinate reference system. Understanding how CRS works prevents many frustrating problems.

What Is a Coordinate Reference System?

A coordinate reference system (CRS) defines how coordinates in a dataset relate to real locations on Earth. Without a CRS, a pair of numbers like 45.4215, -75.6972 is just two numbers — the CRS tells the software that these represent a latitude and longitude in a specific geographic system, placing the point in Ottawa, Ontario.

GIS 101 covers the conceptual foundations of coordinates and projections in detail. This lesson focuses on how CRS appears in software workflows.

Geographic vs. Projected Coordinates

Geographic coordinate systems describe locations using latitude and longitude in degrees. They reference a model of the Earth's shape (a datum). WGS 84 is the most widely used geographic CRS and is the basis for GPS coordinates.

Projected coordinate systems convert the curved surface of the Earth onto a flat plane using a map projection. Projected coordinates are typically expressed in metres or feet, making distance and area calculations more straightforward for local analysis. Examples include UTM zones and provincial coordinate systems used in Canada.

Layer CRS vs. Project CRS

Each layer in your project has its own CRS — the one assigned to the source data. Your project also has a CRS, which controls how layers are displayed together on the map canvas. Most GIS software can reproject layers on the fly for display purposes, so layers with different CRS values can appear together correctly. However, on-the-fly display reprojection does not change the underlying data.

For analysis — especially distance and area calculations — all input layers should be in the same projected CRS appropriate for your study area.

Assigning vs. Transforming a CRS

These are two different operations that are easy to confuse:

• Assigning a CRS tells the software what coordinate system the data is already in. Use this when the data has no CRS defined, or when the wrong CRS was previously assigned. Assigning does not move any coordinates — it only changes the label.
• Reprojecting (transforming) a CRS mathematically converts the coordinates from one system to another. Use this when you need the data in a different coordinate system for analysis or to match other layers.

Assigning the wrong CRS does not fix data with incorrect coordinates — it just mislabels them. If your data appears in the wrong location, check whether the CRS is correctly assigned before assuming the coordinates themselves are wrong.

Choosing an Appropriate Projected CRS

For local analysis in Canada, a projected CRS appropriate for your province or region will give more accurate distance and area results than a global geographic CRS. Common choices include UTM zones (e.g., UTM Zone 17N for southern Ontario) and provincial coordinate systems. Check what CRS is used by authoritative data sources in your area — using the same CRS simplifies combining datasets.

Avoid calculating distances or areas using geographic coordinates (degrees) — the results will be meaningless or misleading.

Common Mistakes

• Assigning a CRS instead of reprojecting, or reprojecting instead of assigning — these are not interchangeable.
• Calculating distances or areas in a geographic CRS (degrees) and treating the result as metres.
• Assuming that layers displaying together correctly means their CRS values match — on-the-fly reprojection can mask mismatches.
• Ignoring a missing CRS warning and proceeding with analysis.
• Choosing a projected CRS designed for a different region, which introduces distortion.

Key Takeaways

• A CRS defines how coordinates relate to real locations on Earth.
• Geographic CRS uses degrees (latitude/longitude); projected CRS uses metres or feet on a flat plane.
• Assigning a CRS labels the data; reprojecting converts the coordinates.
• For distance and area analysis, use a projected CRS appropriate for your study area.
• On-the-fly display reprojection does not change the underlying data.