crs - Data at Reed - Reed College

Understanding Coordinate Reference Systems (CRS)

A Coordinate Reference System (CRS) is a framework that defines how spatial data is mapped on the Earth's surface. Essentially, it allows us to take a 3D object and turn it into a 2D projection. But there are many different ways of doing this, and that can lead to conflicts in our data.

This is often the answer to the perpetual question “why are my points showing up in the middle of the ocean”? You have one set of data in one CRS and you are adding a layer on top that uses a different projection. It’s like laying out a blueprint measured in feet and one measured in meters and expecting them to line up. But you can convert between reference systems just as you would between feet and meters. See below for information on changing the CRS.

Most Common CRSs

WGS84 (World Geodetic System 1984):
- Expresses coordinates in degrees of latitude and longitude
- Commonly used globally
- Location of Reed College: 45.4811° N, 122.6308° W or (45.481499, -122.631014)
NAD83 (North American Datum 1983):
- Expresses coordinates in degrees of latitude and longitude
- Commonly used in North America
- Similar to WGS 84, but is better for very localized mapping (typically WGS84 and NAD83 coordinates are within one meter of one another)
- Location of Reed College: 45.4811° N, 122.6308° W or (45.481499, -122.631014)
UTM (Universal Transverse Mercator):
- Expressed in meters
- UTM divides the world into a series of zones, each with its own projection
- Location of Reed College: UTM Zone 10 (528837.132, 5036507.309)

These are just a few. There are many, many more beyond these and many subdivisions within these.

Changing the CRS

You should try to always work with data in the same CRS. Having your original data files in the same CRS is wonderful but not always possible, but you should always standardize your CRS within projects. Even minor differences like those between NAD83 and WGS84 can add up, so if you have multiple data files that each use a different CRS, you should convert to a standard. You can do this several ways in QGIS: by changing the CRS when you load a file, by setting the CRS when you save a layer, or by reprojection. Reprojection will remap one set of coordinates to be the same type of projection (the same CRS) as another set.