CRS Basics and How to Set Them in QGIS

Introduction

One concept you cannot avoid when working with GIS is the coordinate reference system (CRS). A CRS is the reference that expresses positions on the Earth as coordinates such as latitude and longitude, and setting the CRS correctly is essential for handling geographic information accurately.

This article briefly explains CRSs for GIS beginners, then shows how to set the CRS of projects and layers in QGIS.

CRS basics

This article introduces two CRS types you will use often in QGIS: the geographic coordinate system, which uses latitude and longitude, and the projected coordinate system, which projects the Earth's surface onto a plane.

Geographic coordinate system

A geographic coordinate system expresses positions on the Earth in latitude and longitude. There are world-standard CRSs such as WGS84, used for data sharing at a global scale and for GPS positioning. In Japan, JGD2011 (Japanese Geodetic Datum 2011) is widely adopted as the national standard geographic coordinate system.

Projected coordinate system

A projected coordinate system projects the Earth's surface onto a plane. There are various projections, such as the Mercator and UTM projections. Choosing the best projection for your purpose and area, for example one that makes it easy to measure distances and areas in a specific region, improves the accuracy and convenience of your analysis.

In Japan, the Japan Plane Rectangular Coordinate System based on JGD2011 is used nationwide. It divides the Japanese archipelago into 19 zones and expresses a position as distances in meters from the origin of each zone, in the north-south and east-west directions.

What is an EPSG code?

Every CRS has an EPSG code, a code that uniquely identifies the CRS of geospatial data. This code is widely used as an international standard, and you use it when specifying a CRS in QGIS.

CRSs commonly used in Japan and their EPSG codes

Project CRS basics

Each QGIS project has its own CRS, and this setting determines how the data added to QGIS appears on the map canvas.

For example, displaying OpenStreetMap in Web Mercator (EPSG:3857) shows the whole world as a square map.

If you then change the CRS to WGS84 (EPSG:4326), the display switches to a map in which the Earth looks stretched horizontally.

QGIS also has an on-the-fly feature. Even when a project and its layers use different CRSs, QGIS automatically transforms the layers to the CRS set for the project. Thanks to this mechanism, layers appear in the correct positions relative to each other, and you can create a unified map.

The figure below sets the project CRS to WGS84 and layers OpenStreetMap, which uses Web Mercator, with Japanese administrative area data, which uses JGD2011.

Check and change the project CRS

You can check the project CRS from the EPSG:XXXX button at the bottom right of the window.

Click the button labeled EPSG:XXXX to open the Project Properties dialog, where you can change the project CRS.

Set the CRS for new projects

By default, QGIS automatically sets the project CRS to the CRS of the first layer added to the project.

To change this, follow the steps below. From the menu bar, select Settings → Options.

Next, in the list on the left, select CRS Handling under CRS and Transforms. The CRS for Projects section at the top lets you choose either Use CRS from first layer added or Use a default CRS, whichever you prefer.

Layer CRS

Every layer you add to QGIS carries information about the CRS it was created in. Because of this CRS information, QGIS can overlay the layer with other layers.

Check the layer CRS

When you add a layer in QGIS, it helps to first find out which CRS the layer was created in.

For example, on the download page for the Administrative Area Data of National Land Numerical Information, the “Coordinate system” field reads “JGD2011/ (B, L)” (“B, L” are the initials of the German words Breite (latitude) and Länge (longitude)). This shows that the data was created in the JGD2011 latitude and longitude coordinate system.

Next, check which CRS QGIS uses to draw the layer.

You can check the layer CRS in the layer properties. In the Layers panel, right-click the layer and select Properties.

Then select the Information tab in the list on the left. The Coordinate Reference System (CRS) section shows the CRS of the data.

For this layer, the name is JGD2011 (EPSG:6668) and the units are “Geographic (uses latitude and longitude for coordinates)”. So the layer is set to the JGD2011 latitude and longitude coordinate system (a geographic coordinate system).

This matches the CRS you checked earlier on the National Land Numerical Information page, so QGIS draws this data in the correct CRS.

If the CRS defined in the data itself differs from the CRS in QGIS, change it to the appropriate CRS.

Change the layer CRS

To change the CRS, open the Source tab of the properties. The Assigned Coordinate Reference System (CRS) section shows JGD2011 (EPSG:6668), the CRS you saw earlier on the Information tab.

To change it, click the Select CRS button (the globe icon) and choose a different CRS for the data.

Note that this setting tells QGIS which CRS the data was created in. Changing it does not convert the CRS of the data itself. To change the CRS of the data itself, you must use the reprojection function.

Settings for layers without CRS information

In QGIS, you can choose how the CRS is set when you add a layer that has no CRS information. Change the setting as follows.

From the menu bar, select Settings → Options, then in the list on the left, select CRS Handling under CRS and Transforms.

The CRS for Layers section lists several choices for when a new layer is created or a layer without a CRS is loaded.

If you only work with data in one particular CRS, you may want to select Use project CRS or Use default layer CRS. In general, though, you are likely to use layers in many different CRSs, so selecting Prompt for CRS is a good idea.

Conclusion

This article covered CRSs, from the basic concepts to the specific steps in QGIS. You will often need to change the CRS of projects and layers, so it helps to know how to set it.