How to Reproject a Vector Layer in QGIS

Introduction

GIS data has a coordinate reference system (CRS). To display and analyze geographic information accurately in QGIS, working in an appropriate CRS is essential.

This article explains how to convert the CRS of a layer with the Processing tool Reproject layer.

For more on the basic concepts of CRSs, see the following article.

When you need to convert a CRS

Reproject layer is a tool that converts the current CRS to another CRS. For example, it can convert data from EPSG:4326 - WGS 84, which covers the whole world, to the Japan Plane Rectangular Coordinate System used in surveying in Japan.

Reprojection is needed in situations like these:

• When you need to convert from a geographic coordinate system to a projected coordinate system
  • When you want to measure distances and areas accurately
  • When you want to run distance-based spatial analysis, such as creating buffers
• When you need to match the CRSs of your data
  • When you want to unify the CRSs of the datasets you use in an analysis
  • When you need to match the CRS specified by a public agency or research institution for a deliverable

Steps to convert a CRS with Reproject layer

QGIS offers several ways to reproject data. This article uses the Processing tool Reproject layer.

Prepare the layer and check its CRS

The example uses the designated emergency evacuation site data for Saitama Prefecture, provided by the Geospatial Information Authority of Japan (GSI).

First, open the layer properties and check the layer's CRS in the Information tab. This layer was created in the world geodetic system WGS84 (EPSG:4326), widely used for GPS and other purposes, and its units are Geographic (uses latitude and longitude for coordinates).

This example converts the layer to the JGD2011 Japan Plane Rectangular Coordinate System, widely used for public surveying in Japan. The Japan Plane Rectangular Coordinate System divides Japan into 19 zones, and Saitama Prefecture belongs to zone IX.

Open Reproject layer and configure it

From the menu bar, click Vector → Data Management Tools → Reproject layer.

In the Reproject layer dialog that opens, set the following items.

1. Input layer: Select the layer whose CRS you want to convert.
2. Target CRS: Click the Select CRS button (the globe icon) and choose the target CRS (how to find a CRS is explained later).
3. Reprojected: Click the ... button and specify where to save the output. If you do not specify one, QGIS outputs a temporary layer.

When the settings are done, click the Run button.

Check the result

When the run finishes, QGIS adds a new layer. QGIS displays layers with different CRSs on top of each other on the map thanks to its on-the-fly feature, so the map looks no different.

Open the properties of the output layer and check the coordinate reference system in the Information tab. The name is JGD2011 / Japan Plane Rectangular CS IX and the units are meters, which shows that the CRS was converted correctly.

How to find a CRS

To set Target CRS in Reproject layer, click the Select CRS button (the globe icon) to open the CRS selector.

QGIS has many CRSs registered, so use the filter to find the one you want quickly.

For example, the CRS used in this conversion is the JGD2011 Japan Plane Rectangular Coordinate System. Enter JGD2011 in Filter, and the CRSs that match appear under Predefined Coordinate Reference Systems.

The list is grouped by CRS type: geocentric, geographic, and projected coordinate systems. The Japan Plane Rectangular Coordinate System is classified as a projected coordinate system, so look for “Japan Plane Rectangular” among the projected coordinate systems and select the CRS you want.

When you select a CRS from the list, the geographic area that CRS covers appears at the bottom of the window. Checking this information as you select helps you get the setting right.

It also helps to remember the international code that identifies a CRS, the EPSG code. For example, the JGD2011 Japan Plane Rectangular Coordinate System zone IX used earlier is registered as EPSG:6677.

The filter also accepts EPSG codes, so searching by EPSG code is a smooth way to find the CRS you want.

Conclusion

This article explained how to convert a CRS with Reproject layer. Converting to an appropriate CRS makes more accurate analysis possible. Real-world work often calls for a CRS conversion. Learn how to convert CRSs in QGIS and put it to use in your daily work.