How to define/change coordinate system in QGIS?

One of the most important data processing steps in GIS works is defining/changing coordinate system. It is crucial to get accurate results from our analysis. In this post, I will try to explain how to do it in QGIS with details.

Change Coordinate System of the Project

We first need to change the coordinate system of the project we work on (if we wish). You can see coordinate system information on the right bottom of the project page.

Fig. 1: Left click on it to see other options

Fig. 2: You can search and select the one you are looking for on this window

I will select WGS 84 UTM Zone 35N now for the sake of the data i will use.

Define Coordinate System for the Data

Let’s upload the data we are going to use to the project;

Fig. 3: This is how my data looks on the map

If the data does not have any defined coordinate system, you will get a warning on the top of the page, indicating “undefined CRS”. You can proceed with that warning and defined the CRS you are going to work on.

Change Coordinate System of the Data

We need to right click on the data on Layer Panel and select Properties -> Source to see coordinate system information.

Fig. 4: You can see the assigned coordinate reference system on this window

It is possible to change the coordinate system of that data on this window. However, this only redefines how QGIS interprets the layer’s coordinates. Coordinate values do not change so It does not reproject the actual data. To permanently reproject a layer, we need to use Reproject tool

To reproject the data, you can go to top menu and click Vector -> Data Management Tools -> Reproject Layer

Fig. 5: You can select the target CRS on this window and run the tool

When you run the tool, a new layer with the defined coordinate system will be created and appear on the layer panel;

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