# Projections <a id="projections"></a>

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## How to check EPSG-Code <a id="how-to-check-epsg-code"></a>

:::{Note}
Always check that the Coordinate Reference System (CRS)/EPSG code of your data is the same as the CRS/EPSG-code of your project!
:::

The default CRS/EPSG code of every QGIS project is the World Geodetic System 84 (EPSG: 4326). This CRS is optimized for world maps. So not perfect for most applications, because we mostly use maps for small areas.

### How to check EPSG-Code/CRS of your QGIS Project and change it <a id="how-to-check-epsg-codecrs-of-your-qgis-project-and-change-it"></a>
:::{Note}
To check and adjust the CRS/ EPSG-Code should be the first thing you should do when starting a new QGIS project.
:::

1.  Open a QGIS project
2. In the bottom right corner of QGIS you find the button `EPSG`. The number next to it is the EPSG Code currently used in the project. For more information click on the button.
![](../../../fig/EPSG_Code.png)
3. The window `Project Properties` will open. Here you can view all available CRS/EPSG-Code and their properties.
4. To change the CRS/EPSG-code, select the one you want to use and click `Apply`.

<video width="100%" controls src="https://github.com/GIScience/gis-training-resource-center/raw/main/fig/qgis_change_project_CRS.mp4"></video>

### How to check EPSG-Code/CRS of layer/ data <a id="how-to-check-epsg-codecrs-of-layer-data"></a>
:::{Note}
After loading any spatial data in QGIS, check the CRS/EPSG code of the data to make sure it is the same as the CRS/EPSG code of the project.
:::
1. Right click on the data layer, click on  `Properties`.
2. The “Layer Properties” Window of the data layer will open. Click on `Information`.
3. Under the headline “Coordinate Reference System (CRS)” you find all information about the CRS. The most important are:
    - __Name:__     Here you find the EPSG Code.
    - __Unites:__    Here you can find whether it is possible to use meters with this data layer or latitude and longitude.


## Changing the projection of a vector layer <a id="changing-the-projection-of-a-vector-layer"></a>

1. `Vector` Tab → `Data Management Tools` → `Reproject Layer`.
2. Select target CRS/ EPSG-Code.
3. Save the new file by clicking on the three dots next to `Reprojected`, specify the file name and the location where you want to save the file.
5. Click `Run`.

<video width="100%" controls src="https://github.com/GIScience/gis-training-resource-center/raw/main/fig/qgis_reproject_vector.mp4"></video>


## Changing the projection of a raster layer <a id="changing-the-projection-of-a-raster-layer"></a>

1. `Raster` Tab → `Projections` → `Warp (Reproject)`.
2. Select target CRS/EPSG-Code.
3. Select resampling method.
4. Save the new file by clicking on the three dots next to `Reprojected`, specify the file name and the location where you want to save the file.
5. Click `Run`.

<video width="100%" controls src="https://github.com/GIScience/gis-training-resource-center/raw/main/fig/qgis_reproject_raster.mp4"></video>

## Common mistakes with Coordinate Reference Systems <a id="common-mistakes-with-coordinate-reference-systems"></a>

The website [__I Hate Coordinate Systems!__](https://ihatecoordinatesystems.com/) offers a “a problem-based guide of common CRS issues, root causes, and solutions.”.