Working with Projections (QGIS3)

Map projections - or Coordinate Reference System (CRS) - often cause a lot of frustration when working with GIS data. But a proper understanding of the concepts and access to the right tools will make it much easier to deal with projections. In this tutorial, we will explore how projections work in QGIS and learn about tools available for vector and rasters data layers.

Overview of the task

The task is to re-project selected features of a layer and overlay data layers of different projections together in QGIS.

Get the data

Natural Earth has Admin 0 - Countries dataset. Download the countries

UK’s Ordnance Survey provides open data for download. Download the MiniScale raster product for Great Britain and extract it to a folder on your computer.

For convenience, you may directly download a copy of the dataset from the link below:

ne_10m_admin_0_countries.zip

minisc_gb.zip (Contains only the files required for this tutorial)

Data Sources: [NATURALEARTH] [OSOPENDATA]

Procedure

  1. Open QGIS. Go to Layer ‣ Add Layer ‣ Add Vector Layer….

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  1. Click on next to Source, Browse to the downloaded ne_10m_admin_0_countries.shp file and click Add.

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  1. At the bottom of QGIS window, you will notice the label Coordinate. As you move your cursor over the map, it will show you the X and Y coordinates at that location. At the bottom-right corner you will see EPSG:4326. This is the code for the current CRS (Projection) for the project - also known as the Project CRS.

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  1. To determine a layer’s projection, we can look into the metadata. Right click on ne_10m_admin_0_countries layer and select Properties.

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  1. Switch to the Information tab in the Layer Properties dialog. Expand the Information from provider section. At the bottom, you will see the name of the projection under CRS.

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  1. Now let’s see how we can change the layer’s projection. This operation is called Re-Projection. Rather than re-projecting the entire layer, we can also select a subset of features and re-project them to a new layer. Use the Select features by area or single click tool and click on United Kingdom feature to select it.

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  1. Search for and locate the Vector General ‣ Reproject layer algorithm in Processing toolbox.

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  1. Select ne_10m_admin_0_countries as the Input layer, check Selected features only then click on the globe icon next to Target CRS, search and select EPSG:27700 - OSGB 1936 / British National Grid. In Reprojected, choose the ... and click Save to a file. Now choose the directory and enter the name as united_kingdom.gkpg and click Run.

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  1. A new layer united_kingdom will appear on the Layer Panel. As you see, both the layers still line up exactly with each other - even though they are in different CRSs. This is because QGIS supports On-The-Fly (OTF) CRS transformation. Which means that whenever a layer’s CRS doesn’t match the Project CRS, it will automatically be transformed to the Project CRS so it can be displayed correctly. Now let’s set the Project CRS to match the newly created united_kingdom Layer’s CRS. Remove the ne_10m_admin_0_countries layer and, right click on the united_kingdom layer Layer CRS ‣ Set Project CRS from Layer.

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  1. You will see the Project CRS is updated to EPSG:27700.

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  1. Now let’s add a Raster layer. Go to Layer ‣ Add Layer ‣ Add Raster Layer….

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  1. Click on the ... next to Source, select the layer MiniScale_(standard)_R23.tif. Click Add.

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  1. Now a new layer MiniScale_(standard)_R23 is added to the canvas.

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  1. To make both layers visible, switch the order of the layer by dragging the MiniScale_(standard)_R23 to the bottom in the Layers panel.

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  1. Right-click on the MiniScale_(standard)_R23 layer and click on Properties.

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  1. In the Layer Properties, switch to Information, the CRS is EPSG:27700 - OSBG 1935 / British National Grid - Projected. This confirms that the raster layer’s CRS is the same as the Project CRS.

Note

If you want to re-project a raster layer, you can use GDAL ‣ Raster projections ‣ Warp (reproject) algorithm in Processing toolbox.

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