Docs in progress for ‘QGIS testing’. Visit http://docs.qgis.org/2.18 for QGIS 2.18 docs and translations.

7.1. Lesson: Reprojecting and Transforming Data

Let’s talk about Coordinate Reference Systems (CRSs) again. We’ve touched on this briefly before, but haven’t discussed what it means practically.

The goal for this lesson: To reproject and transform vector datasets.

7.1.1. basic Follow Along: Projections

The CRS that all the data as well as the map itself are in right now is called WGS84. This is a very common Geographic Coordinate System (GCS) for representing data. But there’s a problem, as we will see.

  • Save your current map;
  • Then open the map of the world which you’ll find under exercise_data/world/world.qgs;
  • Zoom in to South Africa by using the Zoom In tool.
  • Try setting a scale in the Scale field, which is in the Status Bar along the bottom of the screen. While over South Africa, set this value to 1:5000000 (one to five million);
  • Pan around the map while keeping an eye on the Scale field.

Notice the scale changing? That’s because you’re moving away from the one point that you zoomed into at 1:5000000, which was at the center of your screen. All around that point, the scale is different.

To understand why, think about a globe of the Earth. It has lines running along it from North to South. These longitude lines are far apart at the equator, but they meet at the poles.

In a GCS, you’re working on this sphere, but your screen is flat. When you try to represent the sphere on a flat surface, distortion occurs, similar to what would happen if you cut open a tennis ball and tried to flatten it out. What this means on a map is that the longitude lines stay equally far apart from each other, even at the poles (where they are supposed to meet). This means that, as you travel away from the equator on your map, the scale of the objects that you see gets larger and larger. What this means for us, practically, is that there is no constant scale on our map!

To solve this, let’s use a Projected Coordinate System (PCS) instead. A PCS “projects” or converts the data in a way that makes allowance for the scale change and corrects it. Therefore, to keep the scale constant, we should reproject our data to use a PCS.

7.1.2. basic Follow Along: “On the Fly” Reprojection

By default, QGIS reprojects data “on the fly”. What this means is that even if the data itself is in another CRS, QGIS can project it as if it were in a CRS of your choice.

You can change the CRS of the project by clicking on projectionEnabled button in the bottom right corner of QGIS.

  • In the dialog that appears, type the word global into the Filter field. One CRS (NSIDC EASE-Grid 2.0 Global, EPSG:6933) should appear in the list below;
  • Click on the NSIDC EASE-Grid 2.0 Global to select it, then click OK;
  • Notice how the shape of South Africa changes. All projections work by changing the apparent shapes of objects on Earth;
  • Zoom in to a scale of 1:5000000 again, as before;
  • Pan around the map;
  • Notice how the scale stays the same!

“On the fly” reprojection is also used for combining datasets that are in different CRSs.

  • Add another vector layer to your map which has the data for South Africa only. You’ll find it as exercise_data/world/RSA.shp.
  • Load it and a quick way to see what is its CRS is by hovering the mouse over the layer in the legend. It is EPSG:3410.

What do you notice?

The layer is visible even if it has a different CRS from the continents one.

7.1.3. moderate Follow Along: Saving a Dataset to Another CRS

Sometimes you need to export an existing dataset in another CRS. As we will see in the next lesson, if you need to make some distance calculations on layer, it is always better to have the layer in a projected coordinate system.

Be aware that the ‘on the fly’ reprojection is related to the project and not to single layers. This means that layers can have different CRS from the project even if you see them in the correct position.

But you can easily export the layer in another CRS.

  • Right-click on the buildings layer in the Layers panel;
  • Select Export ‣ Save Features As… in the menu that appears. You will be shown the Save Vector Layer as… dialog;
  • Click on the Browse button next to the File name field;
  • Navigate to exercise_data/ and specify the name of the new layer as buildings_reprojected.shp;
  • We must change the value of the CRS. Only the recent CRSs used will be shown in the drop down menu;
  • Click on the setProjection button next to the dropdown menu;
  • The CRS Selector dialog will now appear;
  • In its Filter field, search for 34S;
  • Select WGS 84 / UTM zone 34S from the list;
../../../_images/CRSselector.png
  • Leave the other options unchanged;

The Save Vector Layer as… dialog now looks like this:

../../../_images/save_vector_dialog.png
  • Click OK;

You can now compare the old and new projections of the layer and see that they are in two different CRS but they are still overlapping.

7.1.4. hard Follow Along: Creating Your Own Projection

There are many more projections than just those included in QGIS by default. You can also create your own projections.

  • Start a new map;
  • Load the world/oceans.shp dataset;
  • Go to Settings ‣ Custom Projections… and you’ll see this dialog;
../../../_images/custom_crs.png
  • Click on the signPlus button to create a new projection;

An interesting projection to use is called Van der Grinten I.

  • Enter its name in the Name field;

This projection represents the Earth on a circular field instead of a rectangular one, as most other projections do.

  • Add the following string in the Parameters field:

    +proj=vandg +lon_0=0 +x_0=0 +y_0=0 +R_A +a=6371000 +b=6371000 +units=m +no_defs
    
../../../_images/new_crs_parameters.png
  • Click OK;
  • Click on the projectionEnabled button to change the project CRS;
  • Choose your newly defined projection (search for its name in the Filter field);
  • On applying this projection, the map will be reprojected thus:
../../../_images/van_grinten_projection.png

7.1.5. In Conclusion

Different projections are useful for different purposes. By choosing the correct projection, you can ensure that the features on your map are being represented accurately.

7.1.6. Further Reading

Materials for the Advanced section of this lesson were taken from this article.

Further information on Coordinate Reference Systems is available here.

7.1.7. What’s Next?

In the next lesson you’ll learn how to analyze vector data using QGIS’ various vector analysis tools.