QGIS | Coordinate Reference System (CRS)
> Objective
This section introduces students to projections and coordinates
This section shows how to determine the Coordinate Reference System (CRS) of a projection
This section explains how to georeference scanned maps and aerial images
> Index / Table of Contents
Introduction
Tutorial(s)
Introduction to projections and coordinates
Define and/or edit the Coordinate Reference System (CRS) of a projection
Georeferencing
Scanned maps
Image to image georeferencing
Aerial Images
Raster layers
Verification
Visualisations
QGIS Installation
> Summary and Highlights
Introduction
Map projections help to transform the representation of the earth from a spherical shape to a planar shape. Coordinate Reference Systems (CRS) allow every place on the Earth to be defined by a set of three numbers. They are called coordinates. Many CRS exist for different regions and scales (local, regional and global).
A CRS is also specified for the projections/layers in QGIS. In case a projection/layer does not have a CRS, it is possible to define one. This process is called georeferencing. QGIS also allows users to make their own projections, e.g. projection on a globe.
The first set of tutorials covers the Coordinate Reference Systems (CRS) and map projections. The second one explains how to find the Coordinate Reference System for the projection in QGIS. Then, there are tutorials that show how to georeference an image. The last part shows how to project a map on a globe.
Tutorial(s)
Introduction to projections and coordinates
The following tutorials give general explanations about the CRS and map projections.
Introduction to Coordinate Reference systems QGIS A Gentle GIS Introduction: “Coordinate Reference Systems” Language(s): English Chapters: overview, map projection in detail, the three families of map projections, accuracy of map projections, map projections with angular conformity, map projections with equal distance, projections with equal areas, Coordinate Reference System (CRS) in detail, Universal Transverse Mercator (UTM) CRS in detail, on-the-fly projection, common problems/things to be aware of, what have we learned, now you try, something to think about, further reading, what’s next https://docs.qgis.org/3.22/en/docs/gentle_gis_introduction/coordinate_reference_systems.html
Project CRS and layer CRS TOI-pedia: “Coordinate Reference System” Language(s): English Chapters: background, project CRS, layer CRS http://wiki.bk.tudelft.nl/toi-pedia/Coordinate_reference_systems
Coordinate reference systems and layer CRS Erik Meerburg (3D geoinformation research group): “Urbanism GIS Course 6: Projections and Coordinates” Language(s): English Time: 6m 26s https://www.youtube.com/watch?v=4yF4xGjldBc
Difference between On-The-Fly (OTF) reprojection, assigning a projection and reprojection in QGIS Hans van der Kwast: “Projection in GIS” Language(s): English Time: 8m 2s https://www.youtube.com/watch?v=vOEGsFFCz18
Define and/or change the Coordinate Reference System of a projection
The links below show how to define and changes the CRS of a projection.
Project CRS and layer CRS TOI-pedia: “Coordinate Reference System” Language(s): English Chapters: background, project CRS, layer CRS http://wiki.bk.tudelft.nl/toi-pedia/Coordinate_reference_systems
Find the EPSG code for a GIS project Hans van der Kwast: “How to find the EPSG code for your projection?” Language(s): English Time: 6m 17s Chapters: introduction (start-0:14), EPSG code (0:14-3:18), demonstration (3:18-end) https://www.youtube.com/watch?v=gu3WMWTZpY4
Reproject and transform vector datasets QGIS Training manual: “6.1. Reprojecting and Transforming Data” Language(s): English Chapters: follow along: projections, follow along: “on the fly” reprojection, follow along: saving a dataset to another CRS, follow along: creating your own projection, in conclusion, further reading, what’s next https://docs.qgis.org/3.10/en/docs/training_manual/vector_analysis/reproject_transform.html
Reproject selected features of a layer and overlay data layers of different projections together in QGIS Ujaval Gandhi: “Working with Projections (QGIS3)”/”Werken met projecties (QGIS3)” Language(s): English, Dutch Chapters: overview of the task, get the data, procedure EN: https://www.qgistutorials.com/en/docs/3/working_with_projections.html NL: https://www.qgistutorials.com/nl/docs/3/working_with_projections.html
Georeferencing
Georeferencing allows to relate the internal coordinate system of a map or photo to geographic coordinates.
Scanned maps
Georeference a scanned map of southern India from 1870 Ujaval Gandhi: “Georeferencing Topo Sheets and Scanned Maps (QGIS3)”/”Topobladen en gescande kaarten voorzien van geoverwijzingen (QGIS3)” Language(s): English, Dutch Chapters: overview of the task, get the data, procedure EN: https://www.qgistutorials.com/en/docs/3/georeferencing_basics.html NL: https://www.qgistutorials.com/nl/docs/3/georeferencing_basics.html
Georeference a scanned map and use it as a background for digitizing points Hans van der Kwast: “Georeferencing a scanned map and digitizing vectors in QGIS3” Language(s): English Time: 32m 39s https://www.youtube.com/watch?v=m12ZXpGBoDc
Georeference a scanned map Hans van der Kwast: “Georeference a Scanned Map in QGIS” Language(s): English Time: 7m 6s https://www.youtube.com/watch?v=bKfm0crsMBM
Georeference a scanned map Mariëlle Geers-Plasmans (Geojuffie): “Georeferen en digitaliseren van een kaart in QGIS” Language(s): Dutch Time: 25m 43s https://www.youtube.com/watch?v=r_HaMPZOyyM
Georeference a scanned map and digitize vectors in a GeoPackage Hans van der Kwast: “QGISHydro Webinar: Georeferencing and digitizing vectors” Language(s): English Time: 1h 7m 3s Chapters: start QGISHydro Webinar 1 (start-6:01), introduction to the demonstration (6:01-7:07), find the projection of the map (7:07-12:42), the Georeferencer GDAL plugin (12:42-14:38), transformation settings (14:38-18:03), add Ground Control Points (18:03-22:52), check error statistics (22:52-24:46), evaluate the georeferenced layer (24:46-29:10), digitize point vectors (GeoPackage) (29:10-35:35), Digitize rivers (GeoPackage) (35:35-40:59), dissolve tributaries (40:59-42:32), digitize lake polygons (GeoPackage) (42:32-45:40), style vectors (45:40-59:20), shameless plugs (59:20-end) https://www.youtube.com/watch?v=hPLqy-NBgu8
Georeference old map of Hamburg Erik Meerburg (Geogoeroe): “Georefereren oude kaart van Hamburg” Language(s): Dutch Time: 12min 26s https://www.youtube.com/watch?v=QUfWHCxsZho
Image to image georeferencing
Georeference an image to fit its coordinates to the geographic coordinates of another image.
Image to image georeferencing of a scanned map Hans van der Kwast: “Image to image georeferencing in QGIS 3.4” Language(s): English Time: 6m 40s https://www.youtube.com/watch?v=67j_HShwv8Y
Image to image georeferecing Hans van der Kwast: “Image to image georeferencing in QGIS using a docked Georeferencer panel” Language(s): English Time: 12m 14s https://www.youtube.com/watch?v=kBJ-UbBrXg0
Aerial Images
Georeference an image taken from the air to geographic coordinates.
Georeference a high resolution balloon-imagery using reference coordinate from OpenStreetMap Ujaval Gandhi: “Georeferencing Aerial Imagery (QGIS3)”/”Geo-verwijzingen voor satellietbeelden (QGIS3)” Language(s): English, Dutch Chapters: overview of the task, get the data, procedure EN: https://www.qgistutorials.com/en/docs/3/advanced_georeferencing.html NL: https://www.qgistutorials.com/nl/docs/3/advanced_georeferencing.html
Raster layers
A raster layer is a matrix of pixels that contain information, organized into a grid.
Reproject raster layers using the Reproject (warp) tool Hans van der Kwast: “Reproject Raster Layers (warp)” Language(s): English Time: 2m 33s https://www.youtube.com/watch?v=hOSVctFvXAk
Verification
Once an image has been georeferenced, it should be verified to check whether the georeferenced layer is in the correct position.
Copy coordinates and add a backdrop from the QuickMapServices plugin Hans van der Kwast: “Verify a Georeferenced Map in QGIS” Language(s): English Time: 3m 22s https://www.youtube.com/watch?v=DCfDdoq6rjg
Visualisations
After georeferencing, data can be visualised in the correct dimensions, position and curvatures.
Add a custom projection Mariëlle Geers-Plasmans (Geojuffie): “Visualisatie op een globe in QGIS” Language(s): Dutch Time: 10m 26s https://www.youtube.com/watch?v=BXtZlTPlMj8
> QGIS Installation
Do you still need to install QGIS? The QGIS installation and update section explains how to install QGIS on Windows and macOS.
Useful Links
Erik Meerburg (3D geoinformation research group)
Hans van der Kwast YouTube channel
Ujaval Gandhi (QGIS Tutorials and Tips website)
Mariëlle Geers-Plasmans (Geojuffie) YouTube channel
Erik Meerburg (Geogoeroe) YouTube channel
#QGIS, #Coordinate Reference System, #CRS, #Georeferencing
Hans van der Kwast. (2022, May 10). Georeference a Scanned Map in QGIS [Video]. YouTube.