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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. 


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.


Aerial Images 

Georeference an image taken from the air to geographic coordinates. 


Raster layers  

A raster layer is a matrix of pixels that contain information, organized into a grid. 


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. 


> 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


QGIS Training Manual

QGIS User Guide

GIS OpenCourseWare

TOI-pedia 

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.

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