← Methods 3

In-class Exercises

Georeferencing

  1. Go to Urban Reviewer.
  2. Find an urban renewal plan with a map image on the right when you select it. If you have a hard time finding one with a scanned map, use Pratt Institute or West Side.
  3. Click on the image to make it larger, then right click on it and save as to save it to your computer.
  4. Open QGIS.
  5. Set the project's projection (CRS) to EPSG:3857. We do this because the base layer in the next step is in that projection.
  6. Using QuickMapServices, add a satellite base layer with labels. I would suggest the Google Hybrid layer.
  7. Zoom to the general area your image should be in.
  8. Enable Georeferencer GDAL in the plugins area of QGIS.
  9. Go to Raster > Georeferencer > Georeferencer to open the Georeferencer window.
  10. Go to File > Open Raster and select the image you downloaded from Urban Reviewer.
  11. Add 3 or 4 ground control points:
    1. Switch between the Georeferencer and map windows and find a common point between the two (generally a street corner will be best).
    2. Click the Add point button in the Georeferencer window.
    3. Click on the common point in the Georeferencer window.
    4. When prompted to Enter map coordinates, select the From map canvas button.
    5. Click on the common point on the map.
  12. Once you are happy with the ground control points, click the Settings icon in the Georeferencer window.
  13. Set Transformation type to Thin plate spline.
  14. Set Target SRS to 3857 (the projection the reference map is in).
  15. Select a location to save to.
  16. Check Load in QGIS when done.
  17. Click Start Georeferencing in the Georeferencer window (looks like a play button).
  18. The Georeferenced image should show up on your map.
  19. Find a ground control point that doesn't line up as much as you would like.
  20. Delete the ground control point in the Georeferencer window.
  21. Add a replacement ground control point.
  22. Georeference it again—by default your georeferenced image will be overwritten, so change the destination in the georeferencer settings if you want to make a new file.