QGIS – How to Georeference a Large Set of Points

Question

I have a static image of a map:

And I would like to obtain the coordinates of each blue point. I am very new to QGIS but I have done a very similar task but for a much smaller set of point. My approach there was to:

1. Open an OSM standard map

2. Load the image of my map containing the points I want referenced

3. Match points on my map to points on the OSM map

4. After mapping a couple of points my map is aligned with the OSM map

5. Select the appropriate transformation strategy

6. My image is exported as a referenced GeoTIFF

7. Create an empty shapefile to contain the points of interest

8. Turn on editing mode and manually select the points of interest

The problem I have is with point 8. With a small number of points this is clearly not an issue. However, in the map above, manually selecting each point is something I would like to avoid. I tried converting the map to a vector (raster to vector) but the output looks very strange.

Is there a clever way of letting QGIS know that I am interested in ALL blue dots?

EDIT:

I managed to convert the map to a vector (polygonize). And it looks like this:

However, I am still uncertain how I could go about selecting just the circles and then obtain their coordinates.

Answer 1

Convert the raster to polygons, select polygons with a certain area and roundness:

1. Georeference the raster

2. i.segment. I use default settings. If you're using model builder you need to go to advanced settings and set Value on "Name for input raster map with starting seeds" and "Name of input bounding/constraint..." to be able to be able to set them to no value.

3. Raster pixels to polygons

4. Dissolve by the created Value field

5. Extract by expression. Extract the polygons with a specific size and roundness, e.g. the blue circles. I use:

$area>160000 and $area<215000 and (($area*4*pi())/ $perimeter^2)>0.6

6. Centroids

It works for almost all points: