How to automatically convert dots on paper maps to vector points?

Question

I have several paper maps with dots that I need to convert to vector points. I was hoping to get some suggestions on how to accomplish this without recreating each and every point by hand. I know the basic workflow will look something like this:

1. Scan paper maps.
2. Use QGIS Georeferencer plugin to georeference maps.
3. Run some kind of analysis to identify dots and convert to vector points.

I'm fine on 1-2, but a little stuck on 3. I've provided a sample crop of one of the maps below.

Answer 1

You can clean up the scan in Gimp first and make processing a lot easier: Use the 'select by colour' tool and select the map background, adjusting the threshold to give a nice clean selection. Invert the selection, so the lines and points are now selected.

Now 'shrink' the selection down Select > Shrink... by enough pixels so that just you points remain, 'grow' the selection a tiny bit and fill the selection with a solid colour. This stage may take a bit of playing around with shrink and grow values. Undo and try again if needed.

Georeference the raster image as before and then vectorise it Raster > Conversion > Polygonize, filter the polygons to show those with value 0 (black) and create polygon centroids from them.

There may be smarter or more accurate ways to do this, but it really does only take a minute using this method.

Answer 2

I guess the third step would be(among others) use high/low pass filters to eliminate the lines.Then Rasterize the resultant image to get points digitized automatically.