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Importing and rendering Esri dot-density data in QGIS

Geographic Information Systems Asked by Matt Brunt on October 16, 2020

I’m not a professional digital cartographer. I’m a graphic designer who has had to familiarize myself with some processes to fill a real estate broker’s needs. I only offer that as an apology for any ignorance you may read in my post. I’m fairly ignorant!

That being said… I’m having some issues rendering population in the form of dot-density in QGIS 3.12.

  • My process begins with generating the population data in ArcGIS Business Analyst Online. (The client relies on ArcGIS for the
    entirety of their demographic and mapping data.)
  • I share the data out to ArcGIS Online, where I render it as a
    shapefile for download.
  • After download and extraction, I drag the shapefile into my QGIS
    project, where it successfully appears in the layers palette.

From here, I’m encountering some issues. I’ve researched online, and there only appear to be a couple of tutorials for processing the dot-density map that aren’t ancient.

  • I select the appropriate layer (pop_density_greater_houston_2010)
    and, under the “Vector/Research tools” menu, select “Random points
    within polygons”.
  • Select the appropriate input layer
  • Points strategy = point count

enter image description here

  • However, the tool does not give me the option of selecting from which
    field to pull it’s data.
  • I’ve tried changing symbology to “Categorized” and “Graduated”, then
    classifying based on the density field before running the “Random
    points…” tool.

Results I’ve encountered:

  • “Invalid geometry” error messages
  • Endless processing (based on the processing animation at the bottom
    of the window)
  • single points within each census tract

I cannot seem to accomplish a true dot-density.

One Answer

Thanks, Ben W. Again, I'm on QGIS 3.12.

That override fly-out menu was the solution! In there, I could either select the "ThematicVa" field as my dataset... enter image description here

or (and this was definitely more useful for my purposes) via the "Edit..." option, I was able to create a simple expression that divided the raw population value by 50 ("ThematicVa"/50) to accomplish a 1 dot=50 people representation.

enter image description here

That was a good pointer. I appreciate the good counsel, @benw!

Answered by Matt Brunt on October 16, 2020

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