Week 11 - Dot Mapping

This week in GIS4043 we created dot density maps. Dot maps are great for showing the distribution of raw totals of conceptual data, such as for example income distribution or ethnicity within a city. The key here is to keep the enumeration unit small (so census tract areas would be better than a county, which would be better than a state, etc.). The map deliverable for this week shows the population density of southern Florida.

Dot map showing population density of southern Florida.

Technical Details

The dots are meant to be the main focus of the map, so the symbols for my polygon layers (the urban and water areas, and the Florida counties) are in fairly low key pastels. The water and urban area symbols are set to a 45% transparency, and the Florida counties layer is set to 10% transparency. The cities layer isn't transparent, but the symbol is 80% gray as opposed to being solid black. Incidentally the dots showing the population density and the Florida counties come from the same data layer - I just have the symbols separated out into two layers in order to be able to manipulate the map view a bit better.

The dots shown above were generated within ArcMap, under Symbology > Quantities > Dot density. Instead of keeping the default settings I opted to have the dots stay within a fixed location and applied a mask to the data. This mask stipulates that the dots will draw only within an urban area (enter the urban area layer!). Before applying the mask I had messed with the dot size vs. dot value, and eventually settled on a size of 1.87 with each dot representing 25,000 people. I had wanted to keep the dot value within increments of 5,000 and it seemed to me that 25,000 was the best fit while still allowing a reasonable dot size.

Use of the mask was fraught with all kinds of mini-dramas for my poor little laptop. The mask seems to pull on all available computer resources, and when it wasn't getting enough it caused ArcMap to crash! Repeatedly. This became an issue when I wanted to see how a different setting with the mask would look on my map, and also when I wanted to export the image. To get around this I did the following:

• Got over my need to see minor changes to the mask displayed on the whole map. It just wasn't worth it, and I found that those little boxes in the Symbol settings window were there for a reason.
• I decided to drag and drop a new population density layer onto my original map. To do this I re-created the exact original layer settings (even down to the scale) on a blank map.
• I then converted my original legend to graphics, so that I didn't have to mess with making or editing a new legend once I added my fresh population density layer (yes, ArcMap was that touchy).
• Once everything was in place and with two maps open, I then dragged and dropped the new layer onto the old map. After waiting a few minutes for it to draw I then exported it. Several more minutes later I had the .png file that you see above!

At no time during this process did I do anything else on my computer. And once my new population density layer drew correctly, I closed its source map in order to allocate even more computer resources for the map I was trying to export.