[postgis-users] Dot Density idea

John Abraham jabraham at ucalgary.ca
Mon May 3 21:49:32 PDT 2010

One of the things I miss about using ESRI's GIS is the ability to do dot-density maps.  Within a polygon, the number of dots is proportional to a value, and the dots are randomly placed.  I find it useful to be able to present several data values at once (e.g. blue dots for population, red dots for employment).  

I also find that it is a more intuitive way of scaling for zone size than dividing the value by the area of the zone.  That is, the count of the dots represents the actual number, but the density of the dots represents the density of the number.  So I don't have to decide whether to divide the value by the area of the polygon to plot density: both the absolute number and the density are easily visible.

Since my open-source GIS viewing systems (mostly QGIS and Mapserver) won't plot dot-density, I've done without.

But today I realized that I can build these on the server instead.  I can generate random points within the bounding-box of the polygon, throwing out those that aren't contained within the polygon, repeating until I have enough.  Then I can save these points as a separate layer, and display this layer using almost any desktop or web based viewer!

Has anyone done this?  Can I do it in SQL or do I need to write something in PL/pgsql?

John Abraham

PS I just bought the Postgis In Action book; enjoying it so far.

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