Populate all pear-ish
Agricultural density may sound physiological but this scale keeps it real by measuring the rural population per square kilometre of arable land. Clearly, there's some benefit to knowing whether there are folk still willing to till the soil. Finding a neat listing for this category is not so readily available, hint hint, budding researchers.
Residential density is the number of people living in an urban area or area of residential land.
Urban density is the number of people inhabiting an urban area/total area of urban land. It's possible to subdivide categories of measurement here but to the level that a town planner might require. As a layman of the layout, I'm trying to distinguish this from residential density, then I realise that residential density is but one measurement of urban density; which can be measured as precisely as floor area ratio or as encompassing as population density
Last but not least, ecological optimum, the density of population which can be supported by the natural resources.