Calculating population density by neighbourhood

MathieuCain

Hello,

I have a population density raster (pop per square km on 10m grid cells) and a neighbourhoods polygon shapefile. I want to know the population by each neighbourhood (as a field in the neighbourhood table).

There were two ways I could think of doing this:
1. I use Zonal Statistics as Table and join the output table to the neighbourhoods shapefile

2. I convert the density raster to a points shapefile and then do a count by location or some such.

Either way though, I would assume that I would use the Sum field to determine the total population per neighbourhood. The sums appearing in the table though are phenomenally large and unrealistic (i.e., hundreds of millions of people per neighbourhood). Why is it that I need to divide this amount by 10,000 to get the more realistic value?

MathieuCain

A density, Mathieu, is a number of people per unit area.  Therefore you need to multiply the zonal sums by the area of a single cell to obtain estimated populations.

Because your densities are measured in people per square kilometer and each cell is 10 m = 0.01 km on a side, the cell area is (0.01)*(0.01) = 1/10,000 km^2.  That's why you have to divide the sums by 10,000.

I figured it was something along those lines (hence why I gave so much detail on the density raster's make-up). The math makes sense though, so thank you!

Cheers