Allocate polygon population values to a grid of points, and save as delimited text

readme.md

Allocate polygon population values to a grid of points, and save as delimited text

We have population data (total population, male, female, etc.) by polygon (Census tracts), but want a CSV file of the population densities for a regular grid of points at 1km spacing.

Below are the steps to do this using QGIS. In this example, we have already added a polygon layer called "Tracts" to the map, and will calculate the male population densities. (Repeat this process for other subpopulations.)

Step 1: Transform the tract polygons to UTM

In order to calculate densities per square kilometer, we will to transform the tracts to a UTM (meters-based) coordinate system. For New York State tracts, we will use UTM zone 18N.

• In the layers panel, right-click "Tracts" > Save As...
  • File name = 'tracts_utm.shp'
  • CRS = EPSG:26918 (NAD83 / UTM zone 18N)
• Click OK
• When the process completes, right-click the new 'tracts_utm' layer > Set Project CRS from Layer

Step 2: Calculate population density

• Right-click "tracts_utm" > Open Attribute Table
• Click the "Field Calculator" icon (2nd from right in the toolbar)
• Create a new field:
  • Output field name = male_dens
  • Output field type = Decimal number (real)
  • Output Precision = 3
  • Expression = "MALES" / $area * 1000 * 1000 (this takes the total count of males in each tract, and converts it to a density per sq km)

Step 3: Convert to raster with 1km pixels

• Raster menu > Conversion > Rasterize (Vector to Raster)
• Input file = tracts_utm
• Attribute field = male_dens
• Output = male_density.tif (be sure to click the "Select..." button to browse to specify the output location!)
• You'll see an alert that the output file doesn't exist. Click OK.
• Select "Raster resolution in map units per pixel"
  • Horizontal = 1000
  • Vertical = 1000
• To properly encode nodata pixels as -1, we need to edit the generated command:
  • Click the pencil to edit the command
  • Add " -a_nodata -1" immediate after the gdal_rasterize command, so that it starts like this:
    gdal_rasterize -a_nodata -1 -a male_dens -tr 1000.0 1000.0 ...

Step 4: Save as delimited text

• Raster menu > Conversion > Translate (Convert format)
• Input Layer = male_density
• Output file = male_denstiy.xyz (be sure to click the "Select..." button to browse to specify the output location!)

The lines of the .xyz file should look something like this:

626606.42132833065 4851489.903991037 10.925999641418457
627606.42132833065 4851489.903991037 6.8530001640319824
628606.42132833065 4851489.903991037 6.8530001640319824
629606.42132833065 4851489.903991037 6.8530001640319824
630606.42132833065 4851489.903991037 -1

The .xyz file is space-delimited, with 3 columns: x, y, and the raster value, which in this example is the male population density. Note that any lines with a -1 value can be discarded -- these are the nodata pixels that fell outside the original tract polygons.