Table of Contents

Calculating Area Intersections

Next we need to determine how much of the population Dissemination Areas(DA) lie within the Traffic Zones (TAZ's). To do this we need to create intersection areas overlay and then later find aggregate populations in those areas.

Steps

  1. Go to Vector then Geoprocessing tools and click Intersection and calculate the intersection of the zones.
  2. Change invalid feature filtering to Skip (Ignore) Features with Invalid Geometries.
  3. Save intersection to a new shapefile called zonea_zoneb_intersection.
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Figure 1: Creating Intersections
  1. In the new intersection layer, calculate the area following the same process as before, and then create a new attribute to represent the fraction of one zone type that overlaps with the other. For example, if converting DAs to TAZs, you will have to create a field or column called frac_DA_in_TAZ which compares the size of a given intersection to the DA it is a part of. The Calculation for this would be: area_int/area_DA.
  2. The fraction of the DA covered by that intersection is what the DA population will be multiplied by to find the population of that intersection.
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Figure 2: Intersection Populations

Next Steps

The next step is to copy the intersection attribute table over to excel and complete our calculations there.