When you are using custom trade areas in ENVISION (e.g., drive times, rings, drawn, imported spatial files, etc) you may wonder how the underlying data is being measured, and how ENVISION decides whether to include it in your trade area or not.
The answer differs slightly depending on whether you are using a US workspace or a Canadian workspace.
In Canada, data is apportioned at the Dissemination Area (DA) level. The proportion of the DA that falls within your custom trade area is used to determine how much of the data from the DA to include. For example, if you have a DA that is roughly 25% inside your trade area, then roughly 25% of the data for that DA will be included in your trade area.
In the United States, the data is apportioned at the Block level. All block centroids – within a block group located inside a trade area polygon – have their associated population, household or block centroid counts aggregated and expressed as a percentage of the block group’s total. This percentage is then used to apportion and aggregate data for any non-standard polygon.
(Note that a centroid is roughly the middle of an area or polygon.)