Measure coverage without double counting
See this course’s project requirements
Compare an existing and proposed service location in the supplied fictional four-zone town.
Keep your working files in your own notebook or software. Save a private description, checklist, and work link in My Proof.
What to include
- Define the decision and geographic layers with aggregate data.
- Plot a labeled coordinate grid and check units.
- Join attributes by zone ID and document missing-data checks.
- Calculate baseline and closure routes from the given network.
- Compare unique coverage and deliver a map, table and scoped memo.
Review and revise
Use these criteria to check your work and choose what to improve.
- Geographic meaning: layers, units and assumptions are explicit.
- Data quality: IDs and populations match without invented values.
- Calculation: route and unique coverage arithmetic is reproducible.
- Communication: the map supports a limited, accessible decision explanation.
Want more background?
The idea in everyday life
A second service site may add access, overlap existing coverage or both. Counting each site’s reachable population independently can exaggerate the combined result. You will apply a seven-minute threshold to the baseline network and count each zone once. The result is a transparent model comparison, not a claim about actual service quality, capacity or individual travel.
Coverage is a rule applied to locations; overlapping service areas do not create extra people.
One step at a time.
Use the sample or information you have permission to use. Keep your work as you go. You can pause between steps.
- 1
Use the baseline route table and define covered as time <= 7 minutes. For B, mark A at six, B at zero and C at four as covered; D at ten is uncovered.
- 2
Add the populations of covered zones: 30+20+40=90. Divide by the known total of 100 to obtain 90% representative-zone coverage under the model.
- 3
Calculate times to proposed D: D=0, C=6, A=8 and B=10 via C. D covers C and D, totaling fifty represented people when considered alone.
- 4
For B plus D, use each zone’s shorter travel time and count it once. The union covers A,B,C,D, totaling 100. Additional coverage is ten, not fifty, because C was already covered.
- 5
Record the seven-minute rule, how you counted each zone once, and the capacity limit. Try a six-minute cutoff to see whether the answer changes; this is a sensitivity check. Coverage stays the same for these particular travel times.
See how it works.
Fictional coverage calculation: B covers A,B,C, or 90 of 100 represented people. Proposed D alone covers C,D, or 50. Adding 90 and 50 would report 140 and double-count C’s forty people. The combined service set instead covers the union A,B,C,D once, giving 100% in the toy model. D adds only the previously uncovered zone D, ten represented people. Coverage does not show whether staff can serve demand or whether every resident can make the modeled trip. A later real planning study would need those questions answered before choosing a site.
Did you cover these?
- B covers 90 of 100.
- B plus D covers 100 without double counting C.
- Additional coverage is 10 represented people.
What to finish
A one-site/two-site coverage table with overlap and sensitivity calculations.
Watch for this
Adding site totals without removing zones counted by both sites.
How can I tell I understand it?
Explain why adding a fifty-person service area can add only ten new people.
Quick knowledge check
Answer all 3 questions. Get at least 2 right and the lesson completes automatically.
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Source & access notes · checked 2026-09-10
Read point, line, polygon, scale, geometry, layer and attribute explanations in official QGIS 3.44 documentation.
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