ENV103 Chap.9 Buffers, Overlays and Population Density
Buffers, Overlays and Population Density
Lab 3 strings the vector tools together. A 500 m buffer around each Auckland volcano is intersected with census areas and aggregated to estimate the people affected; suburbs receive census population through a spatial join and become a population-density choropleth; and a digitised tsunami line feeds an inset map. The chapter explains each tool, what it returns and where its estimate can mislead.
Worked arithmetic shows how a buffer total is assembled from census areas and how a suburb's density is calculated from its area in square kilometres.
What this chapter covers
- 01
Buffer, intersect and aggregate
- 02
Clip, intersect and difference compared
- 03
Table joins versus spatial joins
- 04
Filtering suburbs with an expression
- 05
Area with $area and population density
- 06
Graduated symbols and labelled filters
- 07
Digitising and inset maps
Worked example · free
Estimate the population inside a 500 m volcano buffer
- +1Intersection keeps the six touched areas, each carrying its full population field.
- +1Aggregate sums them: 1,200 + 2,600 + 3,100 + 4,200 + 700 + 1,500 = 13,300 people.
- +1The buffer covers π × 0.5² ≈ 0.79 km², while the six areas cover 6 × 0.25 = 1.5 km².
- +1Because whole populations are counted for partly covered areas, 13,300 is an upper estimate of the people inside the buffer.
Key terms
- Intersection
- Intersection returns only the area shared by two layers, carrying attributes from both.
- Clip
- Clip keeps the parts of an input layer that fall inside an overlay layer, with the input's attributes only.
- Aggregate
- Aggregate summarises feature values by group, for example summing population for each volcano name.
- Spatial join
- A spatial join transfers attributes between layers according to where features overlap or touch.
- Population density
- Population density is the number of people divided by the area they occupy, here per square kilometre.
- Inset map
- An inset map is a smaller map within a map that shows a zoomed area or wider context.
- Digitising
- Digitising is creating new vector features by drawing them over a basemap or image.
- Field calculator
- The field calculator creates or updates attribute values from an expression, such as an area or a density.
Buffers, Overlays and Population Density FAQ
What is the difference between clip and intersect?
Both keep only the overlapping area, but clip returns the input layer's features and attributes alone, while intersect combines features and attributes from both layers. That is why the volcano analysis uses intersect: each piece needs the volcano's name and the census population together.
Why calculate density instead of mapping total population?
Totals make large areas look important simply because they are big. Dividing population by area in square kilometres turns counts into a rate that can be compared fairly across suburbs of different sizes, which is what a choropleth needs.
What does $area / 1e6 do in the field calculator?
$area returns each polygon's area in square metres, measured on the ellipsoid, and dividing by 1e6, one million, converts it to square kilometres. The lab notes that area(geometry) is a different, planimetric measure that ignores Earth's curvature.
How do I label only the volcanoes affecting many people?
Use a rule-based label filter, such as "Aggregated_VAR_1_1" >= 4000, so only features meeting the condition get labels. Build the label text from the name and the count, and add a white text buffer and callout lines so the labels stay readable over the basemap.
Assessment move
Work through Lab 3 as three separate problems, because each map uses a different chain of tools. For the volcano map, write the chain on a card, buffer, intersection, aggregate, join back, graduated symbols, and next to each step write what the output layer contains.
Then add the caution that intersection copies whole populations, so the total is an upper estimate; being able to say that shows you understand the method rather than just following it. For the density map, practise the field calculator: create an area field with $area / 1e6, then a density field dividing population by area, and check one suburb by hand.
Compare the quantile and equal-interval histograms you saved and write two sentences on which classification represents the data better. For the tsunami map, rehearse digitising a short line, saving edits and making an inset with an overview frame. Revisit the Week 5 online tools table with the labs in mind, and invent one question of your own for each tool.
Finally, keep the label-filter expression and the suburb filter expression in your notes; small syntax details, such as quotation marks around field names, are where most errors in QGIS expressions come from.