S3U

Geography / Map symbols / Undergraduate / gmd01

Raster Aggregation and the Meaning of a Cell

Choose aggregation rules that preserve the quantity a map represents.

All worksheets
5 questions0m 0s

Prerequisites

Area, totals, densities, weighted averages, and categorical map values.

Learn the skill

Before changing raster resolution, identify whether values are totals, intensities, or category codes. Totals add over disjoint cells; densities combine through total quantity divided by total area. Category codes do not become meaningful intermediate classes by arithmetic interpolation.

Worked example

Two disjoint cells of areas 1 and 3 square kilometers with densities 100 and 200 people per square kilometer contain 700 people in total. Their combined density is 700/4=175, not the unweighted average 150.

Model and Assumptions

Invented 2-by-2 raster: four disjoint one-square-kilometer cells contain 40, 20, 10, and 30 residents. A separate categorical layer codes land cover as 1=forest and 2=water. Aggregation combines the four population cells into one cell without changing its footprint.

1. What population count belongs in the combined cell?
2. What is the combined cell's area in square kilometers?
3. What is its population density per square kilometer?
4. What does a bilinearly interpolated land-cover code of 1.5 mean?
5. Which change alone does not create finer observations?

Further inquiry

Recalculate a pooled density for unequal cell areas and explain why area weighting matters. Specify separate aggregation and resampling policies for counts, densities, and categories, including an example where a tempting default would change the quantity being represented.

Review criteria

  • Keep count, area, and density units explicit.
  • Use an area-weighted density when areas differ.
  • Distinguish interpolation choices from additional observed detail.