How Cities Count Their Trees
Aircraft, satellites and volunteers with measuring tapes are producing the first precise maps of urban canopy. The maps show that shade, like most things in cities, is unevenly distributed.
The Daily Edition2 min read
Illustrative. This article is demonstration content written to show The Daily Edition. It does not report actual events. Historical and scientific facts are attributed to real sources in the Deep Dive.
Quick Read · about 22 seconds
- What happened
- Cities increasingly map their tree canopy using aerial laser scanning and satellite imagery, supplemented by street-level inventories. The maps typically find canopy cover varying several-fold between neighborhoods within the same city.
- Why it matters
- Trees lower surface temperatures by several degrees, and heat is the deadliest weather hazard in many cities. Canopy maps turn an aesthetic question into a public health one.
- What remains uncertain
- How much canopy expansion is possible given water, space and maintenance budgets, and whether planting programs survive the years needed for trees to mature.
A tree is easy to see and hard to count. Cities that want to know how much shade they have, and who has it, have turned to three methods, each with its strengths.
From above
Aircraft carrying lidar, which measures distance by laser pulse, produce three-dimensional maps in which trees stand out from buildings by their height and texture. Combined with aerial photography, the result is a canopy map accurate to a few meters. Satellites provide coarser maps more often, which is useful for tracking change.
From the street
Lidar sees canopy but not species, health or ownership. For that, cities send inspectors, or increasingly volunteers, to record each street tree: its species, trunk diameter, condition and location. A full inventory of a large city takes years and is out of date as soon as it is complete, which is why most cities combine periodic inventories with aerial updates.
What the maps show
Nearly every city that has mapped its canopy has found the same pattern: wealthier neighborhoods have more trees, often several times the cover of poorer ones, and the difference tracks historical patterns of investment and disinvestment. Because trees cool their surroundings measurably, the canopy gap is also a heat gap.
What comes next
Many cities have adopted canopy targets and planting programs aimed at the neighborhoods with least cover. The difficulty is time. A tree planted today provides meaningful shade in fifteen to twenty years, longer than most political cycles and many municipal budgets. The maps make the problem visible; they do not make it fast.
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