A citywide heat map built from 1.26 million readings by 312 Dublin food-delivery cyclists understated afternoon temperatures by a median 4.6°C (8.3°F) during hot spells, researchers found. A software update caused the sensors to record the cool interior of delivery bags rather than street air, compromising data gathered over 14 months.
The finding, published on Friday by the Liffey Street Climate Survey, puts a question mark over a map used to rank streets for tree planting and shade projects. In repeat tests, the faulty setting produced errors as large as 7.2°C (13°F) when outside temperatures topped 25°C (77°F).
The wrong temperature channel
The survey paired a small logger on each insulated bag with two probes: one for outside air and one to track conditions inside the bag. A firmware update on 18 June 2025 changed which probe the logger treated as its primary reading. The error went unnoticed because the devices still recorded plausible-looking temperatures.
Researchers at the Dublin Institute for Urban Climate say they identified the problem after testing 36 loggers against calibrated reference instruments at 12 temperatures. The team also compared readings with nearby weather stations. That comparison showed an unexplained cool bias on warm afternoons, but could not by itself pinpoint the faulty channel.
“We have been drawing shade maps from readings that were never street-air readings,” said Norbert Grosu, an atmospheric scientist who led the technical review. He said the fault did not invalidate every entry: readings taken before the update and a portion of later records were unaffected, but the project could not reliably separate them across the full map.
We have been drawing shade maps from readings that were never street-air readings.
Norbert Grosu, atmospheric scientist
Decisions made on bad data
The map helped the city’s climate office shortlist 18 streets for shade improvements, including routes in Ballyfermot and Rathmines. The office has paused use of the rankings while it checks the underlying data. Its planned €1.8 million (about $2 million) first phase is still funded, but officials have not said whether the affected locations will change.
Kirti Prashad-Saxena, an environmental-systems engineer who independently examined the test results, called the failure “serious because it was quiet”. The sensors continued to transmit, she said, and their readings changed smoothly enough to pass routine checks. She recommended that future mobile surveys record both probe channels and flag unusual differences against fixed weather stations.
The project’s designers say couriers were not responsible for the error. Researchers fitted the loggers to bags and asked riders to follow their usual routes; the scheme was intended to capture street-by-street variation that a handful of fixed stations would miss. The team is now retesting stored data and plans to publish a corrected map by February.
Dublin’s dense streets can hold heat after sunset, while open parks and breezier routes may cool more quickly. Diana Lindmaa-Pärn, a climate-planning adviser to the project, said the flawed map should not be used to claim that particular neighbourhoods are safe or at risk. “We need to restore confidence with measurements people can check,” she said.
