Loading the photons…
LiDAR from space · NASA's ICESat-2 satellite
Bucharest measured by a laser from space
NASA's ICESat-2 satellite sends 10,000 laser pulses a second towards the Earth and times every photon that comes back. In the summers of 2019 to 2026 it crossed Bucharest, Romania's capital, 32 times; the page uses the strong beams of each pass. The returning light shows the city in cross-section: the roof of the Palace of Parliament, the water of Herăstrău lake, the tree crowns under the beam's path, and how close to any address the beam passed.
Find a streetor scroll: 2.82 seconds over the city9 July 2024, 12:38 local time
Three seconds over Bucharest#
The ICESat-2 satellite came in from the south, 496 km above the city. One of its six beams drew a line 20.1 km long on the ground, from Jilava to Fort 3 on the Otopeni boundary, in 2.82 seconds.
Every dot in the picture is a photon that made it back to the satellite. Its onboard clock times the light's flight, and the time gives the height: 49,546 photons on this line alone. On screen, heights are stretched against distances, so houses and trees show across 20 km of city.
Kilometre 7
The beam runs through the middle of the Palace of Parliament#
The line crosses the Palace for 216 m. The photons trace two flat roofs: one at 121 m above sea level and, higher, one at 147 m, about 70 m above the streets around it, which lie 74 m (towards Rahova) and 80 m (towards Izvor) above sea level. Along the whole Palace, the laser rarely reaches the ground. The building's highest point, 84 m above the ground, is not on this line; the city's height records are on the page about Bucharest's tallest buildings.
Just north, the tree crowns of Izvor Park reach about 13 m. Beside them, the line crosses the Dâmbovița river.
Kilometres 10–11
The towers near Piața Victoriei, then Kiseleff Park#
The Sanador hospital, Premium Point and Țiriac Tower stand side by side on the line. Where the beam crosses them, all three return photons from about 40 m above the ground.
Then Kiseleff Park: a forest of fine green dots, crowns up to 23 m. Where the leaves are dense, only a few photons reach the grass and come back.
Kilometres 12–14
Herăstrău lake is a straight line#
For 505 m, the photons came back off the lake at one height: 78.8 m above sea level. Still water is the flattest surface the satellite sees.
On the banks, the park: crowns of 15–20 m, then the blocks of Herăstrău and the tower of the old Băneasa airport.
Kilometres 16–18
Băneasa Forest, from the top of the canopy to the ground#
North of the city, the line enters Băneasa Forest. On this line, crowns rise to 29 m, and photons come back from the whole canopy, from the top down to the ground.
All 32 summer passes together show how tall the trees on Bucharest's streets are, and where the laser passed near any address in the city.
Where the laser passed near any address#
Type a street and number. The page finds the nearest photon line of the 32 summer passes and draws 640 m of it, at the measured height. Half of Bucharest's addresses have a line within 171 m; 76% have one within 300 m. A tap anywhere on the map works too.
Try:
The result appears here: the nearest photon line, how far it is and what it hit.
Each line is one strong beam in one summer pass; the brightest were flown at night. Tap anywhere on the map.
How tall are the trees on Bucharest's streets? About as tall as in Budapest, Sofia and Belgrade#
Harsh pruning is an old complaint in Bucharest. According to press figures cited by the AGERPRES news agency, between 2016 and 2020 City Hall approved the felling of 30,000 trees, and 100,000 were pruned. The laser allows a comparison no inventory makes: the same measurements, with one rule, on the streets of Bucharest, Budapest, Sofia and Belgrade.
Crowns taller than 5 m within 15 m of a street, outside parks and forests, at least 15 m from any building; every summer pass of the same satellite from 2019 to 2026, analysed with one rule in every city.
On Bucharest's streets, the typical crown stands 9.9 m tall. In Belgrade, 10.3 m; in Sofia, 9.4 m; in Budapest, 9.0 m: all between 9 and 10.5 m. The laser finds no sign that Bucharest's streets have shorter trees than its neighbours'. In Belgrade, 21% of crowns pass 15 m; in Bucharest, 18%; in Sofia, 13%; in Budapest, 12%.
Băneasa–Tunari forest, the Buda hills (János-hegy), the foot of Vitosha, Košutnjak and Avala.
The forests around the cities differ more than the streets do: the typical crown stands 20.5 m tall in the Buda hills, 28.0 m in the Băneasa–Tunari forest and 30.7 m at the foot of Vitosha. Species, tree age, soil and street width all matter, so the differences between cities cannot be put down to pruning. One more limit: a tree cut back below 5 m does not enter the count at all. The laser shows how tall the remaining trees are, not why.
Exact heights need care. Of 40 random "street crowns" per city checked on aerial photographs, the share that really was a tree ranged from about 45% in Budapest to 70% in Bucharest; the rest were mostly the edge of the street, grass, shadows or small roofs. Keeping only the confirmed crowns, the median comes out between 10 and 12.5 m in all four cities, and Bucharest is still not shorter. The samples are small, so the values are approximate.
How much of Bucharest is under tree crowns? Published answers range from 3% to 22%#
The figures that circulate about Bucharest's greenery do not agree, because each counts something different:
| Source | Share of Bucharest covered by crowns | What it counts |
|---|---|---|
| IUFRO 2024, Mărmureanu et al., 2024 | 3% | conference abstract; method and image year not described |
| GreenSpace / ACRI-ST, 2019 | 6.5% | mean tree cover from a Space Climate Observatory (CNES) project; area boundary not stated |
| Green Guide, Ciornei et al., 2022 | 13.36% | only woodland patches over 0.5 ha (Copernicus) |
| University of Bucharest, Rîșnoveanu and Dușcu, 2026 | 17.84% | points classified by people on 2021 imagery (i-Tree Canopy), tree canopy |
| EEA, European Environment Agency, 2018 | 22% | crowns seen from above, the Copernicus city map |
The laser gives an independent measure, made with one rule across the whole city. Of the 570 km of summer line inside Bucharest, 21.6% runs under crowns taller than 5 m (95% confidence interval 18.9–24.0%), and 34.9% over buildings. The result is close to the European Environment Agency's (22%); the University of Bucharest's point count gives a little less, 17.8%.
A third measure comes from photographs. At 360 random points on the same lines, two raters who did not know what the laser had said saw crowns more often than the laser counts them, because the eye sets no height limit: a shrub or a young tree counts too. Counted that way, cover reaches 33–42% of the line. The other way round, some of the segments the laser calls a crown show in the photographs as small roofs, shadows or bare ground.
Maps made by artificial intelligence from satellite images give the lowest values. Meta's 1 m canopy-height map, read at the same points, finds crowns on only 6.6% of the length of the lines in the city, although in the Băneasa and Tunari forests it finds them on 96% of the line. On streets and in courtyards, where trees stand apart, it misses many of them.
How much of Bucharest is "under trees" depends on what is counted, and with what. The laser, photographs checked by people, the European Environment Agency's map and the university's point count give between nearly a fifth and two fifths of the city. The values of 3% and 6.5% are far below all of them.
How a laser in space measures the height of a tree#
ICESat-2 has flown at 496 km since 2018. Its instrument, ATLAS, sends 10,000 pulses a second of green light (532 nm), split into six beams: three strong and three weak. Each pulse lights a circle of about 11 m on the ground; only a few photons of each pulse come back, and the satellite records the arrival time of each one. The flight time, the satellite's position and the beam's angle give the position and height of the point the light came back from.
On open ground, photons gather at street level; on a tree, from the whole crown; on a roof, from one height. NASA sorts the photons into ground, canopy and top of canopy (the ATL08 product). This page adds building outlines from OpenStreetMap, so that roofs are not counted as trees, and cuts the line into 10 m segments: a segment is "under a crown" if its photons rise at least 5 m above the ground and spread vertically, as they do in foliage.
Heights are given above sea level with the EGM2008 geoid model (over Bucharest the geoid is about 36 m above the satellite's reference ellipsoid). The page uses only passes from May to September, when trees are in leaf, and only the strong beams.
Sources and limits#
- NASA ICESat-2, ATL03 (geolocated photons) and ATL08 (land and vegetation height), version 7 nsidc.org
- SlideRule Earth: public access to ICESat-2 data, no account slideruleearth.io
- OpenStreetMap: building outlines, streets, parks, water, addresses (ODbL) openstreetmap.org
- The EGM2008 geoid model (NGA), via PROJ cdn.proj.org
- Meta and WRI: global canopy height map, 1 m (CC BY 4.0) datasets.wri.org
- European Environment Agency: urban tree cover, 2018 eea.europa.eu
- Rîșnoveanu and Dușcu, Urban Ecosystems, 2026: tree canopy cover of Bucharest doi.org
- Ciornei, Gavrilidis, Calotă, Munteanu: the Green Guide, 2022 silvanews.ro
- GreenSpace (ACRI-ST, Space Climate Observatory): first results spaceclimateobservatory.org
- IUFRO 2024 congress abstracts, p. 3149 iufro2024.com
- AGERPRES, 2 June 2022: trees felled and pruned in Bucharest, 2016–2020, from press figures agerpres.ro
A similar laser, on the International Space Station, also measures the height of Romania's forests.
Photon positions carry an error of a few metres, and the lit circle on the ground is about 11 m wide, so a crown beside a roof can borrow photons from it. Buildings come from OpenStreetMap; a building missing there can be counted as a tree. The check on aerial photographs used Esri World Imagery, which is not shown on the page.