Downtown Helsinki at night in January 2017: 50 lime trees along a city street have been chosen for felling due to maintenance requirements. A group of researchers from the University of Helsinki is moving between the as yet unfelled trees.
The group is installing a tripod on top of which sits a device the size of a milk carton or two. Soon, a pulsating laser beam barely visible to the human eye is scanning the trunks and branches of the trees. Some minutes later, the researchers move the tripod to another location, repeating the process. Over a period of two hours, the images of twelve lime trees are recorded in the device’s memory, as if drawn with infinitesimal dots, to the level of the thinnest of branches.
A few days later, some of these lime trees are strewn about a City of Helsinki depot cut into blocks and piles of branches. The researchers weigh up the trunks and branches separately to find out how much carbon the trees absorbed during their lifetime.
Soon, the only trace left of the trees will be datasets within the computers of University of Helsinki researchers.
Trees in the city have many duties
Urban trees are subjected to severe stress during their lifetime. Their root system and canopy are not always able to spread freely, while excavation work cuts off roots and dents trunks. Air quality is not always ideal either. The urban environment can also be warm or bright at strange times, which may confuse a tree’s perception of seasons and the time of day.
Still, urban trees provide shelter and diverse habitats for many species, just as their country cousins do. Urban trees also absorb carbon, which curbs greenhouse gas emissions and, thus, climate change, decrease temperatures by offering shade and evaporating water, decrease the risk of urban flooding, and provide city residents with aesthetic pleasure and comfort.
On the other hand, trees cause expenses for the city. Saplings are cultivated for years before planting, and the planting spots are carefully prepared to fit alongside municipal engineering services and street zones. Trees are supported and watered for the first few years, while their trunks and root system are protected from damage, they are pruned, and their wellbeing is monitored.
Trees comprise such an important part of urban wellbeing and that of city dwellers that researchers aim to create models, through measurements, on how various species of trees grow and function in varying surroundings. For example, how does a 70-year-old lime tree squeezed between tall buildings grow compared to a peer that grows in open grass or a grove? How do the biomass or solids content of the two specimens differ?
Modelling helps the city in, among other things, choosing the tree species best suited to various surroundings and easiest to maintain. Through modelling, it is also easy to imagine what certain areas and their tree stand will look like in, say, 40 years.
New technologies, such as laser scanning based on remote sensing, are needed to find out such details. The City of Helsinki and the University of Helsinki have conducted cooperation in this field since the beginning of the 2010s.
Laser turns every tree into dots
In laser scanning, the scanner sends a laser pulse at the targeted terrain, after which it measures the time it takes for the pulse reflected from the target to travel back. Based on the time and the location of the scanner, the reflection point for each individual pulse can be determined.
Millions of repetitions create a point cloud, in which altitude differences, as well as the height of trees and buildings can be perceived. The pulse repetition rate, as well as the altitude and speed of the device determine the detail to which the terrain is recorded.
Within the limits of the City of Helsinki, aerial laser scanning has been carried out since 1999. At first, the data produced by scanning was used in analysing the built environment, but since the beginning of the 2010s, the technology has also been utilised in mapping out urban trees.
Register started with 20,000 trees
The original goal of the City’s and University’s joint project was simple: to count trees growing in street zones in a manner that didn’t require footwork.
“We already had a register of street trees available, but we were aware that it most definitely did not include all the trees. What’s more, the register consisted of only a handful of details on many trees, such as their location and possibly an outdated diameter measurement, but not much on their height or canopy width,” says Juha Raisio, a tree expert at the City of Helsinki.
Topi Tanhuanpää from the Faculty of Agriculture and Forestry at the University of Helsinki joined the project as a doctoral student. Based on the aerial laser scans, Tanhuanpää developed an application that made it possible to, for example, count the exact number of trees along a certain street and to perceive the contours of their canopies from above.
To enable this, the location of the trees, as well as field surveys of their height and trunk diameter were required. Eventually, details of the approximately 20,000 trees included in the register were brought up to date.
Accurate information helps in maintenance and reduces costs
Thanks to the increasingly accurate information compiled during the project, Helsinki’s urban tree register is easier to manage than before. Indeed, the need for up-to-date data on urban trees is constantly growing.
As the characteristics of individual trees are better understood, implementing planning, construction and maintenance work as a comprehensive whole in the urban environment also becomes easier. Trees are a growing and evolving element. Through modelling, the development of landscapes and maintenance needs can be forecast, at best, for decades. At the same time, accurate estimates on labour costs can be made in advance.
In the future, remote sensing can also serve the purpose of health check-ups for trees. For example, if the growth conditions of an individual tree change, quick intervention is possible.
“I can imagine that during a long drought, the youngest tree avenues will be watered frequently enough if the results gained from remote sensing indicate a lack of water. However, identifying such problems requires new imaging methods that complement laser scanning. These methods are already being developed at the University,” says Raisio.
Virtual 3D park as the goal
In addition to urban trees growing along the streets, trees growing in other green areas are also being studied in Helsinki. The current objective is to create an application based on laser scanning that can be used to map park trees and compile details on their characteristics.
The data on urban trees usually include information on their location, or the location of the base of the tree. In the case of parks, the situation is more complicated.
“Some parks in Helsinki are very forest-like, with details missing on the exact location of trees. Particularly in the case of aged deciduous trees, canopies are complex, in addition to which some trees may grow in the shadow of another tree. In addition to that, there are all sorts of posts, cables and statues that should be weeded out of the point cloud created by laser scanning. This poses more challenges for developing the application,” notes Topi Tanhuanpää, now a postdoctoral researcher who defended his doctoral dissertation on the 3D modelling of Helsinki’s urban trees with the help of laser scanning in 2016. He is continuing his work in the project under the leadership of Professor Markus Holopainen.
The ultimate objective is the creation of a virtual 3D model of all the parks in Helsinki. The model would help planners and builders plan future construction projects and predict maintenance needs through the simple act of looking at tree models on a display screen. When certain information is available from the air or predictable with a model, the need for experts to go to the field in person will decrease.
A 3D model is also an excellent tool for estimating carbon balances, or how changes in land use affect the amount of carbon absorbed by urban vegetation and soil. This will help create entirely new knowledge on the city’s role in preventing climate change.
Further information on laser scanning and a healthy Helsinki
The Centre of Excellence in Laser Scanning Research investigates and develops laser scanning instrumentation, positioning technologies, data management methods and applications. The centre is comprised of distinguished researchers from four organisations: the Finnish Geospatial Research Institute of the National Land Survey of Finland, the University of Helsinki, the University of Oulu and Aalto University.
The City of Helsinki has two next-generation 3D city models, available as open data. With the help of the city information model, energy consumption, greenhouse gas emissions and the environmental impact of traffic can be calculated, whereas the reality mesh model can be used, for example, as a foundation when planning the location of exit routes, stages and booths during public events. For now, the models do not include three-dimensionally rendered urban trees.