Geospatial precision in the Stockholm Metro Expansion

One of Europe’s most ambitious infrastructure projects is currently taking shape beneath the streets and waterways of Sweden’s capital city. The Stockholm Metro Expansion will add more than 30km of tunnels and new subway stations to a system that has not seen a major extension in decades. At the heart of this vast undertaking lies geospatial expertise, of course. With millimetre precision, surveyors provide the monitoring, control, verification and guidance that ensure the subsurface operations proceed safely, accurately and in harmony with the city and its residents above.

Driven by rapid population growth and an aging network, Stockholm is expanding its metro system. This programme is a long-term investment in mobility that goes beyond tunnelling. As part of a redesign to strengthen interchanges, spread demand and support transit-oriented development, the Stockholm Region government authority is redrawing the map with new lines and extensions that add capacity, shorten cross-city journeys and unlock underserved areas. The project will also deliver new stations and catalyse new neighbourhoods around them. Openings will be staged, with some sections opening from 2027 onwards and others following way into the 2030s.

Surveying is central to the success of such a complex scheme, particularly given Stockholm’s geography. Built on a mosaic of islands, with historic structures and sensitive infrastructure in the central districts, the city poses formidable challenges for engineers and surveyors. Excavating an escalator shaft alongside a 40m residential building illustrates the delicate balance faced by the project team. “Imagine how carefully you have to operate when you’re building a tunnel right next to existing infrastructure or even apartment blocks where people are still living. That’s a responsibility you don’t take lightly,” says Elias Olofsson from Clinton Mätkonsult, one of Sweden’s oldest surveying consultancies spanning construction surveying, monitoring and scan-to-BIM modelling. The surveyors’ expertise ensures progress can continue safely, accurately and efficiently in the demanding underground environment.

Building the survey framework

Clinton Mätkonsult firstly became involved in 2019, establishing the tunnel grid and reference network that would guide the years of construction ahead. GNSS receivers and total stations created a geodetic backbone, from which all subsequent excavation and building could be controlled. “The first thing we worked on was creating the tunnel grid,” Olofsson recalls. “The client required a consistent network across the system, and from there we moved into contractor support.”

For this, the surveyors needed a solid understanding of construction methods, such as drill-and-blast or tunnel boring machines, since every method brought its own requirements for measurement and control. Such knowledge is crucial in complex projects such as the Stockholm Metro Expansion where, in the early stages, the work focused on guiding rock blasting and the excavation of access tunnels. As the project gradually advanced, the surveyors turned their attention to concrete works, earthworks and eventually to finishing tasks. At every step, they ensured that designs translated precisely into reality by working side by side with engineers to provide real-time feedback on alignment, positioning and potential deviations. The ultimate test of accuracy came when separate tunnelling drives had to meet. As Olofsson notes, “One of the most important tasks of the surveyor is making sure that the tunnel is actually built where it’s supposed to be built.”

Underground survey methods

As a sign of how the geospatial industry as a whole – including underground surveying – has been transformed by the digital revolution and technology in recent years, surveyors also increasingly apply laser scanning or inertial systems to ensure precise positioning in GNSS-denied environments. “The use of GNSS is limited in tunnelling because signals are blocked or severely weakened underground. Working in tunnelling was very different 30 years ago,” Olofsson says. “Scanners existed, but they were expensive, complicated and time consuming. Today, scanning is easy.”

In the Stockholm project, laser scanning now accounts for the majority of data collection, supported by total stations for control and verification. Above the surface in the Swedish capital, drones have occasionally been deployed and proved to be a helpful tool, but underground the scanner dominates. Rather than dictating which equipment should be used, the client specifies accuracy and quality thresholds. “It’s up to us whether we use Leica, Trimble or another manufacturer’s equipment,” Olofsson comments. “What matters is delivering scans and measurements that meet the specifications.”

Guaranteeing data quality

Precision is paramount in tunnelling. Continuous instrument checks and frequent updates of the control grid are essential in order to prevent cumulative errors, since even very minor deviations can escalate over long tunnel sections. Such errors can harm the strong safety standards and lead to rework that can be very costly, so surveyors adopt a mindset of redundancy. “For total stations and scanners, we do monthly controls,” Olofsson explains. “But beyond equipment checks, quality is about the surveyor’s mindset. A good surveyor doesn’t measure once – they measure twice, sometimes three times.” Ensuring quality is a collaborative effort. The client actively monitored and validated the tunnel grid, while Clinton is also enforcing its own internal safeguards. In a nutshell, it comes down to double-checking results and cross-verifying data. Following these rigorous workflows has reduced the risk of mistakes.

Another crucial aspect of the surveyor’s role is monitoring. In a dense urban environment like Stockholm, protecting nearby structures is just as important as guiding new construction. “In the past, monitoring could be defined as setting up a total station at the site and measuring a few points,” Olofsson recalls. “Thanks to today’s technology, we can now monitor continuously.”

Hundreds of sensors

Besides the part played by automatic total stations, hundreds of sensors were deployed from various contractors, including Senceive and Geosense. These ranged from tilt sensors on retaining walls to inclinometers installed in soil to measure ground movement. This proved to be a key component in the successful survey works of this massive project. “With sensors, you don’t need line of sight, which is a big advantage in Swedish winters with snow,” he notes. “They measure 24/7, and in our project we set them to log every hour. That way, if the ground moved or a retaining wall was stressed, we would see it immediately.” This constant stream of data allowed contractors to act proactively, adjusting construction methods before risks escalated. Hourly updates flagged subtle trends at a very early stage, supporting timely construction and protecting adjacent buildings and utilities. This ensured that the expansion of the metro system could advance without threatening homes, roads or tramlines above ground.

Applying the geospatial data

The gathered geospatial data was put to work in multiple ways. Scanning validated that excavated tunnels matched the designs and that clearances were respected. Safety depended on ensuring that no protruding rocks or misalignments obstructed future trains or installations. The data also played a key role in quantity checks, as contractors in projects like these are often paid based on the volume of rock excavated or the thickness of the concrete applied.

“Scanning allows us to compare the rock face to the theoretical tunnel, and also to compare the sprayed concrete thicknesses,” Olofsson comments. “That way, we know exactly how much material was used and can control costs.” Beyond contractual purposes, such checks also help to identifying any operators applying more material than necessary, allowing practices to be corrected.

Unexpected challenges

Not all the tasks in the survey works for the Stockholm Metro Expansion project could be solved with standard routines. The construction of steep escalator shafts was a prime example. “These shafts were 30 to 78m long with a slope of around 37 degrees,” Olofsson explains. “We couldn’t blast them like normal tunnels. Instead, we drilled a pilot hole, then used a raise-boring machine with a 5m diameter to ream upwards, before blasting the remaining shape.”

Guiding the pilot hole required extraordinary precision, since even small deviations could lead to major problems. “That was a moment where we as surveyors really had to think outside the box,” he recalls. “Everyone wanted to be involved because it was such an unusual challenge. And let’s be honest, it’s always fun when you’re faced with a problem you haven’t seen before. That’s when surveyors can really show their creativity.”

Teamwork in a 24/7 operation

Needless to say, surveying a metro expansion project is a team effort. Clinton’s crew began with three people and grew to as many as 12 during peak activity. With tunnelling operations running day and night, surveyors also worked shifts to provide constant coverage.

Besides the Swedish project team members from Stockholm Region and Clinton, other participants came from a mix of countries including Slovakia and Italy, with the tunnel crews supplied by Itinera SpA. This multilingual collaboration added another layer of complexity. “Communication was one of the hardest parts,” Olofsson admits. “You need clear systems and data control to avoid misunderstandings.” Handling the vast amount of scanned geospatial data was itself a full-time challenge. With new scans conducted every few metres, the project quickly accumulated enormous datasets, making robust data management essential.

Lessons for the profession

When reflecting on the project, Olofsson stresses that surveyors should prioritize reliable workflows and quality assurance. Redundancy and double-checks prevent costly mistakes, while monitoring technologies make construction both safer and more efficient. He also highlights the importance of communication and organization when working in large, cross-cultural teams. Above all, he emphasizes that surveyors must never lose sight of data management. “When you’re scanning every three or four metres along kilometres-long tunnels, the amount of data is huge. Having a good system to manage it is key to success,” he states.

Central role of surveying in urban infrastructure

The Stockholm Metro Expansion demonstrates the central role of surveying and geospatial expertise in modern urban infrastructure. Using advanced methods, surveyors have laid down the grids that are anchoring construction, guided excavation with scanning and total stations, and safeguarded the city above with nonstop monitoring systems. They have also adapted creatively to unique challenges, such as constructing steep escalator shafts and handling the enormous amounts of data generated underground. Projects of this scale are highly demanding, but they also represent the pinnacle of the surveying profession. For Olofsson and his colleagues, the Stockholm Metro Expansion has been more than just another assignment. The chance to apply their skills in such a significant endeavour was not just a responsibility, it was the icing on the cake in their professional journey.