How Subterra monitors a 15-km railway five minutes at a time
Subterra’s railway modernization project near Prague Airport needs up-to-date survey data for monthly earthworks calculations and visual progress checks. But collecting it means working around airport traffic, sometimes in flight windows of just five minutes. AirView carries out the monthly monitoring, using AVAG to bring thousands of images from separate drone missions into a shared view of the construction site.
Challenges
Mapping an active railway construction corridor close to a busy airport, with access dependent on available flight windows
Collecting data close enough to month-end to support current earthworks quantities and billing
Combining separate missions and maintaining consistent survey control across repeated visits
Providing visual records for site review alongside data for volume measurements
Solutions
Splitting the most constrained section into short, overlapping missions coordinated with airport teams
Uploading the images together into AVAG for processing as a combined dataset
Using RTK drone surveying, ground control points and repeated reference-point checks
Keeping ground control locations accessible in AVAG on a mobile phone for return visits
Providing on-site training for the customer’s team alongside regular monitoring
Results
Large survey datasets of more than 13,000 images brought into AVAG
Data supporting earthworks volume measurements and visual comparison with project documentation
A reusable record of control-point locations that reduces repeated planning in the field
When the construction schedule meets the flight schedule
The Praha-Ruzyne–Kladno modernization will double-track and electrify approximately 15.1 kilometers of railway. For the team monitoring construction near Prague Airport, the hardest part of a drone survey can happen before the drone leaves the ground.
The construction team needs recent measurements near the end of each month. The airport operates to a different timetable. For the mapping team, that means balancing the freshness of the survey against the availability of safe, coordinated flight windows.
A change in runway use made that balance more difficult. According to Ondrej Sejk, the pilot interviewed for this story, the particularly constrained section grew from around 700 meters to approximately 5.5 kilometers. Mapping the railway now requires a series of short missions between aircraft movements.
Waiting can take far longer than flying. Ondrej describes one occasion when four five-minute missions required roughly four hours on site. Choosing a quieter day helps but does not remove the need to coordinate each operation.
The challenge is to turn those limited opportunities into data that the construction team can actually use.
Breaking the survey into workable sections
The team divides the constrained section into missions of approximately five minutes, with overlap between neighboring areas. That overlap allows the images to be processed together after collection.
The approach connects fieldwork carried out in separate windows. Where collection spans different days, the imagery can show differences in lighting. Ondrej explains that those visual differences do not, by themselves, prevent the team from using the resulting data for earthworks volume measurements.
Technical workflow
Drone:
DJI Mavic 3 Enterprise with RTK
Survey cycle:
Monthly, with collection planned close to month-end
Image volume:
More than 13,000 images in each dataset
Example dataset:
13,582 images and approximately 285 GB
Ground control points:
Approximately 190–200 GCPs
Reference checks:
At least five existing reference points remeasured for each survey cycle
One workflow, from field preparation to site review
The process begins on the ground. The team checks the control points needed for the survey and restores markings where necessary. In areas with active construction, points may disappear and need replacing. Points on stable features can often be reused.
Before flying, the pilot coordinates with the relevant airport teams. Less constrained sections can be collected more directly; the section affected by aircraft movements is captured through the available short windows.
The photographs are then uploaded to AVAG. The processing specialist aligns the survey using the ground control points and prepares the data. The construction team then works with it in the platform.
This connects a fragmented collection process with the practical tasks that follow: checking conditions, measuring earthworks and reviewing the site against project documentation.
Finding the same control points, month after month
Repeated surveys do not require the team to redesign the GCP layout every time.
The locations from the initial survey are uploaded into AVAG. On a return visit, the pilot opens the map in AVAG on a phone to find the points that need checking or remarking.
“When I open AVAG on my phone in the field, I know where to go, where to renew the marking and where to check the point.”
Ondrej Sejk
Drone Pilot, AirView
The benefit is practical: less time spent working out where points were placed, and a clearer starting point for the next survey. Where a point is stable and still usable, the team can retain it. Where construction has changed the location, it can be checked, replaced or measured again.
Alongside this, the team remeasures existing site reference points to check the ground-based GNSS receiver. That repeated check supports consistency from one visit to the next.
Current quantities for the monthly billing cycle
The timing of the survey matters because the construction team uses the measurements for monthly earthworks quantities and billing.
Collecting too early can leave changes near month-end outside the survey. Leaving everything until the final day creates a different risk: suitable flight windows may not appear in time.
The team therefore plans around both constraints, preparing control points and allowing additional time for the most difficult section. The aim is to collect sufficiently recent data while leaving room for the unpredictable waiting that airport operations can introduce.
Repeated capture and processing in AVAG give the construction team a basis for working with current site measurements during the monthly cycle.
More than a volume calculation
The project also needs a visual record of the railway as work progresses.
Ondrej explains that the site wants to compare current conditions with project documentation, inspect what has changed and retain a record of construction progress. That is why visual imagery remains an important part of the workflow alongside the geometry used for volume measurements.
A terrain measurement alone would not answer every question. Teams also need to see the site and understand the context around the measured areas.
AVAG brings those survey records into the same project environment, giving the construction team data to work with after the field visit is over.
Building a record across the project
As monthly surveys accumulate, they create a record of changing site conditions. The railway construction program is scheduled for 2026–2029. AirView’s monitoring is intended to continue through construction, with the precise service end date still to be confirmed.
The value lies in connecting each field visit to the next: known control points, repeatable checks, combined survey datasets and visual records that the construction team can return to as work progresses.
AVAG highlights
13,000+ images in each dataset
Approximately five-minute missions used in the most constrained section
Mobile access to GCP locations for repeat field visits
Measurement and visual review supported within the project workflow
On-site training for customer staff alongside monthly monitoring
“What’s especially encouraging on this project is seeing the chief surveyor take to using the data. Surveyors have demanding technical expectations, so seeing them find practical value in what we deliver means a lot.”
Miroslav Staubr
CEO, AVAG Pro
Project details
Praha-Ruzyne – Kladno
Customer
Subterra
Railway project length
Approximately 15.1 km
Construction consortium
Subterra, OHLA ŽS, Elektrizace železnic Praha and Swietelsky
Monitoring provider
AirView
Mapping start
April 27, 2026
Survey frequency
Monthly
Railway construction program
2026–2029
Main uses
Earthworks volumes, monthly billing inputs, visual site review and progress documentation
