CASE STUDY
Understanding Five Years of Railway Performance Using Satellite Remote Sensing
Ground Engineering Awards 2026 Finalist in the Technical Excellence Category
PROJECT IMPACT
Gained long-term understanding of railway corridor performance and ground movement trends across the Cape Town rail network via historical InSAR analysis (2018-2023)
Provided a network-scale railway assessment across four railway corridors, using consistent satellite-based methodology.
Conducted extensive infrastructure screening across the Cape Town metropolitan rail network and a broader study area, extending approximately 30 km south and 30 km east of Cape Town.
Supported early identification of potential infrastructure concerns.
Detected millimetre-level ground movement with Persistent Scatterer InSAR. Identified locations exhibiting cumulative ground displacement of up to 70 mm over the five-year monitoring period.
Improved interpretation confidence and distinguished environmental effects from infrastructure-related issues via combined satellite deformation measurements with phase coherence analysis, NDVI vegetation mapping and high-resolution optical imagery.
Helped prioritise locations for further inspection - integrated analysis identified locations affected by vegetation encroachment, reduced radar coherence, damaged track components.
Complemented conventional inspections by providing scalable, repeatable and cost-effective monitoring for railway asset management.
Project overview
Understanding the condition of large railway networks using conventional inspection methods alone is challenging, particularly where access is difficult and environmental conditions vary significantly.
“Geofem was commissioned to assess sections of the Cape Town metropolitan rail network using historical satellite remote sensing to identify ground movement, vegetation encroachment and signs of infrastructure deterioration over a five-year period.”
Using for·see Rail, Geofem evaluated multiple railway corridors over a five-year period (2018–2023), combining advanced satellite Interferometric Synthetic Aperture Radar (InSAR) with optical imagery and vegetation analysis.
The objective was to provide the client with a comprehensive understanding of infrastructure condition, identify locations requiring further investigation, and demonstrate how remote sensing can complement conventional railway inspection programmes.
The challenge
The Cape Town railway network extends across a diverse range of urban, coastal and vegetated environments, making routine inspection both resource-intensive and operationally challenging.
Many sections of the network are affected by challenging environmental conditions, including dense vegetation and coastal sand accumulation, while ageing infrastructure presents additional operational and maintenance challenges. Together, these factors influence railway performance and complicate conventional monitoring approaches.
“A key challenge was distinguishing genuine ground deformation from areas where satellite measurements were affected by vegetation or a lack of suitable radar targets.”
Additionally, several sections of the railway displayed visible deterioration, including damaged or missing sleepers and rail components, requiring an integrated approach that combined multiple independent datasets to provide reliable interpretation.
A scalable methodology was required to assess the condition of extensive railway corridors while identifying priority locations for targeted field inspections and future maintenance.
Cape Town Railway located in densely vegetated area
The Geofem approach
Geofem developed a comprehensive remote sensing workflow that integrated multiple geospatial datasets to produce a network-scale assessment of the Cape Town railway infrastructure condition.
“The Geofem team processed approximately five years of Historical Sentinel-1 SAR imagery using advanced Persistent Scatterer InSAR techniques to derive millimetre-scale measurements of ground displacement along the railway corridors.”
These deformation datasets were complemented by phase coherence analysis to assess radar measurement quality and identify locations where vegetation or environmental conditions reduced the density of reliable scatterers.
Evaluation of vegetation conditions surrounding the railway using the Normalised Difference Vegetation Index (NDVI) enabled direct comparison between vegetation growth and reductions in radar coherence.
High-resolution optical imagery was then used to visually validate the satellite observations and investigate areas where damaged infrastructure, missing sleepers or track deterioration were suspected.
Rather than analysing isolated locations alone, Geofem assessed complete railway corridors by dividing the network into manageable sections and evaluating deformation, coherence and vegetation characteristics continuously along each route.
Individual sites exhibiting significant deformation or infrastructure deterioration were then investigated in greater detail through displacement time-series analysis and visual interpretation.
“Geofem’s integrated methodology enabled the client to distinguish between genuine ground movement, environmental influences and infrastructure-related issues while providing a comprehensive understanding of railway asset condition across the network.”
Results
The assessment successfully demonstrated the value of integrating satellite remote sensing with complementary geospatial datasets for large-scale railway asset management.
Multiple sections of the railway network were identified as experiencing measurable ground movement, with cumulative displacement reaching approximately 70 mm over the assessment period.
The time-series analysis revealed progressive deformation trends at selected locations, providing valuable information on the evolution of ground movement over time.
The combined interpretation of phase coherence, NDVI and optical imagery also identified numerous sections where reduced radar coherence resulted from dense vegetation rather than infrastructure instability. This significantly improved confidence in the interpretation of satellite-derived measurements.
Visual inspection of high-resolution imagery confirmed several locations where damaged railway tracks, missing sleepers and deteriorated infrastructure corresponded with the satellite observations.
Corridor-wide analysis further highlighted sections affected by vegetation encroachment and coastal sand accumulation, providing additional context for maintenance planning.
The project demonstrated how satellite-based monitoring can efficiently screen extensive railway networks, prioritise high-risk locations for inspection and support proactive asset management while reducing reliance on extensive field investigation.
Displacement time series for the selected railway section shown in figure 1 illustrating progressive ground movement identified through satellite InSAR monitoring over assessment period.
By combining satellite-derived deformation measurements with vegetation analysis and engineering interpretation, Geofem enabled the client to focus field investigations on the locations most likely to require intervention. This reduced unnecessary inspection effort while providing a network-wide understanding of infrastructure performance that would have been difficult to achieve using conventional surveys alone.
Geofem Insights
A data-led approach gives field investigations a much stronger starting point. Satellite-derived movement data can screen the whole rail network consistently, highlighting locations where deformation is actually occurring rather than only where problems have been reported or expected. This helps target boreholes, inspections and monitoring where they will add most value, improving understanding of the cause of defects, reducing missed risks and making investigation budgets more defensible.
"The Geofem team was professional, knowledgeable, and a pleasure to work with throughout the project."
They took the time to understand our requirements and translated a complex satellite dataset into clear, practical engineering insights. Their analysis of the InSAR data provided us with a network-wide understanding of historical ground movement across the Cape Town rail network, delivering spatial and temporal coverage that simply cannot be achieved through conventional inspection methods. This was particularly valuable given that our own physical verification was necessarily limited to targeted spot checks. The analysis quantified ground movement over multi-year time series and identified discrete areas of active deformation, distinguishing progressive settlement from seasonal variation and stable ground conditions. The work delivered value on two fronts. First, it enabled us to target our limited site inspections and geotechnical investigations at locations where the data indicated genuine cause for concern, rather than relying on conventional sampling assumptions. Second, where field verification was undertaken, the observed conditions correlated well with the satellite-derived results, providing confidence in the technique as an effective screening tool for sections of the network that cannot be inspected physically at the same frequency. The analysis also identified several previously unflagged locations that have since been prioritised for further investigation. We were particularly impressed by the quality of the analysis, the responsiveness of the team, and their technical expertise throughout the project. Their work demonstrated that satellite-based remote sensing is a powerful complement to traditional inspection methods, enabling earlier identification of potential issues, more targeted maintenance interventions, and better-informed, proactive asset management.
Christian de Charmoy, Business Unit Manager, PrEng, MEng (Transportation), Bigen
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