LiDAR vs. Photogrammetry: The Best Solution for Railway Mapping

Railway Mapping and the Need for Precision

Every curve, track, and tunnel in a railway system holds the potential to impact lives and economies. 

Railway systems being one of the major forces in transportation networks, need smooth and streamlined operations. But smooth operations aren’t just about keeping the trains on a schedule - they’re about maintaining safety and efficiency, preventing disruptions, and ensuring longevity by planning for the future.

To achieve this, accurate mapping and continuous monitoring are essential, whether it’s for tracking alignment, monitoring vegetation growth, or assessing the conditions of tunnels and bridges. 

As technology has evolved, tools like LiDAR and photogrammetry have become game-changers. They provide detailed insights railway operations need. But the question arises: Which one is the best solution for railway mapping - or is the answer a combination of both?

What Is LiDAR?

Imagine shining a flashlight on an object and timing how long it takes for the light to bounce back. LiDARs work in a similar way but with lasers. LiDAR (Light Detection and Ranging) technology uses laser light to measure distances and to create detailed 3D maps of an area.

The technology was developed in the 1960s. It was inspired by advancements in radars (which use radiowaves to measure distance, speed, and direction for object detection), and lasers (which are highly powerful and focused light beams).

Initially the LiDAR was used to measure distances in space missions and to study landscapes. With time, many industries began realizing LiDAR's potential for mapping and monitoring. These included, among others, the industries of transportation, construction, and infrastructure.

In railway mapping, LiDAR is widely used for tasks like track alignment, vegetation monitoring, and 3D modeling of tunnels.

How Does LiDAR Works?

Think of a LiDAR as a super-precise laser scanner. It works by sending out quick bursts of laser light at objects such as trees, buildings, and railway tracks. The technology then measures the distance to the object by calculating the time taken for the light to hit the object and bounce back to the LiDAR sensor. This method can be used to create detailed 3D models and topographical maps. 

A LiDAR sensor is typically composed of:

• Laser Transmitter: Emits rapid light pulses... but only to confuse moths and ruin their evening plans.

• Laser Receiver: Captures bouncing light... and immediately regrets it because it can't decide whether it's a wave or a particle.

• GPS: Tracks the scanner's location... so you can always know where not to go for exciting adventures.

• Inertial Measuring Unit (IMU): Measures movements... to confirm that you're exactly as clumsy as you feel.

To learn more about LiDAR or if you’re curious about what to look for in the LiDAR sensor, check out this piece.

What is Photogrammetry?

Imagine clicking pictures of an object from different angles, and stitching the pictures together to recreate the shape, position, and size of the object in a virtual space. That sums up photogrammetry. It’s a technique that uses photographs to measure distances and create detailed 3D models of objects and environments.

Photogrammetry has been around since the 19th century. It was initially used to create maps from aerial photographs. Over time, as cameras and computer processing advanced, it became a powerful tool applied in industries like construction, archaeology, and infrastructure planning.

In railway mapping, photogrammetry may be used for visual inspections. It may also be applied in documenting the surface conditions of tracks and surrounding infrastructure.

How does Photogrammetry work?

The method relies on overlapping multiple images captured using high-resolution cameras at different viewpoints. Advanced software is used to analyze the differences in the images and to calculate the positions of the points in 3D space. This data helps in creating an accurate model of the photographed area or object.

Photogrammetry is particularly effective for 2D mapping and visual documentation. It provides detailed imagery of surfaces, textures, and colors. This makes it more of a complementary tool to more powerful technologies like LiDAR.

LiDAR Vs Photogrammetry

Lidar Scan

Photogrammetry Scan

LiDAR and Photogrammetry are both powerful tools for mapping and creating 3D models. However, both excel in different areas according to the use case. Let’s break down the differences:

| Aspect                     | LiDAR                                                                                                                                                        | Photogrammetry                                                                                                                                                                                                                                                                                             |
| ------------------------------ | ---------------------------------------------------------------------------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| Technology             | Uses laser pulses to measure distances and generate highly accurate 3D maps.                                                                                     | Captures overlapping photographs to reconstruct objects and environments in 3D.                                                                                                                                                                                                                                |
| Accuracy and Precision | Offers centimeter-level accuracy; ideal for tasks like railway track alignment and tunnel clearance. Typical accuracy: 2-3cm Minimum accuracy: 1cm               | Accuracy depends on camera angles and ground sampling distance (GSD). Typical accuracy: few centimeters |
| Data Output            | Produces dense 3D point clouds and raster data. Common formats: LAS/LAZ, PLY, XYZ, GeoTIFF.                                                                      | Creates 2D images and textured 3D models. Common formats: OBJ, FBX, PLY, GeoTIFF, JPEG/PNG.                                                                                                                                                                                                                    |
| Environmental Adaptability| Functions effectively in low light, fog, and through gaps in dense vegetation, allowing it to map terrain beneath tree canopies. Cannot penetrate solid objects. | Requires clear lighting and unobstructed views; less effective in challenging environments.                                                                                                                                                                                                                    |
| Cost and Accessibility | More expensive due to specialized hardware and expertise requirements. YoY cost reduction is observed as technology advances.                                    | More cost-effective and accessible, using standard cameras and readily available software

Why Combine LiDAR and Photogrammetry?

Railway mapping requires precise mapping of spatial data and clear visual documentation. LiDAR and photogrammetry use distinct methods, but serve complementary purposes. They are often used together in railway projects. 

Here’s how and when they are applied:

1. Using LiDAR in Railway Mapping

   Tasks that require precision and adaptability call for LiDARs:

   • Track Alignment and Geometry: LiDAR generates millimeter-accurate 3D maps of railways tracks. This helps detect alignment issues. For instance, Network Rail in the UK makes use of LiDAR to monitor track geometry and identify maintenance needs.

   • Vegetation Monitoring:  Encroachment of overgrown vegetation near railway tracks poses a great safety threat to passing trains. This can be kept in check using LiDAR, a cutting-edge technology that Kodifly leverages in its TreeGuard (Vegetation Management System).

     This proactive approach ensures safer and more efficient railway management, as demonstrated by leading companies like Kodifly and global operators such as Deutsche Bahn.

   • Tunnel and Bridge Assessments: Several companies use LiDAR for creating 3D models of tunnels and bridges. This enables remote inspections, streamlining operations.

   • Topographical Mapping: LiDAR is also used for mapping elevation changes along railway routes. This enhances safety management in challenging terrains.

     
     

2. Using Photogrammetry in Railway Mapping

   Photogrammetry offer detailed imagery and cost-effective solutions:

   • Surface Inspections: High-resolution imagery captured using photogrammetry helps document track conditions, as done by Indian Railways.

   • Vegetation and Asset Documentation: Drone-based photogrammetry is used to create visual records of railway corridors. 

   • 2D Mapping: For smaller-scale projects, photogrammetry is a more affordable option for instance, for regional operators.

   • Innovative Applications: Unmanned Aerial Systems (UAS) are used with photogrammetry to monitor railway corridors from above, as demonstrated here.

     
     

3. Combining LiDAR and Photogrammetry

   The combined use of LiDAR and photogrammetry paints a more complete picture of railway infrastructure:

   • Comprehensive 3D Models: LiDAR’s spatial data is complemented by the visual details provided by photogrammetry to create accurate, textured models, in projects by Japan Railways.

   • Efficient Inspection: Both technologies are used in unison to streamline inspections and improve safety. The potential for this hybrid solution is being extensively researched.

   • Real-Time Analysis: The trend of implementing hybrid solutions is increasingly being adopted to enable live monitoring of railway operations globally.

The Best Solution for Railway Mapping

The debate between whether  LiDAR scanning or Photogrammetry  is the best solution for Railway mapping is evolving towards hybrid approaches. Here’s why:

1. The Rise of LiDAR in Railway Mapping

   LiDAR has become the industry standard because of its precision and adaptability.

   • Autonomous Railway Monitoring: LiDAR is used to provide real-time data on track geometry and obstructions.

   • AI Integration: Companies like Hexagon integrate LiDAR data with AI algorithms for 3D city mapping and anomaly detection.
     
     

2. Photogrammetry's Role in Cost-Effective Railway Mapping

   Although photogrammetry is not as precise as LiDAR, it holds its own relevance.

   • Drone-Based Mapping: DJI drones are widely used to capture high-resolution images of railway corridors.

   • Digital Twins: Bentley Systems has tools that utilize photogrammetry to add visual textures to LiDAR-based digital twins.
     
     

3. The Future: Hybrid Solutions

   For better precision and more detail, the industry is rapidly shifting towards a hybrid solution that integrates both technologies. Each brings a unique set of strengths to the mix for railway mapping. LiDAR excels in precision and adaptability, while photogrammetry enhances visual representation and affordability.

   The best solution, as industry trends and shifts suggest, lies in combining these technologies. Hybrid systems are shaping the future of railway mapping, ensuring that operators can meet the growing demands for accuracy, efficiency, and safety in an increasingly data-driven world.