With more than 34,000 kilometers (21,000 miles) of rail lines, DB Netz AG (DB) operates one of the largest rail systems in Europe. Each year, DB carries more than two billion passengers on its network of intercity, regional and local routes. To handle increasing demands for capacity and safety, DB continually maintains and upgrades its infrastructure and rolling stock. The company relies on 3D surveys by in-house crews and contractors to provide precise location data on the structures and installations of the railway system. To manage its assets, DB maintains a system-wide geographic information system known as DB-GIS. 

As changes take place within the DB network, a critical safety requirement is to ensure adequate clearance between trains and the structures adjacent to the tracks. The work is so important that DB uses a separate database named Lichtraum Datenbank (Clear Space Database, or LIRA) to track clearances and encroachments along the railways.

Surveying plays a key part of keeping both the LIRA and DB-GIS databases current. While DB employs some surveyors for its in-house needs, most of the encroachment work is done by contractors. These firms must adhere to DB’s strict safety and operation guidelines while meeting time and cost constraints. To assist in these efforts, researchers at the Technical University of Dresden (TU-DD) have produced a new, more efficient approach of measuring and recording encroachment information and delivering it to the DB databases. 

Traditional Methods

Although the DB system is thoroughly measured and mapped, ongoing changes to buildings, power lines and other structures located near the tracks must be accurately measured to ensure safe passage. All measurements must meet DB’s requirements for accuracy, quality control and compatibility with existing databases. Additionally, DB requires the encroachments to be related to the track alignment as well as tied in to the DB geodetic coordinate system (DB_REF). 

In undeveloped and rural areas, kinematic measurement is the traditional approach to measuring track encroachments. A trolley carrying measuring equipment and an operator rides on the train tracks. GNSS or optical positioning sensors provide the location of the trolley along the track alignment, and the operator uses a laser rangefinder to measure distances to the structures of interest. The kinematic method requires expensive equipment, and tracks must be closed to train traffic while surveys are underway.

In more congested areas such as urban corridors and train stations, static measurements are the norm. By using photogrammetric methods, individual transverse profiles (cross sections) can be collected and referenced to the track alignment. To provide reference information for the tracks, the static methods must use a measuring frame that is placed on the tracks while the photos are collected. The photos are rectified and scaled using the known dimensions of the measuring frame. The method produces good results, but the measuring frames are heavy, difficult to handle and require frequent calibration.

A New Approach

Led by Prof. Dr.-Ing. habil. Michael Möser, a team from the Geodetical Institute at TU-DD has developed a new method for static measurement of encroachments. Seeking to eliminate many of the problems of the photogrammetric approaches, the team uses a Trimble VX Spatial Station with Trimble CU Controller to capture detailed information about encroachments and clearances along the tracks. Similar to the static methods, the Trimble VX captures digital images that are used in the photogrammetric analysis. Additionally, the new method takes advantage of the instrument’s survey positioning capability to streamline work in the field and office.

Next, the team measured directly to the suspected encroachment, again using DR measurements. “The DR capability lets us measure objects that are too high or otherwise difficult to access,” says Dipl.-Ing. Mandy Kolb, one of Möser’s students during the project at TU-DD. "By using the station and offset function in Trimble Survey Controller Software, we computed the station value of the encroachment relative to the two reference points attached to the rail on the ground.”

The team then set out two more reference points, this time placing one point on each rail. These points had to be at the same station as the computed station of the encroachment and were placed using the stakeout-by-station function in Trimble Survey Controller. According to Kolb, the software’s overlay function helped the setout go quickly by providing a visual reference of the setout point in the video display from the Trimble VX. When the new points were in place, the team placed the target on each point and measured their position. These data confirmed that the reference points were located in the same plane--radial to the track alignment--as the encroachment. Back in the office, the reference targets would serve to define the vertical reference plane for rectifying the photos.

Finally, the survey team used the video imaging capability of the Trimble VX to capture digital images of the encroaching drainpipe together with the two reference points. It took only a few minutes to capture images of the entire scene, with all photos and measurements tied to the DB_REF coordinate system. Möser says that the process, which moves swiftly, could be repeated for each encroachment along the track.

In the office, the team used Trimble RealWorks Software to create the rectified images. They made an initial quality check by correlating the reference objects against the images. After confirming that the positions measured using the Trimble VX agreed with positions taken from the rectified images, the team used the image data to compute coordinates of the encroaching objects in the station and alignment system. The coordinates of the instrument, reference objects and encroachment were then transformed into the DB_REF coordinate system. From there, the true stationing of the reference objects and encroachment could be determined from the known track geometry and ties to the DB_REF system. To meet DB requirements, the horizontal and vertical offsets from the alignment must be accurate to within 7 millimeters (0.2 feet).

With the photos rectified to the plane of the encroachments, the team used Trimble RealWorks to create a hybrid view of the scene. They superimposed the clearance envelope onto the image to provide an accurate, detailed picture of the encroachment and took measurements in the image to determine clearances and other dimensions. The information could then be exported into tabular format for analysis using spreadsheet software, and hybrid views could be provided to DB in a standard JPEG image format.

The video capability of the Trimble system has replaced the need for field sketches, and the ability to visualize encroachments against the clearance envelope provides image data as well as measured values. The new approach also provides direct ties to control points and the DB_REF system.

Möser says that the Trimble VX Spatial Station is well suited for the complex work of railway clearance measurements. “The approach is fast; it delivers complete, concise data; and it does not require heavy trolleys, reference frames or other specialized equipment,” he says. “Because of the speed of the process, it reduces the amount of time that survey teams must spend on the track or in hazardous areas.” In addition to its application for railways, the method can be used to locate objects along power lines, roads and other alignment-based installations.

As trains grow larger and move faster, it is critical to ensure clearance along the tracks. DB has developed strict regulations for the clearances, including precise dimensions of the clearance envelopes along the tracks. The envelopes can vary based on the types of equipment that will travel on a particular section of track.

Encroachments are referenced to the centerline of the track alignment, with horizontal and vertical offsets from the centerline. In areas of super-elevation, the clearance envelope tilts along with the track so that it is always perpendicular to the plane of the two rails.

Frequent clearance surveys look for changes along the tracks, and any object that penetrates into the clearance envelope is identified as an encroachment. All the penetrations are gauged against the clearance envelope, and any intrusion must be noted in DB’s clearance database.