Geodetic pre-measurement with total station and track measuring trolley
When pre-measuring with a total station and a track measuring trolley, the track position is recorded by geodetic surveying. This measuring method is particularly suitable if the quality of the reference point field must be checked. This enables the identification of incorrect reference points in addition to the track surveying.
In classical geodetic surveying, the total station is usually mounted on a tripod next to the track and its position is determined by free station setup (resection). Horizontal directions and vertical angles as well as distances to several known reference points are measured by electronic distance measurement directed towards with a prism. For this purpose, at least three track marking points should be measured. Each further measuring point leads to an overdetermination, which enables conclusions to be drawn about the quality of the reference points.
After the position of the total station is known, each prism position in this reference system can be recorded as a three-dimensional coordinate. The prism can also be mounted on a measuring trolley, which simultaneously records the track gauge in the track. This enables the determination of the as-built track position and height for left rail, right rail and track center line with only one measurement.
With integrated measurement systems (e.g. Trimble GEDO Track), the software directly compares the as-built position with the design alignment and displays the deviations on the fly. The user immediately gets an impression of the currently necessary lifting and lining and can react instantly if necessary. In addition, the track tangent points are displayed and can be marked in the location for the synchronization of the tamping machine.
Modern total stations can automatically detect and track a prism and continuously provide the measured values (tracking). With this mode it is possible to continuously record the position of the track while the track measuring trolley is constantly pushed. The measured values are stored fully automatically in the predefined point interval.
Precondition for a successful measurement is an unhindered aiming of the prism by the total station. The area that can be measured from one position depends on the surroundings of the track. Refraction (e.g. by air flickering) can also limit the transmission of the measurement signal. For this reason, it is necessary to change the total station position several times for longer measuring distances.
Due to the limited accuracy of the reference points and the measuring accuracy, each measurement results in a slight difference in the three-dimensional coordinate of a position. In order to obtain a homogeneous transition between the areas recorded from different positions, the connection area is measured twice (from two station setup positions). In the evaluation of the measurement data, the overlap area is then checked and distorted. The deviations between the final as-built track trajectory and the design alignment are used for the preparation of the correctional values for lifting and lining for tamping machines.
Like most manually operated measuring devices, the measuring method is characterized by the fact that the track is not permanently blocked by machines during the measurement. Instead, the measuring trolley can be lifted off the track at short notice at any time and let track construction machines or trains pass without any problems. After the brief interruption, the measurement can be continued.
The advantage of geodetic surveying is mainly the result of the resection. By using several backsights for the station setup, the quality of the reference points can be checked simultaneously, and any unstable markings identified. Therefore, in some countries this measuring method is mandatory as proof of accuracy.
Of course, the target positioning and measurement of the reference points results in an expenditure of time which limits the performance of the system. For easier positioning in the track and for easier transport between positions, the total station can be mounted on a second track measuring trolley instead of a tripod. Another possibility to speed up the measuring process is the use of two total stations. Thus, one total station can be set up while the other – already set up total station – is used for the measurement with the track measuring trolley.
In friendly cooperation with ©Trimble Railway GmbH