| associated netElements coordinates for LPS and measure precision issues [message #3642] |
Wed, 11 June 2025 16:14  |
Fredrik Jönsson
Messages: 14
Registered: June 2024
Location: Sweden
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Junior Member |
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When using linearCoordinateBegin/associatedNetElement/linearCoordinateE nd a measure is given in meters refering and a ref to a linearPositioningSystem via associatedPositioningSystem.
The measure is calculated by taking the value of last km board(km post) times 1000 + meters after said board. Due to the distance between kilometer boards not always being exactly 1000m the measure for a linearCordinate might differ from the actual real value. We've not dealt with a situation like this before as the Swedish equivalent of measure in our legacy system is given in the format of (km + m), for example (44+500) would be 500m after km-board 44, no matter how long or short the "kilometres" are, the location is still "accurate" as it doesn't have to refer back to the start of the LPS only the last board.
Our tool railexporterSE https://www.railml.org/en/software/railexporterse does not know the distance of all kilometers before where the export begins, it only has data from the given scope of the current export. Therefore, railexporterSE does not know the actual distance from the start point (measure = 0) of the LinearPositioningSystem(s) to the start of the given scope of the export.
How important is it that the value of "measure" is 100% spot on? We mostly see this as a data quality/ precision issue.
The way we have used LPS in the Swedish railroad system it's a "secondary way" of locating objects in the network. Our main/master way of locating is using meters from last node/switch. Which is converted to intrinsic coordinates on linearLocation/associatedNetElement in railexporterSE. Therefore, we see the intrinsic coordinates as the "Must have" for precise locating of assets in the network and the LinearCoordinate with measure and LPS-ref to be used for a quick overview where the precision is not that important, the important part is that the assets with lower measure are physically located "before" assets with hight measure.
For example: How big issue would it be if a signal is given a measure of 114600m (yes we have atleast one LPS where this is possible) but the actual measure might differ with a few 100m if you were to accurately measure the distance in meter from the start of the LPS to the end. As long as all objects in the exported railML-file has the same offset to the true distance along the LPS there is no real problem, objects closer to the start of the LPS will get a lower measure than objects closer to the end of the LPS 99.9% of the time.
Example
associatedNetElement/linearCoordinateBegin and linearCoordinateBegin/associatedNetElement/linearCoordinateE nd measure values refer to the green locations.
On the third location you get a discrepancy due to the previous kilometre not being 1000m
Measure = 2010 m (green location)
Real value = 1960 m (red arrow)
950m is an exaggerated value to illustrate the problem, usually the distance between boards is 1000 +/- 5 m.
Has anyone else dealt with a similar issue, and if so, what was your solution?
Fredrik Jönsson
Trafikverket - Swedish Transport Administration
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