Author Topic: What is Local height with reference to Railway Track Segments?  (Read 314 times)


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Track Segment Properties contain a parameter called 'Local Height'.
It is primarily intended as a 'helper' parameter to enable the height of a track segment to be calculated from the NoiseMap ground model contours, but it can also have another use, which is to define when a section of track is above ground level, on a bridge or viaduct.

The ground level profile is defined only by the ground contours.  Whether the track is on level ground, on an embankment or in a cutting, there should be ground contours (in fact ground profiles, as they need not be at a constant level) along the edges of the 'permanent way'.  However, there is likely to be a small distance between the edge of the closest ground profile and the edge of the adjacent track segments.  If 'Local Height' is set to zero, NoiseMap assumes that the ground slopes uniformly from the ground profile to the track segment, as these may not be at exactly the same height.

However, suppose the track segments are on an elevated structure?  In this case, the ground will pass at a lower level, below the track: it will not slope up to meet it.  In this situation, you would set the Local Height at a suitable value.  For example, if the structure is 10 m above the local ground level, set the 'Local Height' parameter to 10.  NoiseMap will then assume that the ground is 10 m below the track and that in essence there is open space beneath it.

1.   To allow NoiseMap to calculate the track height if you don’t have an accurate height profile of the track –  if you click on ‘select method to calculate height', NoiseMap will find the heightof the ground from the ground model and use the local height and to calculate the height of the track segment.
2.   Local Height also defines how the ground rises or falls to the edge of the track.
a.    Thus if 'Local Height' is zero, NoiseMap assumes that the ground rises or falls from the last ground contour until it meets the track with the track at ground level. This avoids having to have a contour that exactly follows the edge of the track.
b.   If Local Height is positive, then the track is assumed to be on a structure with a gap underneath it.
c.   If it is negative, then the track is in a cutting and the ground drops from the last contour until it meets the edge of the track.  The track is not underground.

If you set the height above local ground to a huge value and leave it at that value when the calculation is run,  NoiseMap does not change either the height of the ground or the  track.  It uses the track level already defined in the Object Position coordinates, and also assumes that the ground profile runs smoothly between the contours on each side of the track. In other words, it does not check that the height above local ground is actually correct at the stage the calculation is run.

When you do click on ‘calculate height’ and NoiseMap resets the height of the start and end of the track, it doesn’t reset the height above ground parameter.  If you are using the local height, we would advise that it is best to set this to zero where the tracks are at local ground level.