Support
Why does the grade line elevation not match between the long section and the cross section?
Summary
Differences between the grade line elevations shown in the long section and those shown in the cross sections of the same alignment are common. In most cases this is not a calculation error, but one of two situations: either the point where the grade line elevation is applied is not on the alignment of the template, or the long section was generated and represented with different intervals, so that the elevations at intermediate stations are obtained by interpolation. This article explains how to tell the two apart and correct them.
Applies to
MDT — the documented cases mention MDT 9 and MDT 25; the remaining queries do not state a version. The procedure is still valid in MDT26: the reference manual documents the same template definition dialog, with the pivot point and the point where the grade line elevation is applied.
Long section and cross section functions, and template editing.
Symptoms
The grade line elevations of the cross sections do not match those of the long section of the same street, with a systematic difference (all of them shifted by the same amount, for instance).
The grade line is drawn in the cross sections at a higher elevation than it should really have.
The elevations obtained in the long section for a given station do not match those of that same station in the cross sections.
Cause
The origin differs from case to case:
Pivot point off the alignment. The point to which the grade line elevation is applied within the section is not on the alignment, but has been moved to another point —the curb, for example. This usually happens when trying to change the superelevation pivot point. As a result, the grade line is drawn away from its real position.
Long section generated and represented with different intervals. When the long section is generated with the All option without specifying an interval, and then represented with a fixed interval, the elevations of the stations that were not explicitly generated are computed by interpolation between the previous and the following vertex. That is why they may not match the real surface elevation, which the cross section does show.
Solution
Case: the grade line appears shifted or too high in the cross sections (pivot point off the alignment)
In the template, the grade line elevation is applied at a specific vertex of the platform —the pivot, marked in the graphic with a red arrow—, which is also the point from which superelevations are applied to the left and to the right. If that vertex is not the one you think it is, the grade line appears shifted or too high in the cross sections.
Open the template editing window (Templates > Create/Edit).
Check where the red marker is: it must be on the alignment vector, not on the curb or any other element.
To change it, first select the name of the vector where it should go and then press the button that defines the pivot point of the platform.
Two different buttons
The dialog has one button for the pivot point (from which superelevations are applied) and another one for the point where the grade line elevation is applied. They are independent: the second one lets the grade run along a vector other than the alignment —a curb, for instance— without moving the superelevation distribution. If it is only the elevation that does not add up, check that second button.
On platforms with a median there are two pivot points, one for each roadway.
To change the superelevation pivot point more flexibly there is also a specific function available from version 25 onwards (not available in version 9).
Case: long section elevations do not match the cross section at intermediate stations
If you compare elevations at equally spaced stations (10, 20, 30… for example) that you did not generate one by one, those long section elevations come from an interpolation and need not match the surface. To make them match the cross sections:
When generating the long sections, in addition to All, also select Distances and enter the interval you want (10, for instance).
When representing / drawing the simple long section, specify that same interval instead of All.
The long section elevations will then match those of the cross sections.
If the problem persists
Check that the grade line you are using is the right one and that it is associated with the corresponding alignment.
If the differences remain after reviewing the pivot point and the intervals, contact Aplitop technical support and attach the alignment, surface and template files, stating the exact station where you observe the discrepancy.
Category: Cross sections, slopes and earthworks · Level: Intermediate