Are different Z-Level values for the same bridge or tunnel location correct according to HERE specifications?

Problem / Question

Different z-level values are observed at the same bridge or tunnel location – even along the same road and direction of travel.

For example, a bridge displays z-level values of "0" and "1", although the elevation of the bridge does not change along its course.

Question:
Should the z-level be consistent (e.g., only "0" or only "1")?
Is this behavior an error?

Short Answer (Resolution)

No, this is not an error.
The observed z-level differences are expected behavior according to HERE specifications.

Explanation

According to the HERE data model (e.g., RDF Reference Guide – Section 3.5.6 Attributes), the z-level represents a relative vertical position, not an absolute elevation.

Key Points:

Z-levels are relative values, not absolute height indicators.
They describe the position of a feature relative to other intersecting features at the same geographic location.
The value indicates how many navigable or hydro features are above or below a given element.

How Z-Levels Work

A z-level of "0" represents ground level (grade).
Positive values (e.g., +1, +2) indicate the feature is above other features.
Negative values indicate the feature is below other features.

Why Z-Levels Can Differ on the Same Bridge or Tunnel

Z-level values can vary within the same physical structure due to:

1. Relative Position at Specific Locations

Z-level is calculated at each node or shape point, not for the entire object.
If a road passes over different numbers of underlying features along its path, the z-level changes accordingly.

2. Multi-digitized Roads

Opposing directions of travel may have different z-levels, depending on how many features each side crosses.

3. Crossing Other Features

Example:

At one point:
+ The road crosses one feature → z-level = +1
At another point:
+ The road crosses two features → z-level = +2
If no crossing occurs:
+ The road remains at z-level = 0, even if physically elevated

Important Clarification

Even if a road is visually elevated (e.g., a bridge), its z-level remains "0" unless it actually passes over another mapped feature.

Conclusion

Different z-level values along the same bridge or tunnel are correct and expected, as they depend on:

The number of intersecting features at each specific location
The relative vertical relationship between features

This behavior is fully compliant with HERE specifications and should not be treated as a data error.