Are negative values allowed for ADAS Absolute Height?

ADAS Absolute Height shows negative values for road geometry or GPS coordinates

Symptoms
--------

Customers may observe one or more of the following:

ADAS Absolute Height contains negative values.
Height values appear below 0 metres despite the location being above sea level.
Calculated elevation seems inconsistent with local terrain expectations.
GPS-derived coordinates return negative ellipsoidal heights.
Road features in ADAS data display negative altitude measurements.
Locations in regions such as Florida show negative ADAS Absolute Height values.

Answer
------

Yes. Negative values for ADAS Absolute Height are valid and represent expected behaviour. ADAS height measurements are based on the WGS84 reference system and associated geodetic models. A negative absolute height does not automatically indicate incorrect data or a platform issue.

Root Cause
----------

WGS84 uses a mathematical reference ellipsoid rather than mean sea level as its primary reference surface. The Earth’s geoid, which approximates mean sea level, is an irregular surface caused by variations in the Earth's mass distribution. Because the geoid and reference ellipsoid do not perfectly align, the geoid undulation varies globally and can be either positive or negative. Reported geoid deviations can range from approximately +85 m to -106 m depending on location.

For locations where the geoid lies below the reference ellipsoid, negative ellipsoidal heights may occur. This is a normal geodetic condition and does not indicate corrupted map content or ADAS data.

Example
-------

A documented example uses coordinates near Florida (27.950324185955257, -82.45113675582347), where negative ADAS Absolute Height values were observed. Geoid calculations for that region indicate a geoid height of approximately -26.786 m, demonstrating that negative values can occur naturally because of local geoid separation from the reference ellipsoid.

Impact
------

Negative ADAS Absolute Height values can be valid and usable.
Routing, positioning, and ADAS workflows are not adversely affected solely because a height value is negative.
Data should not be filtered, corrected, or rejected based only on the presence of a negative height.

Expected vs. Unexpected Behaviour
---------------------------------

### Expected

ADAS Absolute Height may be positive or negative depending on the relationship between the local geoid and the WGS84 reference ellipsoid.
Different geographic regions may produce significantly different height offsets.

### Unexpected

Large height discrepancies that cannot be explained by geodetic reference systems.
Missing, null, or inconsistent height values across identical coordinates.
Results that contradict the documented coordinate reference system being used.

Recommended Actions
-------------------

1. Verify the coordinate reference system used in the workflow.
2. Confirm whether the height value is ellipsoidal height or orthometric (mean sea level) height.
3. Compare the location with an independent geoid model or geoid-height calculator when validation is required.
4. Treat negative ADAS Absolute Height values as valid unless additional evidence indicates a data-quality issue.
5. Products: HERE ADAS Content / ADAS Attributes using WGS84-based positioning references.

Keywords: ADAS Absolute Height, negative height, WGS84, ellipsoidal height, geoid height, elevation, altitude, orthometric height, GPS height, geoid undulation, elevation reference, ADAS attributes, map data, Florida height values, geodetic model.