Question: Why does the system require ellipsoid elevations for runway profile elevations?

Answer: The system uses all Airport Control Points (including runway profile points) independent of, and together with, other types of Airport Control Points, as well as with other types of features.

To support the variety of ways in which this data is used, the point must be referenced to a common horizontal and vertical datum. Using the ellipsoid height provides a common vertical reference without the need for conversion or additional processing. It also ensures that additional error is not introduced.

The ellipsoid height or ellipsoid elevation is defined as the distance between a point and the reference ellipsoid taken along the perpendicular to the ellipsoid. Ellipsoid heights result from GPS observations. Ellipsoid heights are positive if the point is above the ellipsoid.

Ellipsoid Height = GEOID Height + Orthometric Height.

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