I have some locations in EPSG:2284:

(3669486.63386,11293013.10592,709.08004),
(3669559.13811,11292972.72831,711.60055),
(3669639.51308,11292851.48264,712.22258),
(3669800.62304,11292949.38118,714.75766),

These are in US Survey Ft, however I don't understand what the elevation is relative to?

The reason this confused me if I have the following image showing these locations from the air and the EXIF data in the image reports an Absolute Altitude (which I assume is height above ellipsoid) of approximately 200m - which would make it just under these points?